Energetics

Question 1

Q: What is an exothermic reaction?

Answer 1

A: A reaction that gives out energy to the surroundings, usually as heat, causing the surroundings’ temperature to rise.

Question 2

Q: What is an endothermic reaction?

Answer 2

A: A reaction that takes in energy from the surroundings, usually as heat, causing the surroundings’ temperature to fall.

Question 3

Q: Give an example of an exothermic reaction.

Answer 3

A: Combustion reactions, such as burning methane.

Question 4

Q: How can you tell if a reaction is exothermic or endothermic?

Answer 4

A: Exothermic reactions cause a temperature increase; endothermic reactions cause a temperature decrease.

Question 5

Q: What is enthalpy change (ΔH)?

Answer 5

A: The overall energy change during a chemical reaction.

Question 6

Q: What are the units of enthalpy change?

Answer 6

A: kJ/mol (kilojoules per mole).

Question 7

Q: What does a negative ΔH value mean?

Answer 7

A: The reaction is exothermic (energy is released).

Question 8

Q: What does a positive ΔH value mean?

Answer 8

A: The reaction is endothermic (energy is absorbed).

Question 9

Q: What is a reaction profile?

Answer 9

A: A diagram showing the energy of reactants and products and how energy changes during a reaction.

Question 10

Q: How does an exothermic reaction appear on a reaction profile?

Answer 10

an exothermic reaction — the products are at a - lower energy than the reactants. The difference in height represents the energy given out (per mole) in the reaction.
ДН is negative here.
) The initial rise in energy represents the energy needed to start the reaction. This is the activation energy (E).

Question 11

Q: How does an endothermic reaction appear on a reaction profile?

Answer 11

This shows an endothermic reaction because the products are at a higher energy than the reactants.
The difference in height represents the energy taken in (per mole) during the reaction. AH is positive here.

Question 12

Q: What is activation energy?

Answer 12

The activation energy is the minimum amount of energy the reactants need to collide with each other and react.

Question 13

Q: Why is a polystyrene cup used in calorimetry?

Answer 13

Polystyrene is a good insulator

It reduces heat loss to the surroundings

Makes results more accurate

Question 14

What does calorimetry measure?

Answer 14

The temperature change of water or solution
Used to calculate heat energy changes in reactions

Question 15

Q: What material is used for combustion calorimetry containers and why?

Answer 15

A: Copper, because it is a good conductor of heat.

Question 16

Q: What equation is used to calculate heat energy transferred?

Answer 16

A: Q = m × c × ΔT

Question 17

Q: What does each symbol mean in Q = m × c × ΔT?

Answer 17

A:

Q = heat energy (J)

m = mass (g)

c = specific heat capacity (J/g/°C)

ΔT = temperature change (°C)

Question 18

Q: What is the specific heat capacity of water?

Answer 18

A: 4.2 J/g/°C

Question 19

Q: How do you calculate molar enthalpy change (ΔH)?

Answer 19

A: ΔH = Q ÷ number of moles reacted

Question 20

Q: Why is ΔH negative for combustion reactions?

Answer 20

A: Because combustion reactions are exothermic.

Question 21

Q: What happens to energy when bonds are broken?

Answer 21

A: Energy is absorbed (endothermic).

Question 22

Q: What happens to energy when bonds are formed?

Answer 22

A: Energy is released (exothermic).

Question 23

Q: How do you calculate enthalpy change using bond energies?

Answer 23

A:
ΔH = total energy to break bonds − total energy released making bonds

Question 24

Q: What does a positive ΔH from bond energies mean?

Answer 24

A: The reaction is endothermic.

Question 25

Q: What is a catalyst?

Answer 25

A catalyst is a substance which increases the rate of a reaction. without being chemically changed or used up in the reaction.

Question 26

Q: How does a catalyst affect activation energy?

Answer 26

A: It lowers activation energy by providing an alternative pathway.

Question 27

Q: Does a catalyst change ΔH?

Answer 27

A: No, it only changes the rate, not the energy change.

Question 28

What is calorimetry?

Answer 28

Calorimetry is measuring the heat energy change in a reaction by recording the temperature change of water or solution.

Question 29

Whats the symbol if the reaction is Endothermic

Answer 29

The value is positive as the reaction takes IN energy

Question 30

Whats the symbol if the reaction is Exothermic

Answer 30

The value is negative because the reaction is giving OUT energy

Question 31

what is a reaction profile

Answer 31

Reaction profiles are diagrams that show the relative energies of the reactants and products in a reaction, and how the energy changes over the course of the reaction.

Question 32

How is a calorimetry experiment carried out for a combustion reaction?

Answer 32

Water is placed in a metal can or copper calorimeter Initial temperature of the water is recorded Fuel is burned underneath the can Final temperature of the water is measured Temperature increases → reaction is exothermic Some heat is lost to the surroundings

Question 33

How do you measure the heat change of a displacement reaction?

Answer 33

A known volume of metal salt solution is placed in a beaker Initial temperature is measured A more reactive metal is added Solution is stirred Highest temperature is recorded Temperature rises → exothermic reaction

Question 34

How is calorimetry used to measure heat change when a solid dissolves?

Answer 34

Water is placed in a polystyrene cup Initial temperature is recorded Solid is added and stirred until dissolved Final temperature is measured Temperature decrease → endothermic Temperature increase → exothermic

Question 35

How is calorimetry carried out for a neutralisation reaction?

Answer 35

Equal volumes of acid and alkali are measured Initial temperatures are recorded Solutions are mixed in a polystyrene cup Maximum temperature is recorded Temperature increases → exothermic reaction