Chapter 9- Enthalpy

Question 1

Define enthalpy

Answer 1

The heat content that is stored in a chemical system

Question 2

What is enthalpy stored in?

Answer 2

kJmol-1

Question 3

Define activation energy

Answer 3

The minimum energy required to start a reaction by the breaking of bonds

Question 4

Give 2 examples of endothermic reactions

Answer 4

Thermal decomposition

Photosynthesis

Question 5

Give 2 examples of exothermic reactions

Answer 5

Combustion

Neutralisation

Question 6

What happens in a exothermic reaction?

3 things

Answer 6

Enthalpy of the product is smaller than that of the reactants
There is heat loss FROM the SYSTEM TO the SURROUNDINGS
The enthalpy change has a NEGATIVE sign because heat is LOST from the chemical system

Question 7

What happens in an endothermic reaction?

3 things

Answer 7

Enthalpy of the product is larger than that of the reactants
There is a heat gain TO the SYSTEM FROM the SURROUNDINGS
The surroundings DECREASE in temperature
The enthalpy has a positive sigh because heat has been gained by the chemical system

Question 8

What 2 things do you need to write in an exam question to prove a reaction profile is endothermic?

Answer 8

The enthalpy of the reactants is LOWER than the enthalpy of the product

The enthalpy change is POSITIVE so energy is TAKEN IN FROM the surroundings

Question 9

What 2 things do you need to write in an exam question to prove a reaction profile is exothermic?

Answer 9

The enthalpy of the reactants is HIGHER than the enthalpy of the product

The enthalpy change is NEGATIVE so energy is GIVEN OUT TO the surroundings

Question 10

Is energy taken in to break bonds or make bonds?

Answer 10

To break bonds

Question 11

Is energy given out to break bonds or make bonds?

Answer 11

Make bonds

Question 12

Explain the relationship between exothermic energy profiles and bonds

Answer 12

More energy ie released when bonds are made than when bonds are broken

Question 13

Explain the relationship between endothermic energy profiles and bonds

Answer 13

Less energy is release when bonds are made than is taken in than when bonds are broken

Question 14

What are the standard conditions in enthalpy changes?

Answer 14

100kPa
298K
1moldm-3 for a solution reaction

Question 15

What are standard states?

Answer 15

The physical state of a substance being under the standard conditions of 100kPa and 298K

Question 16

What is 🔼H?

Answer 16

The enthalpy changes under standard conditions

Question 17

Define standard enthalpy changes of reactions

Answer 17

The enthalpy change that accompanies a reaction in the molar quantities expressed in the chemical equation in their standard states under their standard conditions of 100kPa and 298K

Question 18

Define standard enthalpy changes of combustion

Answer 18

The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen in their standard states under the standard conditions of 100kPa and 298K

Question 19

Define standard enthalpy changes of neutralisation

Answer 19

The enthalpy change that accompanies the reaction of an acid and an alkali or base to form 1 mole of H20 in their standard states and under the standard conditions of 100kPa and 298K

Question 20

Define standard enthalpy changes of formation

Answer 20

The enthalpy change that takes plafe when 1 mole of a compound is formed from its constituent elements in their standard states under the standard conditions of 100kPa and 298 K