Chemical Energy

Question 1

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

Answer 1

Eh = cmΔT

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

Define the enthalpy of combustion of a substance.

Answer 3

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

Question 4

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

Answer 4

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 5

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

Answer 5

1. Heat loss to the surroundings

2. Incomplete combustion

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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 8

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

Answer 8

Bond enthalpy

Question 9

Is energy required or released when bonds are broken?

Is breaking bonds exothermic or endothermic?

Answer 9

Energy is required to break bonds.

Bond breaking is an endothermic process

Question 10

Is energy required or released when bonds are made?

Is making bonds exothermic or endothermic?

Answer 10

Energy is released when new chemical bonds are formed.

Bond making is an exothermic process

Question 11

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

Answer 11

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 12

What kind of reaction is combustion?

Answer 12

Exothermic