A Level Chemistry

Question 1

Copper(II) nitrate decomposes on heating. The reaction is endothermic.

`2Cu(NO3)_2(s) -> 2CuO(s) + 4NO_2(g) + O_2(g)`

i. Draw an enthalpy level diagram (reaction profile diagram) for this reaction.

ii. Calculate the enthalpy change for this reaction using the following enthalpy changes of formation.

`ΔH""_f^theta [Cu(NO3)_2(s)] = –302.9 kJ mol^(–1)`

`ΔH""_f^theta [CuO(s)] = –157.3 kJ mol^(–1)`

`ΔH""_f^theta [NO_2(g)] = +33.2 kJ mol^(–1`

i.

ii.The enthalpy change for this reaction

`ΔH=ΔH_f("products")−ΔH_f("reactants") = 2xx(−157.3)+4xx(33.2)+0−2xx(−302.9) = 424.0 kJ mol^(-1)`

Question 2

Copper(II) sulfate is soluble in water. A student dissolved 25.0 g of copper(II) sulfate in 100 cm³ of water in a polystyrene beaker stirring all the time. The temperature of the water fell by 2.9 °C.

i. Calculate the enthalpy change of solution of copper(II) sulfate. (specific heat capacity of water = 4.18 J g^–1°C^–1; relative molecular mass of copper(II) sulfate = 249.7 g mol^–1)

ii. Suggest one source of error in this experiment and explain how the error affects the results.

i.

The total heat change (Q) based on the temperature change

`Q=−mCΔT`

The mass of water with m = 100 g, specific heat capacity with C = 4.18 `J/(g °C)`, and temperature change with ΔT = -2.9 °C:

`Q=−100×4.18×−2.9°C=1212.2 J=1.2122 kJ`

The number of moles of `CuSO₄`:

`n = 25.0 / 249.7 = 0.100 mol`

The enthalpy change of solution

`ΔH_(dissolution) = Q/n = 1.2122 / 0.100 = 12.122 (kJ)/(mol)`

ii. Heat loss to the environment is one potential source of inaccuracy. Even though the polystyrene beaker used for the experiment reduces heat exchange, some heat may still escape. This would cause the temperature to shift more than is seen, indicating that more energy is absorbed by the dissolution process than is shown and that the enthalpy change of solution is underestimated.

Question 3

Propanone is a liquid. It has the structure

The equation for the complete combustion of propanone is:

`CH_3COCH_3(l) + 4O_2(g) -> 3CO_2(g) + 3H_2O(l)`

a. Use the following bond energies (in kJmol–1) to calculate a value for the standard enthalpy change of this reaction:

E(C-C) = +347

E(C-H) = +413

E(O=O) = +496

E(C=O) = +805

E(O-H) = +465

b. Suggest why it would be more accurate to use bond energies that are not average bond energies in this calculation.

c. The standard enthalpy change of combustion of propanone is –1816.5 kJ mol^–1. Suggest why this value differs from the value obtained using bond energies.

a. The enthalpy for bond breaking = `2 (C-C) + 6 (C-H) + (C=O) + 4 (O=O) = 2 xx 327 + 6 xx 413 + 805 + 4 xx 496 = + 5961 kJ`

The enthalpy for bond making = `- [2 (C=O) + 6 (O-H)] = - [2xx805+6xx465] = -7620 kJ`

The standard enthalpy change =`-7620 +5961=-1659 kJ`

b. Because the same type of bonds are found in different environments and average bond energies are obtained from a number of different bonds of the same type.

c. Bond energies are calculated by using enthalpy changes of gaseous compound to gaseous atoms as well as enthalpy changes of combustion done experimentally using propanone liquid.

Question 4

The standard enthalpy change of formation of propanone is –248 kJ mol^–1.

i. Define the term standard enthalpy change of formation.

ii. Write the equation that describes the standard enthalpy change of formation of propanone.

iii. Explain why the enthalpy change of formation of propanone cannot be found by a single experiment.

i. The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.

ii. `3C ("grapite") + 3H_2(g) +1/2 O_2(g) -> C_3H_6O (l)`

iii. Carbon does not react directly with hydrogen under standard conditions

Question 5

Define standard enthalpy change of combustion.

The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.

Question 6

a. Calculate the standard enthalpy change of formation of methane from the following standard enthalpy changes of combustion:

carbon = –394 kJ mol^–1

hydrogen = –286 kJ mol^–1

methane = –891 kJ mol^–1

b. Calculate the standard enthalpy change of combustion of methane using the following bond energies:

E(C-H) = +412 kJ mol^–1

E(O-O) = +496 kJ mol^–1

E(C=O) = +805 kJ mol^–1

E(O-H) = +463 kJ mol^–1

a. `C + 2H_2 -> CH_4`

`ΔH""_c^theta = ΔH""_r^theta ("reactants") - ΔH""_r^theta("products") = (-394 - 2 xx 286) - (-891) = -75 kJ mol^(-1`

b. `CH_4 + 2O_2 -> CO_2 + H_2O`

The enthalpy for bond breaking = `4 (C-H) + 2 (O=O) = 4 xx 412 +2 xx 496 = + 2640 kJ`

The enthalpy for bond making = `- (2 (C=O) + 4 (O-H)) = - (2xx805+4xx463) = -3462 kJ`

The standard enthalpy change = `2640 - 3462 = -822 kJ mol^-1`

Question 7

Can this equation `H_2SO_4 (aq) + 2NaOH (aq) → Na_2SO_4 (aq) + 2H_2O (l)` represent the enthalpy of neutralisation?

This equation cannot represent the enthalpy of neutralisation because the enthalpy of neutrolisation produces 1 mole of water from an acid and base.

Question 8

Which equation below can represent both an enthalpy change of formation and combustion?

A. `CH_4 (g) + 2O_2 (g) → CO_2 (g) + 2H_2O (l)`

B. `4Na (s) + O_2 (g) → 2Na_2O (s)`

C. `HCl (aq) + NaOH (aq) → NaCl (aq) + H_2O (l)`

D. `S (s) + O_2 (g) → SO_2 (g)`

The answer is D.

The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.

The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.

A is incorrect because this is not enthalpy of formation

B is incorrect because the number of moles of reactant does not match the enthalpy of combustion definition and the number of moles of product does not match the enthalpy of formation definition

C is incorrect because this is neither formation nor combustion.

Question 9

Hydrogen atoms bond covalently to Iodine atoms to form hydrogen iodide as shown in the equation below:

`H_2 (g) + I_2 (g) → 2HI (g)`

Which statement best describes what is meant by the average HI bond enthalpy?

A. The energy stored in a covalent bond.

B. The energy required to break one covalent bond in the gas phase.

C. The energy required to break one mole of the HI bonds in the gas phase.

D. The energy released when two atoms form a covalent bond.

The answer is C.

A is incorrect because this is the wrong terminology.

B is incorrect because the definition is for 1 mole of bonds rather than just a unitless number

D is incorrect because the average bond enthalpy relates to the energy required not released

Question 10

The standard enthalpy change ∆Hፀ for the below reaction is -1776 kJ.

`2N (g) + 6F (g) → 2NF_3 (g)`

What is the bond energy of the N–F bond?

A. -592

B. +592

C. +296

D. -296