Topic 5 Test Energetics IB Chemistry 3/6/17 [30 marks]

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1 Topic 5 Test Energetics IB Chemistry 3/6/7 [30 marks]. Which equation represents the standard enthalpy of formation of liquid methanol? A. B. C. D. C(g) + 2 H 2 (g) + (g) C OH(l) 2 O 2 H 3 C(g) + 4H(g) + O(g) C OH(l) C(s) + 4H(g) + O(g) C H 3 H 3 OH(l) C(s) + 2 H 2 (g) + (g) C OH(l) 2 O 2 H 3 D This was a fair question with half the students answering it correctly. It was a definition question on the standard enthalpy of formation of liquid methanol but almost 40% of the candidates selected choice A where carbon is given in the gaseous state, not its solid under standard conditions. 2. A simple calorimeter was set up to determine the enthalpy change occurring when one mole of ethanol is combusted. The experimental value was found to be. The Data Booklet value is (at Pa K and ). 867 kj mol 367 kj mol During the experiment some black soot formed. Which statements are correct? I. The percentage error for the experiment can be calculated as follows: ( ) 00% II. III. The difference between the two values may be due to heat loss to the surroundings. The black soot suggests that incomplete combustion occurred. A. I and II only B. I and III only C. II and III only D. I, II and III C

2 3. Consider the following two equations. 2Ca(s) + O 2 (g) 2CaO(s) = +x kj Ca(s) O 2 (g) + C O 2 (g) CaC O 3 (s) = +y kj What is, in kj, for the following reaction? CaO(s) + C (g) CaC (s) O 2 O 3 A. B. C. D. y 0.5x y x 0.5 y x y A There was concern about the use of algebraic notation rather than actual numerical data. This has been used since November 200 so candidates should be familiar with this type of question. (In fact, some G2s in the past have suggested it would be better to use algebraic notation!) In the event it was the sixth easiest question; over 8% of candidates gave the correct answer and 9% gave B. 4. Enthalpy changes of reaction are provided for the following reactions. 2C(s) + 2 H 2 (g) C 2 H 4 (g) = +52 kj mo 2C(s) + 3 H 2 (g) C 2 H 6 (g) = 85 kj mo l l What is the enthalpy change, in, for the reaction between ethene and hydrogen? kj mol (g) + (g) (g) C 2 H 4 H 2 C 2 H 6 A. 37 B. 33 C. +33 D. +37 A

3 The Born-Haber cycle for MgO under standard conditions is shown below. The values are shown in the table below. Identify the processes represented by A, B and D in the cycle. 5a. [3 marks] A: sublimation/atomization; B: atomization/half dissociation enthalpy; D: (sum of st and 2 nd) electron affinity; Do not accept vaporization for A and B. Accept H AT or H EA. This was the second most popular question and in general candidates demonstrated a good understanding of the Born Haber cycle. Some candidates identified the process A as vaporization instead of atomization. 5b. Define the enthalpy change, F. enthalpy change when one mole of the compound is formed from its elements (in their standard states); under standard conditions / 25 C/298 K and atm/ 0.3 kpa/.0 05 Pa;

4 Most candidates correctly stated the definition of enthalpy change of formation although some omitted to specify the standard conditions. Determine the value of the enthalpy change, E. 5c. 602 = E 3888 (kj mo ) l ; Do not allow 3889 (given in data booklet). Allow 3888 (i.e no minus sign). Award [2] for the correct final answer. ; The majority of candidates correctly calculated the lattice enthalpy value. Define the enthalpy change C for the first value. Explain why the second value is significantly larger than the first.[4 marks] 5d. energy required to remove one electron; from an atom in its gaseous state; electron removed from a positive ion; decrease in electron-electron repulsion / increase in nucleus-electron attraction; The definition of the first ionization energy was stated correctly by most candidates but in a few cases the term gaseous state was missing. 5e. The inter-ionic distance between the ions in NaF is very similar to that between the ions in MgO. Suggest with a reason, which compound has the higher lattice enthalpy value. MgO; double ionic charge / both ions carry +2 and 2 charge/greater charge compared to + and ; The compound with higher lattice enthalpy was correctly identified including the reason.

5 The standard enthalpy change of three combustion reactions is given below in kj. 5f. 2 C 2 H 6 (g) + 7 O 2 (g) 4C O 2 (g) + 6H 2 O(l) 2 H 2 (g) + O 2 (g) 2H 2 O(l) C 2 H 4 (g) + 3 O 2 (g) 2C O 2 (g) + 2H 2 O(l) = 320 = 572 = 4 [4 marks] Based on the above information, calculate the standard change in enthalpy, C 2 H 6 (g) C 2 H 4 (g) + H 2 (g), for the following reaction. ( C 2 H 6 (g) + 3 (g) 2C (g) + 3 O(l)) 2 O 2 O 2 H 2 ( H 2 O(l) H 2 (g) + O (g)) 2 2 (2C O 2 (g) + 2H 2 O(l) C 2 H 4 (g) + 3 O 2 (g)) ( C 2 H 6 (g) C 2 H 4 (g) + H 2 (g)) = 560; = +286; = +4; = +37 (kj); Allow other correct methods. Award [2] for 37. Allow ECF for the final marking point. The majority of candidates manipulated the thermo-chemical equations and calculated the correct answer of +37 kj although some reversed the sign. S Θ 5g. Predict, stating a reason, whether the sign of for the above reaction would be positive or negative. positive; increase in number of moles of gas; 5h. Using bond enthalpy values, calculate for the following reaction. (g) (g) + (g) [3 marks] C 2 H 6 C 2 H 4 H 2 Bonds broken (C C, 6C H, or C C, 2C H) = 2825/73; Bonds made (C=C, H H, 4C H) = 2700/048; +25 (kj); Allow 25 but not 25 (kj ) for the final mark. Award [3] for the correct final answer. The bond enthalpy calculation had the usual mistakes of using the wrong value from the data booklet, bond making minus bond breaking and 25 kj instead of +25 kj.

6 5i. Suggest with a reason, why the values obtained in parts (b) (i) and (b) (iv) are different. bond enthalpy values are average values; 6. Which equation best represents the bond enthalpy of HCl? A. B. C. D. HCl(g) H + (g) + C l (g) HCl(g) H(g) + Cl(g) H HCl(g) (g) + C (g) 2HCl(g) l 2 H 2 (g) + C l 2 (g) B 7. Use the average bond enthalpies below to calculate the enthalpy change, in kj, for the following reaction. H 2 (g) + I 2 (g) 2HI(g) A B. +0 C. 0 D. 290 C One G2 comment stated that the question should have asked for the enthalpy change in the reaction in units of kj mol instead of kj. This aspect of units has been discussed in detail in several previous subject reports and teachers are referred to these reports for further reference.

7 8. A student measured the temperature of a reaction mixture over time using a temperature probe. By considering the graph, which of the following deductions can be made? I. The reaction is exothermic. II. III. The products are more stable than the reactants. The reactant bonds are stronger than the product bonds. A. I and II only B. I and III only C. II and III only D. I, II and III A There were two G2 comments on this question, both suggesting that the graph given was confusing to candidates. In this question candidates had to use a combination of ideas to ascertain that the correct answer is A, namely I. and II. From the graph shown, candidates need to realise that the reaction is exothermic, and therefore from this information, the products are more stable than the reactants. 55% of candidates got the correct answer. International Baccalaureate Organization 207 International Baccalaureate - Baccalauréat International - Bachillerato Internacional Printed for Southeast High School