Practical 5 - Finding the enthalpy change for the thermal decomposition of potassium hydrogencarbonate

Question 1

Draw and label the apparatus required for this practical

Answer 1

(See notes)

Question 2

What’s the resolution of the thermometers used?

Answer 2

+-0.1 degrees Celsius

Question 3

What’s the issue with mercury thermometers?

Answer 3

Highly toxic

Question 4

How can we identify a mercury thermometer?

Answer 4

Grey tip on the end

Question 5

What acts as a calorimeter in this practical and what does this mean?

Answer 5

Polystyrene cup
A system for measuring the chemical change in a reaction

Question 6

What acts a a calorimeter in this practical and what does this mean?

Answer 6

Polysterne cup
A system for measuring the chemical change in a reaction

Question 7

What is potassium hydrogencarbonate used as?

Answer 7

A dry chemical fire extinguisher

Question 8

What can potassium hydrogencarbonate thermally decompose to?

Answer 8

Potassium carbonate

Question 9

Why is it important to know the enthalpy change of potassium hydrogencarbonate?

Answer 9

Storage temperatures due to the risk of it thermally decomposing to potassium carbonate

Question 10

How is the enthalpy change calculated during this practical?

Answer 10

Indirectly, via 2 enthalpy experiments and then using Hess’s law

Question 11

Describe the method for this practical

Answer 11

Accurately weigh potassium hydrogen carbonate and the weighing bottle and record the mass
Stand the polystyrene cup in the beaker for stability
Use the burette to add 30cm^3 of 2moldm-3 HCl to the polystyrene cup
Record the temperature of the acid every 30 seconds for 2 minutes
At 2.5 minutes, add the contents of the weighing bottle to the polystyrene cup. Stir carefully with a thermometer and do not record the temperature at this point.
Record the temperature at 3 minutes and again every 30 seconds for a further 4 minutes
Rewrite the weighing bottle and lid and calculate the exact mass of potassium hydrogencarbonate added to the cup and record the value
Wash and dry the cup
Repeat steps 1-8 using potassium carbonate
Plot a graph for each - temperature change is between extrapolated lines when solid is added
(Then can use mc(change in)T etc)

Question 12

What is the change in temperature on a temperature- time graph?

Answer 12

The distance between the extrapolated lines when solid is added

Question 13

When do you re-weigh the weighing bottle and lid?

Answer 13

AFTERWARDS to calculate exactly how much mass was added

Question 14

At which point do we not record the temperature?

Answer 14

At 2.5 minutes, directly following adding the solid to the acid

Question 15

Why is the polystyrene cup in a beaker?

Answer 15

For stability

Question 16

What’s the purpose of taking the tempt=erasure before adding the solid?

Answer 16

This is the initial temperature