Unit 5- Energy Changes and reversible reactions

Question 1

State the law of conservation of energy.

Answer 1

Energy cannot be created or destroyed, it can only transferred from one place to another.

Question 2

How does the law of conservation of energy apply to chemistry?

Answer 2

In all chemical reactions, energy is either transferred to the surroundings or from the surroundings.

Question 3

What is an exothermic reaction?

Answer 3

A reaction where energy is transferred to the surroundings.

Question 4

Give two examples of exothermic reactions.

Answer 4

Combustion, respiration

Question 5

What happens to the temperature of the surroundings during an exothermic reaction?

Answer 5

They increase. The thermometer is included in “the surroundings” so shows the temperature increasing.

Question 6

What is an endothermic reaction?

Answer 6

A reaction where energy is transferred from the surroundings.

Question 7

Give two examples of endothermic reactions.

Answer 7

Thermal decomposition reactions, citric acid and sodium hydrogencarbonate.

Question 8

What happens to the temperature of the surroundings during an endothermic reaction?

Answer 8

They decrease. The thermometer is included in “the surroundings” so shows the temperature decreasing.

Question 9

State two uses of exothermic reactions

Answer 9

Self-heating cans, hand warmers

Question 10

State two uses of endothermic reactions

Answer 10

Some cooling sports injury packs

Question 11

What are reactants?

Answer 11

The substances involved in a chemical reaction

Question 12

What are products?

Answer 12

The substances formed when reactants have a chemical reaction

Question 13

What is a reaction profile?

Answer 13

A diagram which shows whether the reactants have more or less energy than the products.

Question 14

If the reactants have more energy than the products, what kind of a reaction must have taken place?

Answer 14

An exothermic one. The missing energy has been transferred to the surroundings.

Question 15

If the reactants have less energy than the products, what kind of a reaction must have taken place?

Answer 15

An endothermic one. The extra energy has been take in by the surroundings.

Question 16

Is breaking bonds endothermic or exothermic?

Answer 16

Endothermic. Chemical bonds are strong so require energy to break (like when you have to put energy in to separate magnets from each other)

Question 17

Is making bonds endothermic or exothermic?

Answer 17

Exothermic. Energy is released when chemical bonds are formed (like how two magnets move together when close and generate kinetic energy)

Question 18

How do we work out the overall energy change of a reaction?

Answer 18

Work out the difference between the energy needed to break all the bonds in the reactants and the energy released to form all the bonds in the products.

Question 19

What is a reversible reaction?

Answer 19

A reaction which can go from reactants to products but also from products to reactants

Question 20

What chemical symbol represents a reversible reaction?

Answer 20

⇌

Question 21

If a reaction is exothermic in the forward direction what will it be in the reverse direction?

Answer 21

Endothermic

Question 22

Two reactants require 30kJ to turn into products. What energy transfer is involved when the products turn into reactants?

Answer 22

30kJ is released

Question 23

What is equilibrium?

Answer 23

The point in a reversible reaction when the forward and reverse reactions are occurring at the same rate

Question 24

How is the amount of reactant changing at equilibrium?

Answer 24

The point in a reversible reaction when the forward and reverse reactions are occurring at the same rate

Question 25

What is Le Chatelier’s principle?

Answer 25

When a reaction at equilibrium is changed, it will seek to counteract that change

Question 26

How is the amount of reactant changing at equilibrium?

Answer 26

It is not changing

Question 27

How is the amount of product changing at equilibrium?

Answer 27

It is not changing

Question 28

A reaction is exothermic in the forward direction. What will occur if the temperature is increased?

Answer 28

The backward reaction will increase as it is endothermic and will reduce the temperature

Question 29

A reaction is at equilibrium when some product is removed. What will occur?

Answer 29

The forward reaction will increase as that will increase the amount of product

Question 30

The reaction below is at equilibrium. What will occur when the pressure is increased? 2H₂(g) + O₂(g) ⇌ 2H₂O(g)

Answer 30

The forward reaction will increase as there are fewer molecules on the right. This will reduce the total number of molecules and therefore the pressure.