Chemical Energetics

Question 1

Examples of exothermic reactions.

Answer 1

1. All combustion reactions
2. Oxidation of carbohydrates in animals and plants (respiration)
3. The reaction of water with quicklime (calcium oxide)

Question 2

Examples of endothermic reactions.

Answer 2

1. All thermal decomposition reactions
2. Photosynthesis in plants
3. Dissolving certain ammonium salt in water

Question 3

What is meant by Enthalpy?

Answer 3

The chemical energy stored in a system that can be converted into heat is known as enthalpy. It is denoted by “H”

Question 4

What are the standard conditions for enthalpy?

Answer 4

101 kPa (1.01 x 10⁵ Pa)

298K (25°C)

1 mol/dm³

All the substances involved in the reaction are in their normal physical state.

Question 5

Define exothermic reaction.

Answer 5

It is a chemical reaction in which chemical energy is converted into heat energy and the temperature of the system rises.

Question 6

Define endothermic reaction.

Answer 6

It is a chemical reaction in which heat energy is converted into chemical energy and the temperature of the system falls.

Question 7

Formula for enthalpy change of a reaction:

Answer 7

Enthalpy of products - Enthalpy of reactants

Question 8

Exothermic and endothermic reactions alter the temperature of the surroundings. What is considered the surrounding?

Answer 8

1. The solvent
2. The air around the container
3. The container itself
4. Anything dipping into the container

Question 9

Is oxidation exothermic reaction?

Answer 9

They are generally exothermic.

Question 10

Define enthalpy change.

Answer 10

The heat energy transferred during a chemical reaction.

It is the amount of energy taken in or given out during any change in a system provided that the pressure is constant.

Question 11

What is a reaction pathway diagram?

Answer 11

It shows the relative enthalpies of the reactants and the products and the enthalpy change as an arrow. It may also include the activation energy.

Question 12

What is activation energy?

Answer 12

The minimum energy that colliding particles must possess to break bonds to start a chemical reaction.

Question 13

standard enthalpy change of reaction.

Answer 13

the enthalpy change when the amounts of reactants shown in the stoichiometric equation react to give products under standard conditions of 298k and 101kPa pressure.

Question 14

standard enthalpy change of formation

Answer 14

the enthalpy change when one mole of a compound is formed from its elements under standard conditions of 298k and 101kPa pressure.

Question 15

standard enthalpy change of combustion

Answer 15

the enthalpy change when one mole of a substance is burnt in excess oxygen in their standard states under standard conditions of 298k and 101kPa pressure.

Question 16

Standard enthalpy change of neutralization:

Answer 16

It is the enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions of 298k and 101kPa pressure.

Question 17

Specific heat capacity of water, c.

Answer 17

4.18 J/g/°C.

Question 18

What assumptions do we make during calorimetry?

Answer 18

1. 1cm³ of solution has a mass of 1g
2. The solution has the same specific heat capacity of water.

Question 19

Equation for energy transfer as heat, q.

Answer 19

q = mc∆T

Question 20

What does Hess’s law state?

Answer 20

Hess’s law states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.