Required practical 2: Enthalpy change

Question 1

In this experiment, we are finding enthalpy change of
CuSO4 +aq —> CuSO4.5H2O
So make into a hess diagram, with ΔH1, ΔH2, and ΔH3

So what is the first stage of this, getting the copper sulphate prepared

These following flashcards are all to find ΔH1, so making copper (II) sulphate solution

Answer 1

Step 1:
Weigh out 3.9g of anhydrous copper sulphate in a dry, stoppered weighing bottle
Keep the stock of solid in a closed container during weighing so none spills

Make sure you have washed and dried the containers before so there is no previous substance in it

Then construct a results table which will allow you to record temperatures every minute up to 15 minutes

Question 2

Then, use measuring cylinder to pour 25cm^3 of distilled water into a polystyrene cup and record initial temperature
And start timer and measure the temperature of just the water for 3 minutes

Why do we do this

Answer 2

. It allows the water to stand for some time, so they all reach room temperature so a better average temperature change is obtained

. Polystyrene cups are good calorimeters because they are good insulators, place it in a beaker for extra insulation and support

. Use clamp stand to clamp thermometer into place so the bulb is in solution

.Make sure to stir the liquid continuously

Question 3

At the 4th minute, what do we do

Answer 3

. Add the powdered copper sulfate into the polystyrene cup rapidly
. But do not record the temperature for it this minute
. At the 5th minute, continue the temperature readings at minute intervals up to 15 minutes

Stir continuously until these measurements are done

Use a ld on polystyrene cup to help minimise heat loss to surroundings, which will maximise change in temperature

Question 4

Once you have your results, how do you plot them on a graph

Answer 4

Plot graph of temperature against time with temp on y axis
Draw two separate lines: One joining the first 3 points before the addition, and one joining the points after the addition

Then extrapolate both lines to the fourth minute, and whatever the difference is between these two lines is the temperature change at the 4th minute

Question 5

What are some errors of the investigation

Answer 5

. Heat loss to surroundings
. This method assumes that all solutions have the heat capacity of water
. There may be incomplete dissolving of the copper sulfate
. Some water could evaporate
. Room temp can change

Question 6

What are some ways to improve accuracy of the investigation

Answer 6

.Use an electronic temperature senser

. Or use flame colorimeter, where flame is enclosed, fuel burns in pure oxygen instead of air, spiral chimney is made of copper

Question 7

Once we have ΔH1, this gives us copper (II) sulfate solution, but now we need to find ΔH2

Answer 7

Weigh out copper (II) sulphate as before, 6.2g of it into a dry weighing bottle

Measure out 24cm^3 of water in a measuring cylinder, but not 25cm^3 this time!
Do same experiment as last time, finding temperature readings every minute up to 15 mins

Then in your table, have two columns, of temperature every minute, and plot each graph

So find temperature change of ΔH2 at minute 4

Question 8

Once you have the temperature changes for both ΔH1 and ΔH2, what do you do

Answer 8

Use equation q=MCΔT find the enthalpy changes for both ΔH1 and ΔH2

Then plug into the hess law diagram to get the overall enthalpy change

ΔH3 = ΔH1- ΔH2

Question 9

How do you calculate percentage uncertainty with temperature change

How do you reduce it

Answer 9

It is uncertainty divided by temperature change
Then X 100

Well because dividing the uncertainty by a bigger number will give a smaller value, try and get a bigger temp change

Eg by adding more mass, or higher concentration of reactants