required practical 2: measurement of an enthalpy change

Question 1

calorimetric method

Answer 1

1. Weigh out between 3.90g and 4.10g of anhydrous copper(II) sulfate in a dry stoppered weighing bottle, keeping the stock of solid in a closed container during weighing. Take care to avoid skin contact. The precise mass should be recorded.
2. Construct a suitable Table of results to allow you to record temperatures at minute intervals up to 15 minutes.
3. Using a measuring cylinder, place 25 cm3 of deionised water into a polystyrene cup and record its temperature at the beginning (t=0), start the timer and then record the temperature again every minute, stirring the liquid continuously.
4. At the fourth minute, add the powdered anhydrous copper(II) sulfate rapidly to the water in the polystyrene cup and continue to stir, but do not record the temperature. At the fifth minute and for every minute up to fifteen minutes, stir and record the temperature of the solution in the polystyrene cup.
5. Plot a graph of temperature (on the y-axis) against time. 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 fourth minute.
6. Use your graph to determine the temperature change at the fourth minute, which theoretically should have occurred immediately on addition of the solid.

Question 2

why do they put the polystyrene cup in a glass beaker?

Answer 2

-for insulation and support

Question 3

why is the mixture stirred every interval?

Answer 3

-ensures that all of the solution is at the same temperature

Question 4

what happens when the reaction is slow?

Answer 4

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

Question 5

how to counteract a slow reaction?

Answer 5

To counteract this we take readings at REGULAR TIME INTERVALS and extrapolate the temperature curve/line back to the time the reactants were added together.

Question 6

why do you take the temperature of the reactants?

Answer 6

We also take the temperature of the reactants for a few minutes before they are added together to get a better average temperature. If the two reactants are solutions then the
temperature of both solutions need to be measured before addition and an average temperature is used.

Question 7

safety

Answer 7

-copper sulphate and it has an environmental warning
-waste will be an issue so solutions should be collected, filtered and allowed to evaporate so that copper suulphate can be recycled

Question 8

Errors in this method

Answer 8

-energy transfer from surroundings (usually loss)
-reaction or dissolving may be incomplete or slow

Question 9

Measuring Enthalpies of Combustion using Flame Calorimetry method

Answer 9

-Enthalpies of combustion can be calculated by using calorimetry
-Generally the fuel is burnt and the flame is used to heat up water in a metal cup.
-Need to measure the mass of spirit burner before and after
-Temperature change of water
-Volume of water in cup

Question 10

Errors in this method

Answer 10

-heat energy losses to surroundings from calorimeter
-Incomplete combustion of fuel
-Incomplete transfer of energy
-Evaporation of fuel after weighing
-Measurements not carried out under standard conditions as H2O is
gas, not liquid, in this experiment