S4 - Measuring enthalpy changes

Question 1

How can you measure enthalpy change?

Answer 1

By using calorimetry

Question 2

What does calorimetry allow you to measure?

Answer 2

The amount of energy transferred in a chemical reaction

Question 3

What reactions does calorimetry measure?

Answer 3

• Dissolving
• Displacement
• Neutralisation

Question 4

How do you measure the energy transferred in these reactions?

Answer 4

• Take the temperature of the reagents (making sure they are the same)
• Mix them
• Measure the temperature of the solution at the end of the reaction

Question 5

What can you put the reagents into when carrying out the experiment?

Answer 5

A polystyrene cup (insulator)

Question 6

Why should the polystyrene cup be put in a beaker of cotton?

Answer 6

Gives more insulation as lots of energy can be lost to the surrounds

• Also a lid can be put on the cup to reduce energy lost by evaporation

Question 7

How can you measure combustion using calorimetry?

Answer 7

Burn a fuel and use the flame to heat up some water - this method uses a metal container, usually copper, as it conducts heat well.

Question 8

What is the method for working out the enthalpy change in combustion using calorimetry

Answer 8

• Put 50g of water in the copper container and record the temperature
• Weigh the spirit burner and lid
• Put the spirit burner underneath the container and light the wick
• Heat the water stirring constantly until the temperature reaches about 50 degrees
• Put out the flame and measure the final temperature
• Calculate the enthalpy change from the measurements

Question 9

What does the combustion experiment involve?

Answer 9

Heating water by burning a liquid fuel

Question 10

What do you need to measure to get how much energy is supplied by each gram of fuel?

Answer 10

• How much fuel you’ve burned

- Temperature change of the water

Question 11

What is specific heat capacity?

Answer 11

The amount of energy needed to raise the temperature of 1 gram of water by 1 degrees in water

Question 12

What is the specific heat capacity for water?

Answer 12

4.2 J/g/degrees - so it takes 4.2 joules to raise the temperature of 1g of water by 1 degrees

Question 13

Work out the mass of methylated spirit burnt using the following figures:

Mass of spirit burner + lid before heating = 68.75g
Mass of spirit burner + lid after heating = 67.85g

Answer 13

68.75 - 67.85 = 0.90g

Question 14

Work out the temperature rise of 50g of water due to heating with the following figures:

Temp of water in copper before heating = 21.5 degrees
Temp of water in copper after heating = 52.5 degrees

Answer 14

52.5 - 21.5 = 31.0 degrees

Question 15

Work out the total energy produced in this experiment

(knowing that 0.90g of fuel produces enough energy to heat up 50g of water by 31 degrees and it takes 4.2 joules of energy to heat up 1g of water by 1 degrees)

Answer 15

4.2 x 50 x 31 = 6510 joules

Question 16

So if 0.9 of methylated produces 6510 joules of energy, how much does 1g of methylated produced?

Answer 16

6510/0.9 = 7233J or 7.233 KJ

Question 17

How do you work out the energy per mole of a substance?

Answer 17

• Calculate the amount of energy transferred
• Work out how many moles of fuel produced the heat (moles=mass/mr)
• Work out the heat produced by 1 mole

Question 18

Work out the energy per mole of methylated spirit

Total energy produced = 6.510 kj
Mr = 44.6
Mass of meths burnt = 0.90g

Answer 18

Moles = 0.90/44.6 = 0.020 moles

-6.510/0.020 = -325.5 kj/mol

Question 19

Why does the total energy produced in the methylated spirit turn into a negative number?

Answer 19

Combustion is a exothermic reaction