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OCR A Jan 2011 Paper 2 Q3
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![3 Enthalpy changes of reaction can be determined by experiment or by using bond enthalpies. (a) What is meant by the term enthalpy change of reaction?[2] (b) Solid ammonium thiocyanate, NH4SCN, reacts with solid barium hydroxide, Ba(OH)2, as shown in the equation below. 2NH4SCN(s) + Ba(OH)2(s) Ba(SCN)2(s) + 2H2O(l) + 2NH3(g) A research chemist carries out an experiment to determine the enthalpy change of this reaction. thermometer 50.0g of water boiling tube mixture of ammonium thiocyanate and barium hydroxide insulated beaker In the experiment, 15.22 g of NH4SCN is reacted with a slight excess of Ba(OH)2. The reaction absorbs energy, cooling the 50.0 g of water from 21.9 C to 10.9 C. (i) Calculate the energy absorbed, in kJ, during this reaction. The specific heat capacity of water = 4.2 J g1 K1. energy = kJ [2]OCR 2011<br />
(ii) Calculate the amount, in moles, of NH4SCN used by the research chemist. (iii) Calculate the enthalpy change of reaction. amount =mol [1] Include the sign in your answer. Give your answer to two significant figures. Hr = kJ mol1 [3] PART (c) CONTINUES ON PAGE 10OCR 2011 Turn over<br />
(c) Standard enthalpy changes of reaction can also be determined using average bond enthalpies. (i) What is meant by the term average bond enthalpy? 10[2] Table 3.1 below shows some average bond enthalpies. bond CH CC C=C average bond enthalpy / kJ mol1 +415 +345 +611 Table 3.1 (ii) Explain the bonding in a C=C double bond. Use the orbital overlap model.[2] (iii) Suggest why the average bond enthalpy of a C=C bond is not twice the bond enthalpy of a CC bond.[1]OCR 2011<br />
(iv) Propane can be cracked to make ethene. 11 Using the average bond enthalpies in Table 3.1, calculate the enthalpy change of this reaction. Hr = kJ mol1 [2] (v) The actual value for the enthalpy change of this reaction is +81 kJ mol1. Suggest a reason why the actual value for the enthalpy change of this reaction is different from the calculated value.[1] [Total: 16]OCR 2011 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JAN11_question_3-0.jpg)
![F322 Question Answer 3 (a) (enthalpy change when) the number of moles of reactants (cid:57) as specified in the (balanced) equation react together (cid:57) (i) Q = 504.211.0 (cid:57) 2.3 (cid:57) (b) (ii) moles = 0.200 (cid:57) (iii) Hr = 2(2.30.200) (cid:57) 23 (cid:57) + sign (cid:57) Mark Scheme January 2011 Mark Guidance ALLOW (enthalpy change when) the number of moles of products ALLOW molar quantities / amounts Enthalpy change that occurs during a reaction is not sufficient ALLOW 2310 J (cid:57) 2300j ALLOW use 4.18 for c which gives 2.299 J ALLOW two marks for 2.31 / 2.310 with no working out ALLOW ECF ie Q divided by 1000 IGNORE any sign quoted ALLOW 0.2 / 0.20 ALLOW ECF from answer from 2[(i)answer to (ii)] Answer from 2[(i)answer to (ii)] must have only 2 sig figs + sign must be written for sign mark + sign is independent of answer ALLOW answers per mole of NH4SCN Hr = 2.30.200 for one mark 12 for the second mark + sign for the third mark NOTE If c = 4.18 has been used in b(i), Hr = +11 by ECF for calculation per mole of NH4SCN<br />
F322 Question Answer 3 (c) (i) (ii) (Enthalpy change) when one mole of bonds (cid:57) of (gaseous covalent) bonds is broken (cid:57) (Sideways) overlap of p orbitals (cid:57) Forming a /pi bond (cid:57) (iii)bond is weaker (than thebond) ORbond is stronger (than thebond) (cid:57) (iv) bonds broken = (+)4010 AND bonds formed = ()3931 Overall enthalpy change = +79 (cid:57) Mark Scheme January 2011 Mark Guidance ALLOW energy required rather than enthalpy change DO NOT ALLOW energy released DO NOT ALLOW bonds formed IGNORE reference tobonds IGNORE incorrect diagram This diagram would score one markthebond needs to be labelled for second mark sideways overlap 2p orbitals There are two types of bonds is not sufficient DO NOT ALLOWbond is stronger than thebond ALLOW the two bonds in double bond are not the same strength ALLOW Bonds broken = (+)690 AND bonds formed = ()611(cid:57) ALLOW 79 without a sign ALLOW 79 for one mark overall ALLOW ECF from incorrect enthalpy changes calculated for bonds broken and made<br />
F322 Question Answer 3 (c) (v) Bond enthalpies may not be the same as the average bond enthalpy OR The idea that bonds have different strengths in different environments (cid:57) Total 16 Mark Scheme January 2011 Mark Guidance DO NOT ALLOW answers involving heat loss OR the use of non standard conditions Average bond enthalpies are used is NOT sufficient 10<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JAN11_answer_3-0.jpg)
