Energetics

What is enthalpy?

• When a chemical reaction occurs, there is usually a change in energy.

• The enthalpy change of a reaction is the heat change in a reaction at constant pressure.

Standard Conditions

• 100 kPa

• 298 K or 25 C

Bond Breaking

• Endothermic Process so change in enthalpy is positive.

Bond Making

• Releases energy- exo process

• negative enthalpy change

Why is mean bond enthalpy used?

• Because bonds of the same type require different amounts of energy to break.

• Mean Bond enthalpy= total energy to break- total energy to make

Enthalpy change of formation

• Enthalpy change when one mole of a substance is made from its constituent elements with all reactants and products in standard states under standard conditions.

  C (s) +2H2 (g) → CH4 (g)

• 0 by definition

Enthalpy change of combustion

• Enthalpy change when one mole of a substance is burned completely burned in oxygen with all reactants and products in standard states.

  CH4 (g) + 2O2 (g) → CO2 (g) +2H20 (l)

Enthalpy change of neutralisation

• Enthalpy change when 1 mole of water is formed in a reaction between acid and alkali under standard conditions.

Bond dissociation energy or bond enthalpy

• Enthalpy change when one mole of covalent bonds is broken in the gas phase.

  HCl (aq) + NaOH (aq) → H2O (l) +NaCl (aq)

What is heat?

• The transfer of energy between objects due to a temperature difference (q).

• Always flows from higher-temperature object to lower-temperature object.

What is calorimetry?

• Measurement of heat flow and a calorimeter is used to carry out the experiments.

• You need 3 variables: mass of substance being heated, the specific heat capacity of substance being heated and change in temperature.

• Used to calculate enthalpy change of combustion.

Heat energy formula

q (J) =m (g) c (J per gram per K) x change in temperature (K)

• Volume of liquid also used for mass

Enthalpy change (per mole)

Enthalpy change + q/1000n

Because heat loss can lead to error in results, what techniques used in calorimetry are designed to reduce heat loss

• Reaction is carried out in an insulated beaker

• Flame calorimeters as they ensure the c of the calorimeter as a whole is measured.

Hess’s Law

• The total enthalpy change of a reaction is independent of the route taken.

• Used to work enthalpy changes you can’t find out by doing an experiment.