iGCSE Chemistry - Energetics

How can changes in heat content be determined and measured?

With a thermometer.

What is an exothermic reaction?

Releases heat into the surroundings. Temperature of the mixture and surroundings increases.

Two examples of exothermic reactions.

• neutralisation

• combustion

What is an endothermic reaction?

Takes in heat energy from the surroundings. Temperature of the mixture and surroundings decreases

Two examples of endothermic reactions.

• thermal decomposition

• electrolysis

What can calorimetry be used to determine?

The relative amounts of energy released by a fuel.

Which reactions in solution can enthalpy change be measured for?

• Neutralisation

• Dissolving in water

• Displacement

Method: enthalpy of a reaction in solution

1. Add a fixed volume of one reagent; measure initial temp with a thermometer.

2. Add the second reagent in excess; stir the solution continuously.

3. Record the max temperature, then calculate the temperature rise.

State the main calorimetry calculation.

Q = m × c × Δt

heat energy change = mass (g) × specific heat capacity × Δ temp

Method: enthalpy of combustion

1. Measure a fixed volume of water into a copper can.

2. Weigh the spirit burner containing the fuel.

3. Measure the initial temp of the water.

4. Burn the fuel and stir the water.

5. Wait until the temp has risen 20 °C, then extinguish the flame.

6. Record the final temp and re-weigh the spirit burner.

What is the principle of the combustion method?

Using the heat released by the reaction to increase the heat content of the water.

Define specific heat capacity (c)

The energy required to raise the temperature of 1 g of a substance by 1 °C

Specific heat capacity of water?

4.18 J/g/°C

Two sources of error in the Calorimetry method.

Some heat is lost to the surroundings and to the calorimeter.

Therefore, not all the heat is transferred to the water.

Two ways to minimise heat loss

Do not place the copper calorimeter too far above the flame

Use shielding to reduce draughts

Assumptions made about both Calorimetry methods

The s.h.c. is the same as pure water, 4.18 J/g/°C.

Density of the solution = density of water.

The container’s s.h.c. is ignored.

The reaction is complete.

Heat losses are negligible.

What information is needed to calculate heat energy change (Q)?

Mass of the substance being heated (m)

Temperature change (Δt)

Specific heat capacity of the substance (c)

Why calculate a molar enthalpy change?

So we can compare the energy released per gram and per mole for different fuels

Two steps to a molar enthalpy change.

Step 1: Calculate the heat energy released (Q).

Step 2: Calculate the energy released per gram.

Energy released per gram = ?

energy released + mass of fuel burned

Bond breaking — what happens to energy?

Energy must be taken in to break bonds.

Therefore bond breaking is an endothermic process.

Bond making — what happens to energy?

Energy must be released when new bonds form.

Therefore bond making is an exothermic process.

In an exothermic reaction…

• energy released when new bonds form >/< energy taken in to break bonds

• change in energy is — because energy in reactants >/< energy in products

• it has a — ΔH value

• >

• >

• Negative

In an endothermic reaction…

• energy released when new bonds form >/< energy taken in to break existing

• change in energy is — because energy in reactants >/< energy in products

• it has a — ΔH value

• <

• <

• Positive

State the bond energies calculation

enthalpy change (ΔH) = energy taken in − energy released