Chemistry: M3S2; Physical Chemistry Essay

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Process to calculate standard enthalpy change of combustion from experimental results? (4)
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Use q=mc∆T, convert to kJ. Find the moles of the fuel. Divide enthalpy change in kJ by moles of fuel.
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What is enthalpy change? (1)
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The heat energy transferred in a reaction at constant pressure.
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What symbol and letter is used as shorthand for enthalpy change? (1)
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∆H.
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What are standard conditions in terms of temperature and pressure? (1)
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100 kPa and 298 K.
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What is meant by standard state of an element? (1)
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An element in its physical state under standard conditions.
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What happens in an exothermic reaction on a general level? (2)
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Energy is given out. ∆H is negative.
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What happens in an endothermic reaction on a general level? (2)
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Energy is absorbed. ∆H is positive.
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Why are standard enthalpy changes used? (1)
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Enthalpy change is affected by temperature and pressure.
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In what two ways can enthalpy changes be found? (1)
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From data books or experimentally.
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What is standard enthalpy change of reaction? (2)
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The enthalpy change when a reaction occurs in the molar quantities shown by its chemical equation, with all products and reactants in their standard states under standard conditions.
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What is standard enthalpy change of formation? (2)
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The enthalpy change when 1 mole of a compound is formed from its constituent elements, in the their standard states under standard conditions.
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What is standard enthalpy change of combustion? (2)
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The enthalpy change when 1 mole of a substance is completely burned in oxygen, with all reactants and products in their standard states under standard conditions.
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What is standard enthalpy change of neutralisation? (2)
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The enthalpy change when an acid and alkali react together to form one mole of water, in their standard conditions.
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What is the shorthand used for standard enthalpy change of reaction? (1)
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∆↓rH°
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What is the shorthand used for standard enthalpy change of formation? (1)
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∆↓fH°
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What is the shorthand used for standard enthalpy change of combustion? (1)
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∆↓cH°
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What is the shorthand used for standard enthalpy change of neutralisation? (1)
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∆↓neutH°
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Is breaking bonds endothermic or exothermic? (1)
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Endothermic.
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Is making bonds endothermic or exothermic? (1)
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Exothermic.
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What is bond dissociation enthalpy? (1)
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The amount of energy needed to break one mole of bonds in the gas phase.
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What is average bond enthalpy? (2)
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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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Why is average bond enthalpy used? (1)
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The same bond in a different position has a different bond enthalpy.
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What is the process for an experiment which allows standard enthalpy change of combustion to be found? (2)
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Burn fuel in combustion chamber of bomb calorimeter. Find temperature change of water.
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What is the process for an experiment which allows standard enthalpy change of neutralisation to be found? (2)
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Combine known quantities of acid and alkali in an insulated container, measure the temperature change.
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What is the equation that allows enthalpy change to be found? (1)
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q=mc∆T
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In the enthalpy change equation, what does q stand for? Include units. What must be true for q to be the same as enthalpy change? (2)
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Heat lost or gained in joules. Same as enthalpy change if pressure is constant.
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In the enthalpy change equation, what does m stand for? Include units. (1)
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Mass of water in calorimeter or soluton in insulated container in grams.
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In the enthalpy change equation, what does c stand for? Include units. (1)
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Specific heat capacity of water or solution in J/g/K.
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In the enthalpy change equation, what does Delta T stand for? Include units. (1)
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Change in temperature of water or solution in K.
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What is the process to find enthalpy change of combustion from experimental results? (3)
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Find q, convert to kJ. Find mol of fuel. Divide q by mol of fuel.
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What is Hess’ law? (1)
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The total enthalpy change of a reaction is always the same, no matter which route is taken.
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How is Hess’ law useful? (1)
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Can be used to find enthalpy changes that can’t be found directly from an experiment.
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How to calculate standard enthalpy change of reaction using Hess’ law and given values of enthalpy change of formation? (3)
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Write elements, balance, use as standard enthalpy change of formation of both reactants and products. Route to reactants then products= route straight to products and solve.
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How to calculate enthalpy change of formation using Hess’ law and given enthalpy changes of combustion? (2)
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Write products of combustion and use enthalpy change of combustion for both reactants and products. Route from reactants to products to combustion products=route from reactants to combustion products and solve.
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What is the equation to find enthalpy change of a reaction using given average bond enthalpies? (1)
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Enthalpy change of reaction=Total energy absorbed-Total energy released.
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What is collision theory? (1)
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Particles will only react if they collide in the right direction with a minimum kinetic energy.
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What is activation energy? (1)
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The minimum amount of kinetic energy particles need to break bonds and start a reaction.
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What does an energy profile diagram show? (1)
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The change in enthalpy as a reaction goes on.
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How to show activation energy on an energy profile diagram? (1)
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Arrow from peak enthalpy to enthalpy of reactants.
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How to show enthalpy change on an energy profile diagram? (1)
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Arrow from enthalpy of reactants to enthalpy of products.
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What does a Boltzmann distribution show? (1)
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The proportions of molecules in a substance with different kinetic energy.
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What are the different properties of a Boltzmann distribution? Explain where appropriate. (6)
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X-axis kinetic energy, Y-axis number of molecules. Curve starts at 0,0 as no molecules have no energy. Few molecules have little kinetic energy. Most have moderate kinetic energy. Some have high kinetic energy- more than activation energy, which is labelled on an x-axis.
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What are 4 things that increase rate of reaction? (4)
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Increasing temperature, increasing concentration of reactants in solution, increasing pressure of reactants that are gases, using catalysts.
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How does increasing temperature increase rate of reaction? What will this do to the curve of a Boltzmann distribution? (4)
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Particles have more kinetic energy so move faster and collide more frequently. Also have a greater kinetic energy on average so more have greater than activation energy and are able to react. Increases chance of successful collisions. Pushes curve right.
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How does increasing concentration of reactants in solution increase rate of reaction? What will this do to the curve of a Boltzmann distribution? (4)
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More particles so closer together on average, so will collide more frequently, increasing rate of successful collisions. Moves curve up.
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How does increasing pressure if reactants are gasses increase rate of reaction? (3)
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Particles closer together on average, so collide more frequently, increasing rate of successful collisions.
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How does using a catalyst increase rate of reaction? How to show on a Boltzmann distribution curve? (4)
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Provides an alternate reaction pathway of lower activation energy, so more particles are above activation energy, so increased chance of successful collisions. Curve unchanged but activation energy further back on x-axis.
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What is a catalyst like compared at the end of the reaction as compared to the start of the reaction? (1)
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Chemically unchanged.
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What is the Haber process? (1)
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The production of ammonia from nitrogen and hydrogen. An iron catalyst in used.
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How many reactions can catalysts usually work on? (1)
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One.
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How to show the effect of a catalyst on an enthalpy profile diagram? (2)
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Dotted line of a lower peak, always beneath other curve but finishes at same place on products.
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What is a heterogeneous catalyst? How does it work? (2)
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A catalyst in a different phase/physical state to the reactants. Reaction occurs on the surface of the catalyst.
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What is a homogeneous catalyst? How does it work? (3)
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A catalyst in the same physical state as the reactant. Forms an intermediate species with the reactants, which then reacts to form the products and reform the catalyst.
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What effect does increasing the surface area of a heterogeneous catalyst have on rate of reaction? Why? (2)
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Increasing rate of reaction. Increases the number of molecules that can react at the same time.
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How do catalysts benefit industry? (3)
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Lower production costs, give more product in a shorter time, can sometimes make products with better properties.
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How do catalysts benefit the environment? Include one specific example. (4)
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Lower temperatures and pressures can be used so energy is saved and less CO2 released. Also fossil fuels better preserved. Can reduce waste by allowing for a different reaction with better atom economy to be used. Catalytic converters are used on cars to reduce the pollution released into the atmosphere.
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What reaction do catalytic converters speed up? (1)
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2CO+2NO->2CO₂+N₂
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What is the equation to calculate rate of reaction? (1)
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Rate of reaction=Amount of reactant used or product formed/time.
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What is the experiment that finds rate of reaction from amount of reactant used? (4)
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If the product of a reaction is a gas its formation can be measured using a mass balance. Start reaction and start stop clock or timer. Take mass measurements at regular time intervals. Stop when reaction is finished- when the reading on the mass balance stops changins.
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What is the experiment that finds rate of reaction from amount of product formed? (2)
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Use a gas syringe to measure the volume of a gas formed. Start a stopwatch when reaction begins and measure the volume of the gas in the syringe at regular intervals.
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How to calculate rate of reaction from a straight line on a graph? (1)
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Change in Y/Change in X.
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How to calculate rate of reaction at a given point from a curved line on a graph? (2)
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Draw a tangent at that point, Change in Y/Change in X.
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What is a reversible reaction? (1)
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A reaction in which the products can also react to form the reactants.
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What happens to the rate of the forward reaction as reactants are used up in a reversible reaction? (1)
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Slows down.
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What happens to the backward reaction as more product is formed in a reversible reaction? (1)
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Speeds up.
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What is dynamic equilibrium? (2)
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When the concentration of products and reactants are constant, in a reversible reaction in a closed system.
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What does equilibrium position refer to? (1)
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Whether a greater concentration of reactants or products are present at dynamic equilibrium.
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What does an equilibrium position that is further right mean? (1)
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Greater concentration of products than reactants at dynamic equilibrium.
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What does an equilibrium position that is further left mean? (1)
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Greater concentration of reactants than products at dynamic equilibrium.
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What is Le Chatelier’s principle? (1)
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A change to an equilibrium system will result in a shift in equilibrium position to counteract the change.
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What shift in equilibrium occurs with an increase in the concentration of the reactant? (2)
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The forward reaction is favoured and equilibrium shifts right, reducing the concentration of the reactant.
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What shift in equilibrium occurs with an increase in the concentration of the product? (2)
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The backward reaction is favoured and equilibrium shifts left, reducing the concentration of the product.
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What shift in equilibrium occurs with an increase in the pressure of the system? (2)
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Shifts to the side with fewer gas molecules, reducing the pressure.
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How do catalysts affect equilibrium position? Why? (2)
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They don’t, as they speed up the forward and backward reaction equally.
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What shift in equilibrium occurs with an increase in the temperature of the system? (2)
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Shifts to the endothermic side, favouring the endothermic reaction.
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What is the reaction and conditions for the hydration of ethene? (4)
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C₂H₄+H₂O⇌C₂H₅OH. 300 degrees, 60-70 atm, a phosphoric acid catalyst.
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What compromises are made in the industrial production of ethanol by the hydration of ethene? (5)
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Forward reaction is exothermic, but high temperature is used anyway so that reaction rate is sufficiently high, so temperature used is a compromise between yield and reaction rate. High pressure favours products but expensive and unsafe, so pressure used is a compromise between yield and expense/safety.
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What is the equation for the equilibrium constant using general equation aA+bB⇌dD+eE? (1)
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K↓c=[D]↑d[E]↑e/[A]↑a[B]↑b
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What do the square brackets in the equilibrium constant equation mean? (1)
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The concentration of the substance at dynamic equilibrium.
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What does K↓c value show in terms of equilibrium position? (2)
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Larger=further right, smaller=further left.
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What reaction can be used to investigate equilibrium position with changing temperature? (5)
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2NO₂⇌N₂O₄. NO₂ is brown, N₂O₄ colourless. Forward reaction is exothermic. Put reaction mixture in two sealed tubes in water baths. Increasing the temperature will change it brown.
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What reaction can be used to investigate equilibrium position with changing concentration of reactants and products? (7)
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Fe+3SCN⇌Fe(SCN)₃. Fe yellow, FE(SCN)₃ red. 4 test tubes. Control has reddish colour. Add iron(III) nitrate to second, turns deep red. Add potassium thiocyanate to third, turns deep red. Add iron(III) thiocyanate to fourth. Turns yellow.

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