chemistry 8.2 - more on enthalpy changes

What do you need to know to work out enthalpy change?

The temperature change to work out the heat energy change and the moles (energy/moles)

What is heat change (q)?

The enthalpy change if reactions carried out and constant pressure

What are the steps to find the temperature change of the reaction between citric acid and sodium bicarbonate (endothermic)?

1. Add a set volume of a know concentration of citric acid to polystyrene cup

2. Put lid on cup and measure temp every 30 seconds until temperature has stabilised

3. Add a set mass of sodium bicarbonate and stir

4. Measure the temperature every 30 seconds until a minimum has been returning to the initial temperature for a few minutes

5. Draw a graph of temperature against time

6. To account for heat being gained from the surroundings extrapolate a line from where the reaction started to return to the initial temperature back toward the time the reaction started

7. Work out the temperature change using the minimum temperature worked out form the extrapolated line

What is calorimetry?

Measuring heat changes

What makes it harder to work out the the temperature change?

Gas reactants

What is the equation to work out heat energy?

Heat energy (q) = mass of water (m) x specific heat capacity of water (c ) x temperature change (ΔT)

J = g x Jg^-1K^-1 x K

What is the specific heat capacity of water?

4.18

What are the steps to work out the enthalpy of combustion of a flammable liquid?

1. Set up equipment so fuel is below water and light the fuel

2. As the fuel burns it will heat the water and the heat absorbed by water roughly = energy released by fuel

3. Use q=mcΔT