AQA Physical Chemistry and Practical

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What is enthalpy of formation?
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The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.
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What is the enthalpy of combustion?
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Enthalpy change when 1 mole of a substance undergoes complete combustion in an excess of oxygen with all elements in their standard states under standard conditions.
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What is the enthalpy of neutralisation?
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Enthalpy change when one mole of water is produced from a reaction between an acid and alkali with all substances in their standard states under standard conditions.
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What is the first ionisation energy?
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Enthalpy change when each atom in one mole of gaseous atoms loses one electron to form 1 mole of gaseous 1+ ions
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What is meant by the term first electron affinity?
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The enthalpy change when each atom in one mole of gaseous atoms gains one electron, becoming one mole of gaseous 1- ions.
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What is meant by the term enthalpy of atomisation?
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Enthalpy change when one mole of gaseous atoms is formed from an element in its standard states under standard conditions.
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What is hydration enthalpy?
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Enthalpy change when one mole of gaseous ions become hydrated.
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What is enthalpy of solution?
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Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other.
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What is bond dissociation enthalpy?
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Enthalpy change when one mole of covalent bonds is broken in the gaseous state.
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What is lattice enthalpy of formation?
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Enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gas phase.
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What is lattice enthalpy of dissociation?
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Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase.
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What is enthalpy of vapourisation?
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Enthalpy change when one mole of a liquid is turned into a gas.
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What is enthalpy of fusion?
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Enthalpy change when one mole of a solid turned into a liquid.
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In a Born Haber cycle how is enthalpy of formation calculated?
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It is the sum of all the other enthalpy values.
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In a Born Haber cycle which direction does the arrow for lattice enthalpy of formation point?
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Down
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In a Born Haber cycle which direction does the arrow for lattice enthalpy of dissociation point?
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Up
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Why do some ions show covalent character?
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There may be distortion (they are polarised), meaning they aren’t perfectly spherical.
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Describe the Born Haber cycle
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Why might the lattice enthalpy be different in theory to the experimental value?
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covalent character
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What is entropy measured in?
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J/mol/K
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What state do substances have the most entropy in?
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Gases because particles move more rapidly and randomly.
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What is the 2nd law of thermodynamics?
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Over time entropy will naturally increase.
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How is °C converted into K?
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+273
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How would you carry out a titration?
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Rinse out conical flask with distilled water Rinse burette with reagent (known conc.) Run solution to fill tap Rough titration to estimate end-point Reading should be 2 d.p. end in 0 or 5 Average titre of concordant results Conical flask on white tiles
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How would you test for a carboxylic acid
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Add solid sodium carbonate +ve test fizzes and gives off gas that turns cloudy when bubbled through limewater
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How would you test for alkenes?
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Add bromine water Shake Turns orange to colourless
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How would you carry out a flame test for calcium?
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Dip nichrome wire loop in conc. HCl, dip into unknown compound, hold over flame, +ve test will give brick red flame
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What colour flame would strontium give in a flame test?
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Red
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What colour flame would barium give in a flame test
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Pale green
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How would you test for ammonium ions?
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Add dilute NaOH solution Heat Ammonia given off (turns damp red litmus blue)
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How would you test for sulfate ions?
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Add dilute HCl Add barium chloride +ve forms white precipitate
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How would you test for hydroxide ions?
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Red litmus paper into solution turns blue
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How would you test for the halide ions?
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Add dilute nitric acid Add silver nitrate solution Bromide ions form cream ppt. (dissolves in conc. ammonia) Chloride ions form white ppt.(dissolves in dilute ammonia) Iodide ions form yellow ppt.(won’t dissolve in conc. ammonia)
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How would you test for carbonate ions?
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Add dilute HCl +ve will fizz
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When is a dynamic equilibrium reached in a reversible reaction?
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When the conc. of reactants and products are constant, and the forward and backward reactions are going at the same rate.
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What requirements are needed for a dynamic equilibrium to form?
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Reaction must be reversible Must be in a closed system Constant temperature
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What is le Chateliers principle?
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If a reaction at equilibrium is subjected to a change in pressure, temperature or concentration, the position of equilibrium will move to counteract the change.
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Give an equation for Kc
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Kc= [D]^d x [C]^c/[A]^a x [B]^b Where lower case is the number of moles
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Describe how Rutherford determined his model of the atom?
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Alpha particles are fired at an atom, most pass through empty space, some are deflected very strongly by the nucleus.
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What is the general trend in 1st IE down a group?
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Decrease
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What is the general trend for atomic radius across a row?
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Decrease
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Which elements deviate from the atomic radius trend across the row from lithium to boron?
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Boron because it goes from a 2s orbital to a 2p orbital which has a higher energy level.
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Why is the melting point of carbon high?
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Macromolecular Many covalent bonds Covalent bonds are very strong
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What increases van der waal forces?
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Greater Mr Greater size Greater electron cloud
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What is the trend in melting point of the group 2 elements?
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Decrease because ionic radius becomes greater
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Why does aluminium not folllow the general trend of 1st IE?
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lowers because it goes from a 3s to a 3p orbital and the 3p is shielded by the 3s orbital
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What is the trend in m.p of the period 3 metals?
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Increases because there is a higher charge density, so stonger attraction between +ve ions and -ve electrons.
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What are the axis on a mass spectrometre?
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% abundance on Y axis mass/charge on X axis
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How do you calculate relative atomic massof an element from a mass spectrometre?
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Multiply the abundance of each peak with the mass/charge Add the totals Divide by 100
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What is the relative mass of an electron?
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0.0005
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What is the mass number?
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The number of protons and neutrons bound within the nucleus
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What is the atomic number?
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Number of protons bound within the nucleus
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What is an isotope?
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Atoms of the same element with different numbers of neutrons.
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Why do istopes have the same chemical properties?
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They have the same electron configuration
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Why do isotopes have different physical properties?
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Relative masses are different
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Why is the mass number of carbon 12.00000?
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By definition
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What is the relative atomic mass?
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The weighted mean mass of an atom of an element compared to 1/12 the mass of a 12C atom. Always given to 1 d.p
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What is relative isotopic mass?
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The actual mass of an atom of an isotope of an element compared to 1/12 the mass of a carbon atom. Always an integer value.
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What are the stages in a mass spectrometry?
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Vaporisation Ionisation- Sample dissolved in a polar solvent and pushed through a small nozzle at high pressure. A high voltage is applied so each atom loses an electron. Acceleration- Ions pass through an electric field, given an equal acceleration. Lighter ions accelerate more. Time measurement- drift with a constant speed Detection- smaller ions hit detector first and times are recorded to calculate mass/charge.
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What are the isotopes of bromine?
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79Br and 81Br
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What is an orbital?
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A region of space in which we find electrons
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What is Avogadro’s constant?
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6.02 x10^23
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How would you calculate the number of particles in a substance?
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No. of moles x Avogadro’s constant
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What is the ideal gas equation?
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PV=nRT P is in Pa Volume is in m³ R is gas constant 8.31 J/K/mol Temperature is in K
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What is atom economy?
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Atom economy is a measure of the proportion of reactants that become useful products
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How is atom economy calculated?
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relative atomic mass of desired product/total relative atomic mass of reactants x100
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Why is a high atom economy important?
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Shows how many atoms are ‘wasted’ Shows which processes are efficient
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What is empirical formula?
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The empirical formula of a compound shows the simplest/smallest whole number ratio of atoms of each different element in the molecule.
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What is the bond angle in a tetrahedral molecule?
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109.5℃
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What is the structure type for diamond, graphite and silicon dioxide?
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Macromolecular crystal type
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What is electronegativity?
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The ability of an atom in a covalent bond to attract the bonding pair of electrons towards itself.
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What three factors affect electronegativity?
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Proton number Amount of shielding Atomic radius
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What are intermolecular forces?
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Weak forces of attraction acting between adjacent molecules
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What is the structure of a tetrahedral molecule?
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What is the structure of a trigonal bipyramid molecule?
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What is the structure of an octahedral molecule?
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What is the structure of a pyramidal molecule?
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bond angle is 107
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What is the structure of a bent molecule?
What is the structure of a bent molecule?
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bond angle is 104.5
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What is the structure of a square planar molecule?
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b.a is 90
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State the trends in melting point across period 3 and which elements deviate from this
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Increases then decreases after silicon Al and s deviate
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What are the four types of crystal types?
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Ionic Simple molecular Giant covalent (macromolecular) Metallic
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Are simple molecular substances soluble?
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Polar simple molecular substances dissolve in polar solvents Non-polar simple molecular substances dissolve in non-polar solvents
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What equation would you use to calculate enthalpy?
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Q=mcΔt
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What is Hess’s law?
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Enthalpy change for a reaction is independent of the route taken.
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How would you calculate overall enthalpy change when given the enthalpy values for the products/reactants?
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making-breaking
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What does an exothermic enthalpy profile diagram look like?
What does an exothermic enthalpy profile diagram look like?
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What does an endothermic enthalpy profile diagram look like?
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When calculating enthalpy of a neutralisation reaction, what mass should be used?
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The mass of the water
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What does the maxwell-boltzmann graph look like?
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no. of particles on y axis
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Give two requirements for a reaction to occur between molecules in the gas phase?
answer

molecules must have EA have to collide with each other
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Why does the volume need not be known in calculating the value of Kc?
answer

The volumes cancel out because there are equal molar quantities on both sides
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What is the Gibbs free energy equation?
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ΔG=ΔH-TΔS ΔS is entropy
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Why may a feasible reaction not actually take place?
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The activation energy may be too high
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What value must ΔG be for a reaction to e feasible?
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Zero or below
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Why is entropy zero when the temp. is zero Kelvin?
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Particles are stationary Perfect order
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What is the cycle of enthalpy of solution, ionic solid and dissolved ions?
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ΔsolH= ΔhydrationH-Δlattice enthalpy of dissociationH
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What are the assumptions made when carrying out a clock reaction?
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concentration of each reactant doesn’t change significantly during reaction Temperature stays constant Reaction hasn’t proceeded too far when the endpoint is seen
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What is the purpose of a clock reaction?
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Measure how long it takes for a set amount of product to form and how that varies when you change the conc. of one of the reactants.
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What is the first part of the iodine clock reaction?
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A small amount of sodium thiosulfate solution and starch are added to an excess of hydrogen peroxide and iodide ions in acid solution. H₂O₂(aq) + 2I¯(aq) + 2H⁺(aq) ➝ 2H₂O(l) + I₂(aq)
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What is the second part of the iodine clock reaction?
answer

Starch turns blue/black in presence of iodine Sodium thiosulfate reacts with any iodine formed 2S₂Oз(aq) + I₂(aq) ➝ 2I¯(aq) + S₄O₆(aq)
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What is the third part of the iodine clock reaction?
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Once all the sodium thiosulfate has reacted, iodine will turn the starch blue/black. This is the end of the clock reaction. Then can vary [iodide] or [hydrogen peroxide]
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What is the rate equation?
answer

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What is meant by the term order of reaction?
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The order of reaction with respect to a given reactant is the power to which the concentration of reactant is raised in the rate equation.
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How is overall order of reaction calculated?
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sum of the powers of the concentration terms in the rate equation
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What is the Arrhenius equation?
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What does a concentration time graph look like for a reactant that is zero order?
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What does a concentration time graph look like for a reactant that is second order?
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What does a concentration time graph look like for a reactant that is first order?
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What does a concentration rate graph look like for a reactant that is zero order?
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What does a concentration rate graph look like for a reactant that is second order?
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What does a concentration rate graph look like for a reactant that is first order?
What does a concentration rate graph look like for a reactant that is first order?
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What affects the rate constant, k?
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Temperature (increases)
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What is the equation for kp?
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where p=partial pressure
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How is partial pressure calculated?
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mole fraction x total pressure
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What units are used for kp?
answer

K Pa
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What is the equation for kw?
What is the equation for kw?
answer

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Why is kw used instead of kc for the ionic product of water?
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Water conc. so high it acts as a constant Equilibrium lies far to the left
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How can hydrogen ion conc. be calculated from kw?
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[H+]=square root of kw
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Why is water always neutral even if the pH changes with temperature?
answer

[H+]=[OH-]
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Why does the value of kw increase with temperature?
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Dissociation is endothermic Equ. moves right to absorb heat
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What is the equation for ka?
answer

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What is a buffer solution?
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A buffer solution is one which resists changes in pH when small quantities of an acid or alkali are added to it.
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What happens when an acid is added to a weak acid buffer solution?
answer

H+ removed by salt, e.g. ethanoate ions to make ethanoic acid
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What happens when alkali is added to a weak acid buffer?
answer

Will either: OH- react with acid to make salt + water H+ from ionisation of acid combine with OH- to make water
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What happens when an acid is added to a weak alkali buffer?
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H+ reacts with alkali, e.g. NH3 forms NH4+
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What equilibrium is going on in an ammonia and ammonium chloride buffer solution and where will equ. be?
answer

NH3(aq) + H2O(l) ====> NH4+(aq) + OH-(aq)
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How can pH be calculated?
answer

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How can pKa and Ka be used in the same equation?
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so Ka = 10^-pKa
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How would you construct a pH curve?
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Measure initial pH of acid Add alkali in small amounts (note vol. added) Stir to equalise pH Record pH to one d.p Add until alkali in excess Calibrate meter by measuring known pH of a buffer solution.
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What does a strong acid strong base graph look like?
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What does a weak acid strong base graph look like?
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What does a strong acid weak base graph look like?
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What does a weak acid weak base graph look like?
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What is the trend in melting point across period 3?
What is the trend in melting point across period 3?
answer

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What is the trend in 1st ionisation energy for period three?
What is the trend in 1st ionisation energy for period three?
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What is the rate determining step?
answer

Slowest step in a reaction
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Describe how an acid reacts with water?
answer

HA(aq) + H₂O(l) → HзO+ + A ̄(aq)
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Describe how a base reacts with water?
answer

B(aq) + H₂O(l) ➝ BH+(aq) + OH ̄(aq)
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Give an example of a strong acid?
answer

HCl
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Give an example of a weak acid?
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Any carboxylic acid
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Give an example of a strong base
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Anything with hydroxide in it
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Give an example of a weak base
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Ammonia
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What are the units for kw?
answer

mol²dm¯⁶
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What is a diprotic acid?
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release two mol of H+ when 1 mole of acid dissociates.
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What happens to reactivity down a group?
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Increases because ionisation energy decreases
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What happens to a group 2 element when it reacts with water?
answer

M(S) + 2H₂O(l) ⟶ M(OH)₂ (aq) + H₂(g)
question

Give a use of barium sulfate?
answer

Barium meals
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What is used to extract titanium from its ore?
answer

Magnesium which acts as a reducing agent TiCl₄(aq) + 2Mg(l) ➝ Ti(s) + 2MgCl₂(aq)
question

How is sulfur dioxide removed from flue gases?
answer

wet scrubbing where slurry (calcium oxide/carbonate and water. Forms calcium sulfite. CaO(s) + 2H₂O(l) + SO₂(g) ➝ CaSOз(s) + 2H₂O(l) CaCOз(s) + 2H₂O(l) + SO₂(g) ➝ CaSOз(s) + 2H₂O(l) + CO₂(g)
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What is the trend of boiling points down group 7?
answer

Size and mr increases so van der waal forces are stronger.
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Which of the group 2 ions is sparingly soluble when bonded to hydroxide ions?
answer

Magnesium
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Which of the group 2 ions is most soluble when bonded to hydroxide ions?
answer

Barium
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Which of the group 2 ions is most soluble when bonded to a sulfate ion?
answer

Magnesium
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Which of the group 2 ions is least soluble when bonded to a sulfate ion?
answer

Barium
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What is a use of barium sulfate?
answer

Can be used to test for sulfate ions because they’ll react with barium chloride to form insoluble barium sulfate.
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Write an equation to show barium chloride reacting with sulfate ions?
answer

BaCl₂(aq) + FeSO₄(aq) ➡ BaSO₄(s) +FeCl₂(aq)
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What is a use for calcium hydroxide?
answer

Neutralise acidic soils in agriculture
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What is a use for magnesium hydroxide?
answer

Neutralise excess stomach acid
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When will a halogen displace a halide?
answer

If the halide is below it in the periodic table
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What happens to reactivity down group 7?
answer

Decreases because atoms are bigger so less attraction between electrons and protons. So they are less oxidising down the group.
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What happens to electronegativity down group 7?
answer

Decreases because more shielding
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How is bleach made?
answer

2NaOH(aq) + Cl₂(g) ⟶ NaClO(aq) + NaCl(aq) + H₂O(l) Chlorate(I) ion formed in NaClO and kill bacteria
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Give a use of chlorine?
answer

Kill bacteria in water
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What happens to chlorine when it reacts with water?
answer

Undergoes disproportionation Cl₂(g) + H₂O(g) ⇌ 2H+ + Cl-(aq) + ClO-(aq) In sunlight, it will decompose: Cl₂(g) + 2H₂O(l) ⇌ 4H+(aq) + 2Cl-(aq) + O₂(g)
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What happens to reducing power down group 7?
answer

Increases because more shielding and electrons are further away.
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What happens when sodium fluoride or sodium chloride reacts with sulfuric acid?
answer

Forms a hydrogen halide and NaHSO₄
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What happens when sodium bromide reacts with sulfuric acid?
answer

HBr and NaHSO₄
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What happens when sodium iodide react with sulfuric acid?
answer

Hydrogen iodide and NaHSO₄
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What happens when hydrogen iodide reacts with sulfuric acid?
answer

Iodine, sulfur dioxide and water
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What happens when hydrogen iodide reacts with sulfur dioxide?
answer

H₂
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Why does equilibrium shift when temperature increases?
answer

It moves towards the endothermic reaction; to oppose the change (to oppose increase in temp.)
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With reference to the Maxwell-Boltzmann distribution, explain why an increase in temperature increases the rate of a chemical reaction.
answer

Increase in the number of molecules with greater energy than the activation energy; So more frequent successful collisions
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Explain how a catalyst increases rate of reaction?
answer

Catalysts provide an alternative reaction pathway; with a lower Ea
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Suggest why a students value for enthalpy of combustion for methanol is different from that in a Data Book (besides incomplete combustion or heat transfer to the environment)
answer

Data book value came from use of mean bond enthalpy data
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Explain why a thermometer with an uncertainty of 0.5% is adequate for this experiment?
answer

Error on thermometer is less than other errors in the experiment
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Explain why a thin layer of a catalyst is spread over a honeycomb ceramic support.
answer

save cost; honeycomb increases surface area
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Explain why mass spectrometry cannot distinguish between propanal and prop-2-en-1-ol
answer

both have the same molecular formula; both would have the same mr
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Give one reason why bonds in molecules such as carbon dioxide absorb IR?
answer

IR radiation excite electrons in covalent bonds
question

reflux apparatus
answer

question

suggest how anti bumping granules work
answer

provide nucleation points for bubbles to form on
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why is a conical flask used in titration?
answer

to reduce any loss due to splashing
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why would you rinse the sides of the conical flask with distilled water near the end point?
answer

to ensure all splashes of acid and alkali are rinsed down and have to chance to react
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why can the conical flask be rinsed with distilled water?
answer

there is already a measured quantity of moles in conical flask so addition of water does not affect this
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why must the burette and pipette be rinsed with the correct solutions and not distilled water?
answer

to ensure no dilution of the solution
question

When analysing the purity of aspirin tablets, what is the biggest experimental error?
answer

not all 2-ethanoylhydroxybenzoic acid may be hydrolysed in simmering.

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