4 Enthalpy change

Question 1

Define enthalpy change

Answer 1

The heat energy change measured under conditions of constant pressure

Question 2

What units is enthalpy change (ΔH) measured in?

Answer 2

kJ mol^-1

Question 3

What are the standard conditions?

Answer 3

• 298K (25°C)
• 100kPa
• solutions at 1 moldm^-1

Question 4

Reactions can be ____thermic or _____thermic

Answer 4

Exothermic or endothermic

Question 5

What happens in an exothermic reaction?

Answer 5

Energy is transferred from the chemicals to the surroundings

- enthalpy change is negative

Question 6

In exothermic reactions, do the products have more or less energy than the reactants?

Answer 6

Less energy

Question 7

What are some examples of exothermic reactions?

Answer 7

• combustion
• neutralisation

Question 8

What happens in an endothermic reaction?

Answer 8

Energy is transferred from the surroundings to the chemicals

- enthalpy change is positive

Question 9

In endothermic reactions, do the products have more or less energy than the reactants?

Answer 9

More energy

Question 10

What are some examples of endothermic reactions?

Answer 10

• thermal decomposition

Question 11

Define standard enthalpy of formation

Answer 11

The enthalpy change when 1 mole of a substance is formed from its elements under standard conditions, all reactants and products being in their standard states

Question 12

Formation example - magnesium chloride

Answer 12

Mg _(s) + Cl_{2 (g)} → MgCl_{2 (s)}

Question 13

Define standard enthalpy of combustion

Answer 13

The enthalpy change when 1 mole of substance is combusted completely in oxygen under standard conditions, all reactants and products being in their standard states

Question 14

Combustion example - methane

Answer 14

CH_{4 (g)} + 2O_{2 (g)}→ CO_{2 (g)} + 2H₂O _(l)

Question 15

What does incomplete combustion produce?

Answer 15

• soot (carbon)
• carbon monoxide
• water