Unit 3-Chemical Energy

Question 1

Why is it important for chemists to be able to predict the quantity of heat energy taken in or given out?

Answer 1

Endothermic reactions may incur costs in supplying heat energy in order to maintain the reaction rate.

Exothermic reactions may require removal of heat to prevent the temperature rising

Question 2

What is meant by an enthalpy change?

Answer 2

Enthalpy change is the difference in energy between the products and the reactants.

Question 3

What equation is used to calculate the enthalpy change for a reaction?

Answer 3

Eh = cmΔT

Question 4

Define the enthalpy of combustion of a substance.

Answer 4

The enthalpy of combustion of a substance is the enthalpy change (energy released) when one mole of the substance burns completely in oxygen.

Question 5

A spirit burner and a metal beaker can be used to experimentally measure the energy released by a fuel when burned.

Describe how to set up the experiment.

Answer 5

Question 6

What measurements need to be taken when determining the energy released when a fuel burns?

Answer 6

1. the start and the final temperature of the water which allows you to calculate the temperature change
2. the mass of the burner before and after the experiment which allows you to calculate the mass of alcohol burned.

Question 7

Give a reason why your experimental enthalpy value differs from the textbook value.

Answer 7

1. heat loss to the surroundings
2. incomplete combustion

Question 8

What does Hess’s law tell us about the enthalpy change of a reaction?

Answer 8

Hess’s law states that the enthalpy change for a chemical reaction is independent of the route taken.

Question 9

When rearranging equations to calculate an enthalpy change, what two rules have to be followed?

Answer 9

1. The enthalpy change (ΔH) is proportional to the quantities of reactants and products. For example, burning twice as much fuel will result in twice the enthalpy change for the process.
2. If a reaction is reversed then the sign of the enthalpy change must also be reversed.

Question 10

ΔH_T = Ca(s) + 2H₂O(ℓ) → Ca(OH)₂(s) + H₂(g)

Calculate the enthalpy change, in kJ mol⁻¹ , for the reaction above by using the data shown below.

H₂(g) + ½O₂(g) → H₂O(ℓ) ΔH₁ = −286 kJ mol⁻¹

Ca(s) + O₂(g) + H₂(g) → Ca(OH)₂(s) ΔH₂ = −986 kJ mol⁻¹

Answer 10

ΔH_T = ΔH₂ − 2ΔH1

ΔH_T = −984 −​ (2 x −286)

= −986 + 572

= −414 kJ mol⁻¹

Question 11

The energy required to break a covalent bond between two atoms is called the ​

Answer 11

Bond enthalpy

Question 12

Is energy required or released when bonds are broken?

Is breaking bonds exothermic or endothermic?

Answer 12

Energy is required to break bonds.

Bond breaking is an endothermic process.

Question 13

Is energy required or released when bonds are made?

Is making bonds exothermic or endothermic?

Answer 13

Energy is released when new chemical bonds are formed.

Bond making is an exothermic process.

Question 14

Bond enthalpies quoted in the data book are the energies required to break what.

Answer 14

The bond enthalpies quoted in the data book are the energies required to break one mole of a particular bond between a pair of atoms in the gaseous state.

Question 15

Why can the values in the databook only be used to calculate an approximate enthalpy change?

Answer 15

Some values are mean molar bond enthalpies which are average values quoted for bonds which occur in different molecular environments.

Energy to break C-C in ethane will be different to the energy needed to break C-C in decane.

Question 16

The mean bond enthalpy of a C – F bond is 484 kJ mol⁻¹.

In which of the processes is ΔH approximately equal to +1936 kJ mol^-1?

A CF₄(g) → C(s) + 2F₂(g)

B CF₄(g) → C(g) + 4F(g)

C CF₄(g) → C(g) + 2F₂(g)

D CF₄(g) → C(s) + 4F(g)

Answer 16

ΔH comes from 4 x C – F bonds being broken (1936 = 4 x 484) and no bonds being made.

Rules out A and C

Changing the physical state of any reactant (or product) will involve an enthalpy change which rules out D.

Answer is B