Energy changes

Exothermic reactions

A reaction which transfers energy to the surroundings so that the temperature of the surroundings increases.

The energy released from forming new bonds is greater than the energy needed to break existing bonds

ΔH= -kJ/mol

Examples of Exothermic reactions and everyday uses

Combustion, many oxidation reactions and neutralisation reactions.

Everyday uses include self heating cans and handwarmers

Endothermic reactions

A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases.

The energy needed to break existing bonds is greater than the energy released from forming new bonds.

ΔH= +kJ/mol

Examples of endothermic reactions and everyday uses

Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate

Some sports injury packs are based of endothermic reactions

Activation energy

The minimum amount of energy that particles must have to react.

Exothermic energy level diagram

Overall energy change lower

Endothermic reaction profile

Overall energy change is higher

Equation for enthalpy change

ΔH = Σ(bond energies of reactants) - Σ(bond energies of products)

All units in kJ/mol

The difference between the sum of energy needed to break the bonds and the sum of the energy released when bonds in the products formed is the overall energy change.

Equation to determine heat exchange with the surroundings

Q=mcΔT

Q is energy (J)

m is mass of surroundings (kg)

c is specific heat capacity (4.2)

ΔT is change in temperature (*C)

Draw two lines of best fit

Temperature on x-axis, volume of NaOH on y-axis.

Method for neutralisation required practical (1-4, until pouring NaOH)

1. Use 50cm³ measuring cylinder to put 30cm³ dilute hydrochloric acid into the polystyrene cup

2. Stand the cup inside the beaker for stability

3. Measure the starting temperature of the acid using the thermometer and record it

4. Put 5cm³ of sodium hydroxide solution into the 10cm³ measuring cylinder

Method for neutralisation required practical (5-8)

5. Pour sodium hydroxide (NaOH) into the cup. Fit the lid and gently stir the thermometer through the hole. Whem the reading on the thermometer stops changing record the temperature

6. Repeat steps 4 and 5 to add further 5cm³ amounts of NaOH to the cup. A total of 40cm³ needs to be added

7. Repeat steps 1-6 and record the results

8. Calculate the mean maximum temperature reached for each of the NaOH volumes

Risk assessment and apparatus

Safety googles ahould be worn throughout

Apparatus:

• 2M dilute HCl

• 2M NaOH solution

• Expanded polystyrene cup and lid

• 250cm³ beaker

• 10cm³ measuring cylinder

• 50cm