energetics i

Enthalpy change

The heat energy change in a reaction at constant pressure, units are kj mol -1

Standard conditions- 100 kpa, 298 k

Why do you need standard conditions

Enthaply affected by temp and pressure

Standard enthalpy change of reaction

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions

Standard enthalpy change of formation

is the enthalpy change when 1 mol of compound is formed from its element in their standard states under standard conditions

Standard enthalpy change of combustion

1 mole of substance burned completely in oxygen under standard conditions

Standard enthalpy change of neutralisation

When an acid and alkali react together under standard conditions to form 1 mole of water

Experiment to measure temp change

Just mixing together = heat loss to surroudings

1. Set volume and conc of citric acid in polystyrene cup

2. Put lid on beaker and measure temp every 30 s until stabilised

3. Add set mass of sodium bicarbonate and stir

4. Measure temp

5. Graph, extrapolate

Calorimeter

Combustion, as fuel burns it heats water

Enthalpy change formula

Q = mc delta t

Heat loss to surroundings and can

Breaking bonds

Endothermic

Making bonds

Exothermic

Bond enthaply

The amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase

Why are mean bond enthalpy are not exact

Mean, include range for all other compounds