3.1.4 - ENERGETICS

Enthalpy change

The heat energy transferred in a reaction at constant pressure

Symbol: ΔH

Units: kJ mol⁻¹

Standard conditions

Pressure: 100kPa

Temperature: stated in method

What does ΔH^⦵ mean?

Substances in standard states and measurement made under standard conditions

Exothermic reaction

• Give out energy

• Negative ΔH

• Temp increases

Endothermic reaction

• Absorb energy

• Positive ΔH

• Temp decreases

Standard enthalpy of combustion

Enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions

Standard enthalpy of formation

Enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

How do you find the enthalpy of combustion of a liquid?

1. To find enthalpy of combustion of flammable liquid, burn it in calorimeter

2. As fuel burns, it heats water → can calculate heat energy absorbed by water using mass of water, temp change and specific heat capacity

Why are enthalpy of combustion values often inaccurate?

• Heat lost to surroundings

• Some incomplete combustion

• Some fuel lost to evaporation (flammable liquids often volatile)

How do you measure enthalpy changes in solution?

Calorimetry can be used to find enthalpy change for reactions in solutions, such as neutralisation, dissolution (dissolving), displacement

1. To find enthalpy change of neutralisation reaction, add a known volume of acid to insulated container (e.g. polystyrene cup) and measure temp

2. Add known volume of alkali and record temp at regular intervals (stir solution to distribute heat)

3. Find temp change for experiment

How do you use a graph to find temperature change?

1. During experiment, record temp at regular intervals, starting 3 mins before reaction begins

2. Plot graph of results - draw two lines of best fit: one through points before reaction started, and one through points after it started

3. Extend both lines so they both pass the time the reaction started

4. Distance between two lines at time when reaction started = accurate temp change

Formula for enthalpy change

q = mcΔT

What does each symbol in enthalpy equation stand for?

q = heat lost/gained (J)

m = mass of water/other solution (g)

c = specific heat capacity of water (4.18 J g^-1 K^-1)

ΔT = temp change (K)

Hess’s Law

Total enthalpy change of reaction is independent of the route taken

Hess’s Law example

Calculating enthalpy change using enthalpy of formation

• Need to know Δ_fH^⦵ of all reactants + products that are compounds

Hess’s Law equation

Δ_rH^⦵ = sum of Δ_fH^⦵ products - sum of Δ_fH^⦵ reactants

Calculating enthalpy change using enthalpy of combustion

Hess’s Law diagram with backwards arrow example

Bond breaking

• Endothermic - requires energy to break bonds

• ΔH positive

• Stronger bonds take more energy to break

Bond making

• Exothermic - releases energy by breaking bonds

• ΔH negative

• Stronger bonds release more energy when they form

Bond enthalpy

The energy required to break bonds

Mean bond enthalpy

Average energy needed to break a certain type of bond, over a range of compounds

Always positive - breaking bonds = endothermic

Bond enthalpy variation

Energy needed to break a bond depends on its environment → same type of bond may require different amounts of energy to break

Enthalpy change of reaction formula

Enthalpy change of reaction = total energy absorbed - total energy released

Example bond enthalpy calculation