1.6 Energetics

Section A

For each of the questions only one of the lettered responses (A - D) is correct.

Select the correct response in each case and mark its code letter by connecting the dots as illustrated on the answer sheet.

1.C Given the bond energies:

C - C 348 kJ mol^-1 C - H 412 kJ mol^-1

C = C 612 kJ mol^-1 H - H 436 kJ mol^-1

calculate the enthalpy change for the reaction

C₃H₆ + H₂ C₃H₈

A -248 kJ mol^-1

B +248 kJ mol^-1

C -124 kJ mol^-1

D + 124 kJ mol^-1

2.C Given the bond energies:

C - C 348 kJ mol^-1 C - H 412 kJ mol^-1

C = C 612 kJ mol^-1 H - H 436 kJ mol^-1

C = O 803 kJ mol^-1 O – H 464 kJ mol^-1

O = O 497 kJ mol^-1 O – O 146 kJ mol^-1

Which of the following is the calculated enthalpy change for the reaction below?

CH₄ + 2O₂ CO₂ + 2H₂O

A -104 kJ mol^-1

B -323 kJ mol^-1

C -820 kJ mol^-1

D -6104 kJ mol^-1

3. The following table shows the enthalpy change of neutralisation for some acid-base reactions:

acid	base	enthalpy change of neutralisation / kJ mol^-1
HCl	NaOH	-57.2
X	NaOH	-55.2
HCl	Y	-52.2
HNO3	Z	-57.2

The identities of X, Y and Z are

X Y Z

A ethanoic acid ammonia potassium hydroxide

B sulphuric acid ammonia potassium hydroxide

C ethanoic acid sodium hydroxide ammonia

D sulphuric acid sodium hydroxide ammonia

4. Given the bond dissociation energies:

C-C 343 kJ mol^-1 C-H 414 kJ mol^-1

C=C 611 kJ mol^-1 H-H 435 kJ mol^-1

What is the enthalpy change for the reaction?

CH₃CH=CH₂ + H₂ CH₃CH₂CH₃

A -560 kJ

B -125 kJ

C +125 kJ

D +289 kJ

5. If the enthalpies of combustion of carbon, hydrogen and methane are -395, -287 and -881 kJ mol^-1 respectively, what is the enthalpy of formation of methane in kJ mol^-1?

A -199

B -88

C +88

D + 199

6. If the enthalpies of combustion of graphite, hydrogen and ethanol are -393, -286 and - 1367 kJ mol^-1 respectively, which one of the following is the enthalpy of formation of ethanol?

A -139 kJmol^-1

B -277 kJ mol^-1

C +277 kJ mol^-1

D +554 kJmol^-1

Section B

1. Methanol is manufactured by passing a mixture of hydrogen and carbon monoxide, under pressure, over a catalyst at 400^oC. An equilibrium is set up as shown in the equation.

CO + 2H₂ CH₃OH

(a) Calculate the enthalpy change for the reaction using the bond energies below.

Bond Energy kJ mol^-1

Cº O 1077

H ¾ H 436

C ¾ H 413

C ¾ O 336

O ¾ H 464

[2]

(b) Why does no further heat have to be provided to maintain the temperature at 400^oC once the reaction has started?

[2]

2. (a) Define the term standard enthalpy change of combustion.

[3]

(b) Propan-l-ol, C₃H₇OH, burns in excess air to form carbon dioxide and water. Calculate the standard enthalpy change of formation of propan-l-ol given the following data.

Substance	Standard Enthalpy of Combustion/kJ mol^-1
C3H7OH(l) C(s) H2(g)	-2010 -394 -286

[4]

(c) The standard enthalpy change of formation of propan-l-ol was calculated to

be -340 kJ mol^-1 using bond enthalpy values. Comment on the difference between this figure and that calculated in part (b).

[2]

3. The reaction between hydrogen peroxide and hydrazine was used to power the Messerschmidt 163 in World War Two.

The equation for the reaction is

2H₂O₂ + N₂H₄ 4H₂O + N₂

Using the bond energies given below, estimate the enthalpy change for the above reaction in the gas phase.

Bond	Bond energy/ kJ mol-1
O-H	463
N-H	391
N-N	159
O-O	143
N=N	945

[4]

4. Camping gas contains propane which reacts exothermically with oxygen.

(a) Write an equation for the complete combustion of propane in oxygen.

[1]

(b) On the axes below, draw a labelled enthalpy level diagram for this reaction.

Enthalpy

Progress of reaction [3]

5. Use the following bond enthalpies to calculate the enthalpy change for the following reaction:

SF₆ + 3H₂O SO₃ + 6HF

bond bond enthalphy/ kJ mol^-1

S-F 326

O-H 464

S-O (in SO₃) 469

H-F 568

enthalpy change = kJ mol-1 [3]

6. The enthalpy change for the decomposition of calcium carbonate,

CaCO₃(s) CaO(s) + CO₂(g)

is impossible to determine directly so the following method was devised:

Two pieces of natural chalk, calcium carbonate, were chosen, each of mass

1.25 g. One piece, A, was put into 40.0 cm³ of dilute hydrochloric acid, the temperature of which rose by 2.0 ^oC.

The other piece of chalk was heated in the hottest flame available for 10 minutes to decompose it to calcium oxide, B. It was then allowed to cool to room temperature before it was added to 40.0 cm³ of dilute hydrochloric acid. The temperature of the acid rose by 12.0 ^oC.

(Molar mass of CaCO₃ = 100 g mol^-1)

(a) (i) Calculate the energy produced by the reaction of each solid sample with the acid. Use the relationship:

Energy produced in Joules = 4.18 x volume of solution x temperature rise

A Original chalk sample

4.18 x 40.0 x 2 = 334.4 (joules)

B Calcium oxide sample

[2]

4.18 x 40.0 x 12 = 2006.4 (joules)

method [1] two correct answers [1]

(ii) How many moles of chalk were there in each of the original pieces?

1.25/100 = 0.0125 (moles) (1)

[1]

(iii) Use your answers to (a)(i) and (a)(ii) to calculate enthalpy changes for the two reactions with the acid, ΔH_a^θ (from CaCO₃) and ΔH_b^θ (from CaO). Your answers should be given to 2 significant figures and include signs and units.

ΔH_a^θ

ΔH_a^θ = -334.4/0.0125 = -26752 = -27 kJ mol^-1

ΔH_b^θ

ΔH_b^θ = -2006.4/0.0125 = -160512 = -160 kJ mol^-1

method (1) two "correct" answers (1) SF and units, signs (1)

[3]

(b) A Hess cycle based on these reactions is

2HCl(aq) + CaCO₃(s) CaO(s) + CO₂(g) + 2HCl (aq)

CaCl₂(aq) + CO₂(g) + H₂O(l)

Use this and your answers in (a) to calculate a value for ΔH^θ_reaction .

Your answer should be given to 2 significant figures and include sign and units.

ΔH^θ_reaction = ΔH_a - ΔH_b^θ

= -27 + 160

= +133kJ mol^-1

cycle (1) fully correct answer (1)

[2]

(c) The value for ΔH^θ_reaction in reference books is + 178.3 kJ mol-1.

Suggest THREE reasons for the difference between this value and the value you calculated in (b).

[4]

Three reasons include:

1 original chalk was impure so moles used were less than hoped;

2 the chalk was not fully converted to calcium oxide so there was less

than hoped;

3 the reaction with acid was incomplete;

4 temperatures rises recorded were low due to heat losses during

slow reactions.

Disallow any suggestions of mistakes in experimental procedure.

Any three reasons [3]

Fewer moles of reaction/heat losses results in lower value for ΔH^θ_reaction than reference value. [1]