ADVANCED SUBSIDIARY GCE 83/0 CHEMISTRY How Far, How Fast? THURSDAY JANUARY 007 Morning Additional materials: Scientific calculator Data Sheet for Chemistry (Inserted) Time: 45 minutes INSTRUCTIONS TO CANDIDATES Write your name, Centre number and Candidate number in the boxes above. Answer all the questions. Use blue or black ink. Pencil may be used for graphs and diagrams only. Read each question carefully and make sure you know what you have to do before starting your answer. Do not write in the bar code. Do not write outside the box bordering each page. WRITE YOUR ANSWER TO EACH QUESTION IN THE SPACE PROVIDED. ANSWERS WRITTEN ELSEWHERE WILL NOT BE MARKED. INFORMATION FOR CANDIDATES The number of marks for each question is given in brackets [ ] at the end of each question or part question. You may use a scientific calculator. A copy of the Data Sheet for Chemistry is provided as an insert with this question paper. You are advised to show all the steps in any calculations. FOR EXAMINER S USE Qu. Max Mark 0 4 3 0 4 TOTAL 45 This document consists of 8 printed pages and a Data Sheet for Chemistry. SP (SLM/CGW) S96696/5 OCR 007 [A/00/345] Registered Charity Number: 066969 [Turn over
Answer all the questions. A group of students investigated the effect of concentration on the rate of a reaction. They used the reaction between magnesium carbonate and dilute hydrochloric acid and measured the rate at which the gas was collected. (a) (i) An incomplete equation for this reaction is given below. Complete the equation by balancing it and inserting state symbols. MgCO 3 (...) + HCl(...) MgCl (...) + CO (...) + H O(...) [] Write the ionic equation for this reaction. (b) The students added dilute hydrochloric acid to some magnesium carbonate. The students collected the gas and measured the volume, at regular intervals, until after the reaction was complete. They then plotted a graph of their results. [] volume of gas / cm 3 0 0 time / s OCR 007 Use collision theory to explain the changes in the rate of the reaction as it proceeds.... [3]
3 (c) The students repeated the experiment using a weak acid instead of hydrochloric acid. Assume the concentration of both acids and all other conditions are the same. (i) On the axes in (b), sketch the graph the students obtained from this experiment. [] State and explain what effect changing the acid has on the rate of the reaction.... [] [Total: 0] OCR 007 [Turn over
4 Propane, C 3 H 8, is a gas at room temperature and pressure. It is used in blow torches to melt the bitumen needed to apply the felt to flat roofs. (a) Write the equation for the complete combustion of propane. (b) Define the term standard enthalpy change of combustion.... [3] (c) A blow torch was used to determine the enthalpy change of combustion of propane. The apparatus is shown below. [] thermometer water flame propane and oxygen blow torch In the experiment, 00 g of water were used. The temperature of the water changed from 8.0 C to 68.3 C when.00 g of propane was burnt. (i) Calculate the energy produced in kj. The specific heat capacity of water is 4.8 J g K. energy = kj [] OCR 007
Calculate the number of moles of C 3 H 8 burnt during the experiment. 5 mol [] (iii) Deduce the enthalpy change of combustion, in kj mol, of C 3 H 8. enthalpy change of combustion = kj mol [] (d) Values of enthalpy changes of combustion can be used to calculate enthalpy changes of formation. The enthalpy change for the reaction in equation. is the enthalpy change of formation of propane. 3C(s) + 4H (g) C 3 H 8 (g) equation. The table below shows the enthalpy changes of combustion of carbon, hydrogen and propane. enthalpy change of combustion / kj mol carbon 394 hydrogen 86 propane 9 (i) Use these data to calculate the enthalpy change of formation of propane. enthalpy change of formation = kj mol [3] Suggest why the enthalpy change of formation of propane cannot be measured directly.... [] OCR 007 [Total: 4] [Turn over
6 3 Esters are used as flavourings. They are made by a reversible reaction between a carboxylic acid and an alcohol. carboxylic acid + alcohol ester + water (a) Give two features of a reversible reaction, when a dynamic equilibrium has been set up.... [] (b) The production of esters is catalysed homogeneously by the presence of acids. (i) What is meant by a catalyst?... [] What is meant by homogeneous?... [] (iii) Using the fact that acids are needed to catalyse this reaction, deduce the formula of the ion that acts as the catalyst.... [] (iv) Catalysts do not affect the position of an equilibrium. Explain why not.... [] OCR 007
(c) (i) 7 On the axes below, draw and label the Boltzmann distribution to show the energies of molecules in a gas at a fixed temperature. [] Use the graph to explain the effect of a catalyst on the rate of a reaction.... [] [Total: 0] OCR 007 [Turn over
8 4 Ammonia, NH 3, is made industrially by the Haber process. This is an equilibrium reaction. N (g) + 3H (g) NH 3 (g) ΔH = 9 kj mol (a) State the pressure and temperature that are used in the Haber process. pressure... temperature...[] (b) Describe and explain why these conditions are a compromise between rate and equilibrium....[9] [Total: ] END OF QUESTION PAPER Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. OCR is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge. OCR 007
83/0 Mark Scheme January 007 Abbreviations, annotations and conventions used in the Mark Scheme / = alternative and acceptable answers for the same marking point ; = separates marking points NOT = answers which are not worthy of credit ( ) = words which are not essential to gain credit = (underlining) key words which must be used to gain credit ecf = error carried forward AW = alternative wording ora = or reverse argument Question Expected Answers Marks (a) (i) MgCO 3 (s) + HCl(aq) MgCl (aq) + CO (g) + H O(l) balancing state symbols MgCO 3 + H + Mg + + CO + H O/ CO 3 + H + CO + H O (b) (as the reaction proceeds) the concentration decreases (rate) of collision decreases reaction stops when all of one reagent is used up 3 (c) (i) sketch to show slower rate of production ie less steep (must not be straight line) final volume the same but reached later rate is slower because weak acid is partially ionised/ dissociated lower concentration of H + in weak/ higher concentration of H + in strong/ HCl Total: 0 4
83/0 Mark Scheme January 007 Abbreviations, annotations and conventions used in the Mark Scheme / = alternative and acceptable answers for the same marking point ; = separates marking points NOT = answers which are not worthy of credit ( ) = words which are not essential to gain credit = (underlining) key words which must be used to gain credit ecf = error carried forward AW = alternative wording ora = or reverse argument Question Expected Answers Marks (a) C 3 H 8 + 5O 3CO + 4H O formulae balancing ignore state symbols (b) (enthalpy/ energy/ heat change) when mole of substance/ element/ compound (NOT absorbed) is completely burnt/ burnt in excess oxygen under standard conditions ( if conditions stated they must be correct) 3 (c) (i) use of mcδt 00 X 4.8 X 50.3 correct answer 4./ 4.0/4 ( or more sig figs) final answer must be in kj for nd mark moles =.00 = 0.07/ 0.03 44 (iii) 4. = 850 (kj mol - ) 0.07 sign ie (d)(i) cycle multipliers x 9 = 3( 394) + 4( 86) answer 07 (kj mol - ) carbon and hydrogen would react to give more than product/ do not react together easily/ the reaction has a high activation energy 3 Total 4 5
83/0 Mark Scheme January 007 Abbreviations, annotations and conventions used in the Mark Scheme Question 3 (a) / = alternative and acceptable answers for the same marking point ; = separates marking points NOT = answers which are not worthy of credit ( ) = words which are not essential to gain credit = (underlining) key words which must be used to gain credit ecf = error carried forward AW = alternative wording ora = or reverse argument Expected Answers any two from rate of forward reaction = rate reverse reaction macroscopic properties remain constant/ concentrations remain constant closed system needed Marks (b) (i) a substance that alters the rate of a reaction without being used up / a substance that lowers the activation energy (for a reaction) by providing an alternative route catalyst is in the same state/ phase as reactants (iii) H + (iv) they alter the rate of the forward and the reverse reaction by the same amount (c) (i) axes labelled y as number/ fraction/ % of molecules/ particles and x as energy/ enthalpy/ velocity/ speed correct shape to include origin, hump and position wrt x axis two vertical lines drawn both to the RHS of hump (at least one labelled Ea) (labels reversed cannot score) greater proportion of collisions have energy greater than Ea/ more molecules exceed Ea Total 0 6
83/0 Mark Scheme January 007 Abbreviations, annotations and conventions used in the Mark Scheme / = alternative and acceptable answers for the same marking point ; = separates marking points NOT = answers which are not worthy of credit ( ) = words which are not essential to gain credit = (underlining) key words which must be used to gain credit ecf = error carried forward AW = alternative wording ora = or reverse argument Question Expected Answers Marks 4 a) pressure 50 000 atm temperature 00 600 o C b) rate (increased) pressure increases rate because molecules are closer together/ more concentrated (increased) temperature increases rate because molecules are moving faster/ have more energy equilibrium increased pressure pushes equilibrium to RHS because fewer (gas) moles/ molecules on RHS increased temperature pushes equilibrium to LHS because (forward) reaction is exothermic compromise if temperature is too high, low yield if temperature is too low, slow rate if pressure is too high, increased costs/ safety issues 9 Total: 7