Required Practical 2

Question 1

Describe a calorimetric method.

Answer 1

• One type of experiment is one in which substances are mixed in an insulated container and the temperature rise is measured.
• This could be a solid dissolving or reacting in a solution or it could be two solutions reacting together.

METHOD

1. Wash the equipment (cup and pipettes etc.) with the solutions to be used and dry the cup after washing.
2. Put polystyrene cup in a beaker for insulation and support.
3. Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup.
4. Clamp the thermometer into place making sure the thermometer bulb is immersed in solution.
5. Measure the initial temperature of the solution or both solutions if 2 are used for about 2-3 minutes.
6. At minute 3, transfer second reagent into cup. If using solid reagent, then use ‘before and after’ weighing method.
7. Stir mixture to ensure that all of the solution is at the same temperature.
8. Record temperature every minute after addition of reagent for several minutes.

Question 2

How would you use the results from a calorimetric method in order to calculate a value for temperature change (ΔT)?

Answer 2

1. On a graph, plot temperature (in °C) on the y-axis and time (in minutes) on the x-axis.
2. Draw two separate best-fit lines; one, which joins the points before the addition, and one, which joins the points after the addition, extrapolating both lines to the time at which you added the reagent.
3. Calculate the difference in temperature to determine value for ΔT.

(If two solutions are used, calculate an average temperature)

Question 3

What possible errors could there be in a calorimetric method?

Answer 3

ERRORS

• Energy transfer from the surroundings (usually loss).
• Approximation in specific heat capacity of solution.
  
  • The method assumes all solutions have the heat capacity of water.
• Neglecting the heat capacity of the calorimeter.
  
  • We ignore any energy absorbed by the apparatus.
• Reaction or dissolving may be incomplete or slow.
• Density of solution is taken to be the same as water.

Question 4

Enthalpies of combustion can be calculated using calorimetry. Generally, the fuel is burnt and the flame is used to heat up water in a metal cup.

What should you measure in this experiment?

What possible errors could there be in this method?

Answer 4

NEED TO MEASURE…

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

ERRORS

• Energy losses from the calorimeter.
• Incomplete combustion of fuel.
• Incomplete transfer of energy.
• Evaporation of fuel after weighing.
• Heat capacity of calorimeter not included.
• Measurements not carried out under standard conditions as H₂O is gas not liquid, in this experiment.

Question 5

Why will results worked out experimentally using a calorimeter be much lower than calculated ones?

Answer 5

• There will be significant heat loss.
• There will also be incomplete combustion which will lead to less energy being released.

Question 6

Describe a general method to calculate the enthalpy change of a reaction, ΔH, from experimental data.

Answer 6

1. Using q = mcΔT calculate the energy change for quantities used.
   
   • The heat capacity of water is 4.18 J g⁻¹K⁻¹. In any reaction where the reactants are dissolved in water, we assume that the heat capacity is the same as pure water.
   • Also assume that solutions have the density of water which is 1g cm⁻³ e.g. 25cm³ will weigh 25g.
2. Work out the moles of the reactants used.
3. Divide q by the number of moles of the reactant not in excess (i.e. the limiting reagent) to give ΔH.
4. Add a sign and unit (divide by a thousand to convert Jmol⁻¹ to KJmol⁻¹).

Question 7

Answer 7

Question 8

Answer 8

Question 9

Answer 9