Y12 physical chemistry – Flashcards
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Activation Energy
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the minimum amount of energy required to start a chemical reaction
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Collision theory
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States that atoms, ions, and molecules must collide in order to react Conditions: Collide with correct orientation Must have sufficient energy to overcome activation barrier
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How does increasing the concentration affect rate of reaction?
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Increases number of particles in same volume therefore collide more frequently and higher chance of a successful collision
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How does increasing the pressure affect rate of reaction?
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Increase in pressure increases number of gas particles within a particular volume, therefore collide more frequently and increases the chance of successful collisions
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How does increasing the temperature affect the rate?
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Increases the kinetic energy of particles therefore they move faster and more particles have enough Ea for successful collisions
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Catalysts
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Provides and alternative reaction pathway with a lower Ea so increase rates of reactions for specific reactions
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Heterogenous catalyst
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Catalyst in a different phase to reactant Reaction happens on surface of catalyst
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Homogenous catalyst
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Catalyst in same physical state as reactions Usually an aqueous catalyst Works by forming an intermediate species and then later being reformed
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Benefits of catalysts
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Lower production costs Can increase yield Can change properties of a product Catalysts allow reactions to be done at lower temps and pressures Energy is saved Less CO2 produced Can have better Atom economy/ reduce waste Catalytic converters in cars reduce air pollution of poisonous gases
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dynamic equilibrium
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A state of balance between continuing processes. Can only happen in a closed system so nothing gets in or out
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Le Chatelier's Principle
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If there's a change in concentration, pressure or temperature in a dynamic equilibrium reaction, the equilibrium will shift to help counteract the change
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POE to the left=
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More reactants
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POE to the right=
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More products
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Dynamic equilibrium and concentration
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Increase conc of reactant, equilibrium try to get "rid" of extra reactant therefore more product formed and equilibrium shifted to right Vice Versa for increase in conc of products Decreasing product has opposite effects
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Dynamic equilibrium and pressure
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Increase pressure shits equilibrium side with fewer gas molecules reducing pressure Decreasing pressure has opposite effect
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Dynamic equilibrium and temperature
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Increase temperature shifts POE to the endothermic direction in order for it to absorb heat Opposite effect for decrease temp
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Do catalysts affect the POE?
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No They speed up both forward and backward reactions so just enable the equilibrium to be reached faster
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Example of a reaction and dynamic equilibrium
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Ethene and Steam > Ethanol 60-70atm 300oC H3PO4 catalyst Lower temp = better ethanol yield but very slow High pressure = faster rate of formation of ethanol but very expensive and unsafe Conditions a compromise
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Bond enthral py
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The energy required to break 1 mile of bonds
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Is breaking bonds endothermic or exothermic?
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Endothermic therefore positive enthalpy value
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Stronger bond enthalpy =
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Stronger bonds
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Enthalpy change of reaction
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Reactants - products
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Enthalpy change of formation
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Products - reactants
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Enthalpy change of combustion
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Reactants - products
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Number of moles =
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Number of particles/avogadros constant
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Grams =
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Moles x Mr
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Moles =
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Concentration x Volume
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First ionisation energy
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The energy needed to remove one mole of electrons from one mile of gaseous atoms
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Is ionisation energy exothermic or endothermic?
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Endothermic
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How nuclear charge affects ionisation energy
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More protons = more +ve charged nucleus therefore stronger attraction to electrons
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How atomic radius affects ionisation energy
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Electrons closer to nucleus therefore more strongly attracted to nucleus
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How shielding affects ionisation energy
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As number of electrons between outer electron shell and nucleus increases, there's less attraction to the nucleons for outer electrons Inner shells are more greatly attracted
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How does Ionisation energy change going down a group
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Decreases As you go down, more electron shells The extra election shells mean a larger atomic radius and outer electrons are further away from the nucleus, reducing attraction. Inner shells shield outer electrons from charge of nucleus The Extra protons present is overridden by electron shielding
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How does ionisation energy change across a period
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Increases Number of protons increase Positive charge of nucleus increased Electrons pulled closer to nucleus decreasing atomic radius No extra electron shielding as extra electrons provided to outer energy level
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Exceptions to increasing ionisation energy across a period
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Drop in group 3 In 3rd electron in p orbital which is higher energy than s orbital In same shell but further from nucleus Overrides effect of increased nuclear charge Addition shielding from s orbital Drop in group 6 Electron being removed in group 6 is being removed from orbital with two electrons Repulsion between electrons makes removal of electron easier
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What does successive ionisation energies show?
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Shell structure Ionisation energy increases as more electrons are removed due to increasingly positive ion There's always a big jump in ionisation energy when a new shell is broken into
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Group 2 (alkaline earth metals) key properties
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Forms 2+ ions Reactivity increases down group Ionisation energy decreases down group
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When group 2 metals react, they....
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Lose electrons Form cations Easier to lose electrons = more reactive
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Group 2 oxides and hydroxides =
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Bases Oxides react readily with water to form metal hydroxides Oxides and hydroxides formed are more strongly alkaline as you go down group as OH- becomes more soluble
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Use for group 2 metals and examples
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Neutralise acidity Eg. Ca(OH)2 - used in agriculture to neutralise acidic soils Mg(OH)2 and CaCO3 - used in indigestion tablets as antacids
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Fluorine colour and state
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Pale yellow Gas
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Chlorine colour and state
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Green Gas
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Bromine colour and state
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Red/brown Liquid
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Iodine colour and state
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Grey Solid
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Group 7 Halogens key properties
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Boiling and melting points increase down group Due to increased strength of London forces as size of atoms increase As down group, volatility decreases As down group, less reactive React by gaining electron Atomic radius increase Electron shielding increase so harder for atom to attract electron it needs Oxidising agent (as gets reduced)
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Displacement and halogens
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Halogens displace less reactive halogens Colour changes occur during displacement reactions Used to identify a halogens
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Disproportionation
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The oxidation and reduction of the same element in a redox reaction
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When does disproportionation occur with a Halogen?
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When it reacts with an alkali e.g. NaOH
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Chlorine + Sodium Hydroxide =
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Sodium chlorate solution + sodium chloride + water Sodium chlorate solution = bleach
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Chlorine + Water =
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Hydrochloride acid + hypochlorous acid Disproportionation
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What do chlorate ions do?
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Kill bacteria so is used to make water safer to drink and swim in
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Benefits of chlorine in water
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Kills disease Prevents reinfection Prevents growth of algae Removes discolouration
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Risks of chlorine in water
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Gas harmful if inhaled Liquid chlorine causes burns Chlorine can react with organic compounds in water and make carcinogens Ethical implications that it is "mass medication"
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Alternatives to chlorine in drinking water
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Ozone Strong oxidising agent BUT Expensive to produce + short half life UV light Damages DNA of microorganisms BUT Ineffective in cloudy water Not permanent
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Orbital
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Space that electrons move in
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Max electrons in s subshell
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2
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Max electrons in p subshell
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6
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Max electrons in d subshell
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10
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Max electrons in f subshell
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14
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Key points about electron configuration
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Fill lowest energy levels first e.g. 4s before 3d Fill orbitals with same energy singly first before sharing orbital Remove/add electrons to highest energy occupied shell
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ionic bond
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A strong electrostatic attraction between two oppositely charged ions
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Giant ionic lattice
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When two oppositely charged ions are packed together alternately in a regular structure (Latrice) Forms because each ion is electrostatically attracted in all directions to ions of opposite charge
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Physical properties of ionic compounds
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Conduct electricity when molten/dissolved due to delocalised electrons High melting and boiling point Tend to dissolve in water (due to polarity in water and ions)
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Covalent bond
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Strong electrostatic attraction between a shared pair of electrons and the Nuclei of the bonded atoms
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Dative covalent bond
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Where both electrons in the bond come from one atom
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Shape of molecule with 2 electron pairs
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Linear 180°
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Shape of molecule with 3 electron pairs
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Trigonal planar 120°
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Shape of molecule with 4 electron pairs
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Tetrahedral 109.5°
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Shape of molecule with 4 electron pairs including 1 lone pair
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Trigonal pyramidal 107°
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Shape of molecule with 4 pairs of electrons including two lone pairs
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Non-linear 104.5°
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Shape of molecule with 5 electron pairs
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Trigonal bypyramidal 120° and 90°
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Shape of molecule with 6 electron pairs
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Octahedral 90°
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Electronegativity
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An atoms ability to attract the electron pair in a covalent bond
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Differences in electronegativity in a bond=
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Polarised bond therefore a permanent dipole
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If polar bonds arranged symmetrically, molecule is....
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Non polar
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If polar bonds arranged asymmetrically, molecule is.....
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Polar
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Strength of intermolecular forces
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Overall very weak
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London forces
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Found between all atoms and molecules Electrons in charge clouds always moving really quickly so at a moment, electrons in an atom might be at one side more than another, forming a temporary dipole Temporary dipole can cause another temp dipole in opposite direction and atoms become attracted Can cause further temp dipoles Dipoles created and destroyed constantly
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Stronger London forces=
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Higher boiling points
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Iodine and London forces
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London forces can hold iodine molecules in a molecular lattice
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Hydrogen bonding only occurs when hydrogen is covalently bonded to...
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Fluorine Nitrogen Oxygen
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Hydrogen bonds
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Hydrogen very high charge density as so small and F N O very electronegative Bond so polarised that weak bonds form between hydrogens of molecules and the lone pair of electrons on F N O on other molecules
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Intermolecular forces can explain trends in....
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Boiling points
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Simple covalent compounds
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Low melting and boiling points Polar molecules soluble in water Conduct no electricity
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Do covalent bonds break during melting and boiling?
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No, its the overcoming of intermolecular forces
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Enthalpy change
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Heat energy transferred in a reaction at constant pressure
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Standard enthalpy change of reaction
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Enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions
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Standard enthalpy change of formation
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When 1 mole of a compound is formed from its elements in their standard states, under standard conditions
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Standard enthalpy change of combustion
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When one mole of a substance is completely burned in oxygen, under standard conditions
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Standard enthalpy change of neutralisation
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Enthalpy change when an acid + alkali react together, under standard conditions, to form one mole of water
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Average bond enthalpy
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The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds
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Equation used when given experimental data for enthalpy change
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Q= mc/^T Q heat gained/lost (J) /^ temp change (K) c 4.18 m mass of water in calorimeter or solution (g)
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Hess' Law
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The total enthalpy change is independent of the route taken
