Energetics (T4)

Question 1

What is enthalpy and enthalpy change?

Answer 1

Enthalpy change is the term used to describe the energy exchange that takes place with the surroundings at a constant pressure and is given the symbol ΔH.

Enthalpy is the total energy content of the reacting materials. It is given the symbol, H.

ΔH = ΔH products - ΔH reactants

The units are kilojoules per mole (kJmol-1)

An exothermic enthalpy change is always given a negative value, as energy is lost to the surroundings.

ΔH = -xkJmol-1

An endothermic enthalpy change is always given a positive value, as the energy is gained by the system from the surroundings.

ΔH = + ykJmol-1.

Question 2

When the bonds between atoms are being made, energy is released and most of these reactions are..

Answer 2

Exothermic

Question 3

When bonds between atoms are being broken, heat is required so these are..

Answer 3

Endothermic

Question 4

What is the formula for ‘heat given out’?

Answer 4

Heat given out = mass x specific heat x temperature rise

Question 5

Heat given out = mass x specific heat x temperature rise

In the above formula, what is the specific heat referring to?

Answer 5

The amount of heat needed to raise the temperature of 1 gram of a substance by 1 degree centigrade