4. Thermochemical reactions

Question 1

A thermochemical equation includes a change in what?

Answer 1

Enthalpy term.

When we include the amount of heat produced or absorbed by a reaction, often referred to as the change in enthalpy of the reaction (ΔH), in a chemical equation we have created a thermochemical equation.

As with other chemical equations, the number of atoms on each side of the equation should still balance.

Question 2

The change in enthalpy associated with a thermochemical equation must include a sign that is either what?

Answer 2

Positive or negative.

Question 3

A positive change in enthalpy (e.g. ΔH=+20kJ mol−1) is associated with what type of reaction?

Answer 3

A positive change in enthalpy (e.g. ΔH=+20kJ mol−1) is associated with an endothermic reaction while a negative change in enthalpy (e.g. ΔH=−78kJ mol−1) is associated with an exothermic reaction.

Question 4

We can identify the relative stability of the reactants and products in a reaction from what?

Answer 4

The thermochemical equation.

The change in enthalpy term in a thermochemical equation will classify the reaction as either exothermic or endothermic. In endothermic reactions the change in enthalpy is positive and the reactants are relatively more stable than the products.

Question 5

The enthalpy for a general reaction (H) is measured in what?

Answer 5

Kilojoules per mole (kJ mol−1).

Question 6

We measure solids in mass. What do we measure gases in?

Answer 6

Volume.