RI Y3 Chem 2022 FE Revision Energetics Kinetics Equilibrium ANS

1 Raffles Institution Raffles Programme Year Four Chemistry Name: ____________________________ ( ) Class: ______ Date: _______ 2022 FE Revision – Chemical Energetics, Reaction Kinetics, Equilibrium ANSWERS Chemical Energetics Exothermic Process Endothermic Process 1 heat is released heat is absorbed 2 enthalpy change (ΔH) value is negative enthalpy change (ΔH) value is positive 3 the container/surroundings feels hot the container/surroundings feels cold 4 bond making bond breaking 5 energy content of products < energy content of reactants energy content of products > energy content of reactants 6 e.g. combustion, neutralization, respiration, freezing e.g. photosynthesis, thermal decomposition, melting Practice Question 1 (a) C6H12O6 + 6O2(g)  6CO2(g) + 6H2O(l) (b) (i) During respiration, energy is absorbed to break the bonds in C 6H12O6 and the O=O bonds of oxygen. Energy is released when the C=O bonds of carbon dioxide and the O-H bonds of water are formed. The reaction is exothermic because the amount of heat released is greater than the amount of heat absorbed. (ii) C6H12O6 + 6O2 6H2O + 6CO2 Ea ∆H progress of reaction energy

2 (iii) ∆H = energy absorbed – energy released for bond breaking by bond making = [+9546 + 6(496)] – [6(2x740) + 6(2x460)] = −1878 kJ Reaction Kinetics 1 Every reaction has an activation energy (E a), which is the minimum energy that colliding particles must possess in order to result in a reaction. The smaller the Ea the faster the reaction. 2 Factors that increase the speed of a reaction: factors collision theory explanation increase temperature this increases the number of reactant particles having the Ea, causing particles to collide harder and more frequently increase pressure (only for gases) this causes particles to be closer thus more frequent effective collisions between them increase concentration this causes more particles per unit volume thus more frequent effective collisions between reactant particles increase surface area (by decreasing particle size) this causes more frequent effective collisions between reactant particles presence of a catalyst • this provides an alternative pathway with a lower Ea, resulting in more reactant particles having the Ea thus more frequent effective collisions between them 4 Increasing the temperature, pressure, surface area or catalyst would increase the slope (gradient) of the graph but the amount of product would finally still be the same as that of the control experiment. However, increasing concentration MAY result in inc reased speed as well as increased amount of products, since the initial amount of reactant is also increased when its concentration is increased. Practice Questions 1 (a) CuCO3 (s) + 2HCl (aq)  CuCl2 (aq) + H2O (l) + CO2 (g) (b) (b)

3 (c) Dilute HCl has less particles (or H +) per unit volume than concentrated HCl. As such the frequency of effective collisions between acid particles and CuCO3 particles (or CO32−) is decreased, resulting in a slower reaction 2 (a) increase temperature of HCl, increase concentration of HCl, use finer powder of MnO 2, add a catalyst (b) This is due to the decrease in the HCl concentration as the reaction proceeds. (c) When one of the reactants is totally used up the reaction stops. Equilibrium 2 When the speed of the forward reaction is equal to the speed of the backward reaction, a state of equilibrium is attained. Thus an equilibrium mixture always contains both the reactants and the products. 3 The reaction does not stop when equilibrium is attained. It’s just that the amount of reactants and the amount of products are constant. 5 Le Chatelier’s Principle states that if a system at equilibrium is subjected to a disturbance, the equilibrium’s response is to counteract the disturbance to minimize its effect. 6 We can use Le Chatelier’s Principle to manipulate the yield of a chemical process. Haber process: N 2 (g) + 3H2 (g) 2NH3 (g) ∆H = negative (a) Disturbance = Remove some NH3 from the equilibrium mixture. System’s response = favour the forward reaction to replace the lost NH3, thus position of equilibrum shifts right, increasing yield of NH3. (b) Disturbance = Increase temperature of the equilibrium mixture. System’s response = favour the backward reaction to remove the excess heat, thus position of equilibrium shifts left, decreasing yield of NH3. (c) Disturbance = Increase pressure of the equilibrium mixture. System’s response = favour the forward reaction to decrease the total number of molecules to relieve the pressure, thus position of equilibrium shifts right, increasing yield of NH3. 7 The Haber process is carried out at a high pressure of 200 atm and a moderate temperature of 450 oC, using iron (a transition metal) as a catalyst. Although high pressure would increase yield, operating at pressures above 200 atm is costly and dangerous (due to high risk of accidents). Although low temperature would increase yield, operating at temperatures below 450 oC would result in too slow a reaction. Practice Questions 1 Consider the equilibrium mixture: CrO42− (aq) + 2H+ (aq) Cr2O72− (aq) + H2O (l) Would each of the following steps shift position of equilibrium to the right or to the left? (a) Add a few drops of an acid position of equilibrium shifts right (b) Add water position of equilibrium shifts left (c) Add a few drops of an alkali position of equilibrium shifts left (d) Evaporate away some water position of equilibrium shifts right

4 2 How would a decrease in pressure affect the yield of the product in each of the following equilibria? (a) N2O4 (g) 2NO2 (g) increase yield of product (b) H2 (g) + I2 (g) 2HI (g) no change in yield of product (c) CO (g) + 2H2 (g) CH3OH (g) decrease yield of product 3 Carbonated beverages are made by bubbling carbon dioxide gas into aqueous solution under pressurized condition. The chemical equation depicting this process is shown below: 2H2O (l) + CO2 (g) H3O+ (aq) + HCO3─ (aq) By using Le Châtelier's Principle, predict: (a) how the concentration of H 3O+ (aq) changes when a can of carbonated beverage is opened and then quickly closed to be air-tight again. Explain. When the can is opened, CO2 escapes, so the system counteracts this by favouring the backward reaction to replace the lost CO 2, thus position of equilibrium shifts left and H3O+ concentration decreases. [2] (b) how the concentration of CO2 (g) changes when the pressure in the container is increased at constant temperature. Explain. When pressure is increased , the system counteract s this by favouring the forward reaction so as to decrease the total number of gas (CO2) particles, thus position of equilibrium shifts right and concentration of CO2 decreases. [2] More Practice Questions 1 2 3 4 5 6 C B D C C C 7 Ethanol is made from hydration of ethene. C2H4 (g) + H2O (g) CH3CH2OH (g) How would you alter the pressure to increase the yield of ethanol? Explain your reasoning. Pressure should be increased. When pressure is increased, the system counteracts this increase by favouring the forward reacti on, to decrease t he total number of particles and relieve the pressure. Thus, position of equilibrium shifts right, and yield of ethanol is increased. 8 The following is an important step in the manufacture of sulfuric acid. 2SO 2 (g) + O2 (g) 2SO3 (g) ∆H = -197 kJ mol-1 (a) How would the equilibrium react to a decrease in temperature? Give your reasoning. When the temperature is decre