Chemistry // Module 3 // Enthalpy Changes

Question 1

What is the definition of an Endothermic Reaction and give an example?

Answer 1

Chemical system absorbs energy from the surroundings. Bonds are broken. Positive ΔH. Surrounding temperature decreases.
E.g. Thermal Decomposition

Question 2

What is the definition of an Exothermic Reaction and give an example?

Answer 2

Chemical system releases energy to the surroundings. Bonds are formed. Negative ΔH. Surrounding temperature increases.
E.g. Combustion

Question 3

What is the definition of Standard Enthalpy Change of Combustion (ΔHθC)?

Answer 3

When one mole of a substance reacts with oxygen under standard conditions, with all reactants and products in their standard states.

Question 4

What is the definition of Standard Enthalpy Change of Formation (ΔHθF)?

Answer 4

When one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

Question 5

What is the definition of Standard Enthalpy Change of Neutralisation (ΔHθneut)?

Answer 5

When one mole of water is formed in an acid base reaction under standard conditions, with all reactants and products in their standard states.

Question 6

What is the definition of Activation Energy?

Answer 6

The minimum amount of energy needed for a chemical reaction to take place.

Question 7

What are the Standard Conditions and what is meant by standard state?

Answer 7

Pressure- 100KPa
Temperature- 298K
Concentration Solution- 1 moldm^-3

Standard State - Physical State of a substance under standard conditions

Question 8

What is the definition of Enthalpy Change?

Answer 8

Heat change in a reaction at constant pressure.

Question 9

What is Average Bond Enthalpy?

Answer 9

Average energy required to break one mole of a particular type between two atoms in a gaseous state.