Energetics Revision Exam Questions

8/11/2019 Energetics Revision Exam Questions

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Energetics Revision Questions

1. This question is about methanol and the energy changes that accompany some of its reactions.

(a) Complete the diagram (using dots and crosses) to show the bonding in methanol, CH 3OH. oushould show outer electrons only.

(2)

(b) The Hess cycle below can be used to calculate the standard enthalpy change of combustion ofmethanol, using standard enthalpy changes of formation.

(i) Complete the cycle by filling in the empty bo!. (2)

(ii) "efine the term standard enthalpy change of formation of a compound, ma#ing clear themeaning of standard in this conte!t.

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Doha College 1


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(iii) $se your cycle and the data below to calculate the standard enthalpy change of combustion of methanol,%Hc

&

%Hf & '# mol *

CO +(g) 3 3.-

H+O(l) + -.

CH 3OH(l) +3 .*(2)

(c) /n e!periment was carried out, using the apparatus below, to estimate the standard enthalpychange of combustion of methanol.

/fter burning the methanol for a few minutes, the temperature of water in the bea#er had risen by +0.1 2Cand the mass of methanol burnt was 0. g.

(i) Calculate the amount of energy transferred to the water.

4nergy transferred ( ) 5 mass of water 6 .* 6 temperature change(1)

(ii) Calculate the number of moles of methanol, CH 3OH, burnt during the e!periment.(1)

(iii) $se your answers to (c)(i) and (ii) to calculate the e!perimental 7alue for the standard enthalpy change of combustion. 8nclude a sign and units in your answer, which should be gi7en to three significant figures.

(1)

(i7) Compare your answers to (b)(iii) and (c)(iii) and gi7e T9O reasons to e!plain any differences.

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(Total 14 marks)

Doha College 2


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2. 9hich equation represents the reaction for which the enthalpy change is the standard enthalpy change offormation, % H f

: , of sodium nitrate, ;a;O 3 +;a;O 3(s)

B ;a(s) = ?; +(g) = *?O +(g) > ;a;O 3(s)

C ;a(s) = ;(g) = 3O(g) > ;a;O 3(s)

D ;a(g) = ?; +(g) = *?O +(g) > ;a;O 3(g)(Total 1 mark)

3. @agnesium chloride can be made by reacting solid magnesium carbonate, @gCO 3, with dilutehydrochloric acid.

(a) 9rite an equation for the reaction, including state symbols.(2)

(b) Ai7e T9O obser7ations you would ma#e when the reaction is ta#ing place.

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(c) 8n an e!periment to ma#e crystals of hydrated magnesium chloride, @gCl +.BH+O, magnesium

carbonate was added to +- cm 3 of hydrochloric acid with concentration +.0 mol dm 3. The molar

mass of magnesium carbonate is .3 g mol *

.

(i) How many moles of acid are used in the reaction

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Energetics Revision Exam Questions