R.P 2 (Measurement of enthalpy change)

Question 1

How do we calculate enthalpy change
Write the 2 equations

Answer 1

Q = mcΔT
Q - J
m - g
c - J g-1K-1
ΔT - K (or degrees doesn’t matter as change will be same)

Then

ΔH = -q/n

Question 2

Outline the stages in the method of measuring enthalpy change using polystyrene cup

Answer 2

 washes the equipment (cup and pipettes etc) with the solutions to be used

 dry the cup after washing

 put polystyrene cup in a beaker for insulation and support  Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup

 clamp thermometer into place making sure the thermometer bulb is immersed in solution

 measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 3 minutes

 At minute 4 transfer second reagent to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance.

 If using a solid reagent then use ‘before and after’ weighing method

 stirs mixture (ensures that all of the solution is at the same temperature)

 Record temperature every minute after addition for several minutes

Question 3

When plotting a graph, why do we take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together

Answer 3

Because If the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction

Question 4

State 3 sources of error in the method of measuring enthalpy change using polystyrene cup

Answer 4

Density of solution is taken to be the same as water

HEAT energy loss to surroundings (OR HEAT ENERGY GAIN for the endothermic reaction)

Approximation in specific heat capacity of solution. The method assumes all solutions have the heat capacity of water.

Question 5

When using calorimetry to measure enthalpy of combustion, what do we need to measure

Answer 5

Mass of spirit burner before and after
Temperature change of water
Volume of water in cup

Question 6

State 4 sources of error in the method of measuring enthalpy change using polystyrene cup/calorimetry

Answer 6

Incomplete combustion of fuel
Evaporation of fuel after weighing
Heat capacity of calorimeter not included (energy to heat up calorimeter)
Energy losses to surroundings

Question 7

What does the exothermic graph look like for this practical, what is on the axis?

Answer 7

Question 8

What does the endothermic graph look like for this practical, what is on the axis?

Answer 8

Question 9

Out of the spirit burner practical, and the 2 polystyrene cup practicals with CuSO4, which is exothermic and which is endothermic

Answer 9

Spirit burner is combustion which is always exothermic

For hydrated salts like CuSO₄·5H₂O, adding water is endothermic, but for anhydrous CuSO₄, dissolving/(hydrating) it is exothermic.

Question 10

Why is dissolving hydrated CuSO4 endothermic

Answer 10

Dissolving hydrated CuSO₄ is endothermic because the energy needed to break the ionic lattice and release the water of crystallisation is greater than the energy released from hydrating the free ions

Question 11

What colour is hydrated vs anhydrous CuSO4

Answer 11

Hydrated: Blue crystals
Anhydrous: White powder