To determine the heat of reaction of hydrochloric acid with sodium hydroxide

Question 1

Why are liquids mixed in an insulated container

Answer 1

To prevent heat loss to surroundings

Question 2

Why is it important that heat is not lost to the surroundings

Answer 2

We wish to ensure all the heat produced in the reaction is used to raise the temperature of the solution

Question 3

Why is it important that both solutions should be at the same temperature before they are mixed?

Answer 3

Since the calculations involve measuring the rise in temperature of the final solution, it is important that both solutions must be at the same temperature in order to measure the rise of temperature accurately

Question 4

When adding the base to the acid care should be taken to avoid splashing and explain why it is important

Answer 4

Splashing would give rise to loss of liquid and hence cause an inaccuracy in the calculations

Question 5

What is the purpose of placing a lid on the plastic cup?

Answer 5

Prevents loss of heat to the surroundings

Question 6

Why is the maximum temperature recorded?

Answer 6

In order to accurately calculate the heat liberated if it is not recorded then the value for temperature rise will be incorrect.

Question 7

Why is the mixture of liquid stirred continuosly?

Answer 7

To ensure that the heat liberated is uniformly distributed throughout the solution i.e to make sure that the temperature measured does not depend on the position of the thermometer in the solution

Question 8

How is the heat liberated calculated?

Answer 8

Mass of solution x specific heat capacity x temperature rise

Question 9

Why is a plastic container particularly suitable for use in the experiment?

Answer 9

Plastic absorbs very little heat i.e specific heat capacity of plastic is negligible

Question 10

When carrying out the calculations associated with this experiment, what assumptions are made about the density of the solutions and the density of water and why is it valid?

Answer 10

Density of solutions is the same as the density of water and it is valid as the solutions used are not highly concentrated

Question 11

List the sources of experimental error in this experiment

Answer 11

Splashing
Mixing solutions which are not at the same initial temperature
Forgetting to wash the thermometre after measuring the temperature of the acid and before measuring the base
Not measuring the maximum temperature rise
Forgetting to place a lid on the final mixture

Question 12

Would you expect a similar result if nitric acid were used instead of HCL?

Answer 12

Yes, a similar result would be obtained since nitric acid is similar to HCl as both are strong acids i.e both dissociate in water

Question 13

Why does the heat of neutralisation of a strong acid and strong base always have a constant value?

Answer 13

Because the same reaction is taking place in all cases i.e H+ +OH- -> H2O
The other ions are ‘spectator ions’ and are not involved in the reaction

Question 14

Define heat of reaction

Answer 14

Heat change when the numbers of moles of reactants in the balanced equation react completely

Question 15

State one advantage and one disadvantage of the use of a burette instead of a graduated cylinder for measuring out the base and adding it to the reaction vessel

Answer 15

Adv - More accurate (delivers measured volume)

Disadv- Slow addition and difficult to measure 75cm^3 (you would need two burettes)

Question 16

State two ways to ensure that the rise in temperature was measured as accurately as possible

Answer 16

Avoid splashing
Accurate reading of thermometer (reading to .2C)
Stir continously

Question 17

How many moles of HCl were neutralised in the reaction with NaOh (1.0M in 75cm^3)

Answer 17

1.0 / 1000 x 75 = .075M

Question 18

Calculate the heat produced in the reaction vessel as a result of the HCl with the NaOH. Take the density and the specific heat capacity of the reaction mixture as 1.0g/cm^3 and 4.2Kjkg/K respectively. (75cm^3 of HCl and NaOh each.
Temperatures - HCl (13) NaOh (15) with highest temp (20.9)

Answer 18

1) MASS -> 75 + 75 = 150cm3 / 1000 = .15kg
2) TEMP RISE -> 13+15/2 = 14
20.9 - 14 = 6.9K
3) HEAT PRODUCED
.15 x 4.2 x 6.9 = 4.347kJ

Question 19

Calculate the heat of reaction for neutralisation reaction between HCl and NaOh with heat produced being 4.347 and no.of moles of HCl being .075M

Answer 19

Heat produced divided by no.of moles

4.347 / .075 = 57.96

Question 20

The solutions used in this experiment were moderately concentrated. Identify the hazard associated with the use of these solutions

Answer 20

Corrosive

Question 21

What experimental problem would have been encountered if .1M NaOH and .1M HCl solutions had been used instead of 1.0M solutions

Answer 21

Small temperature rise and therefore harder to measure

Question 22

The expected result is -57KJmol/l but the result is lower, why?

Answer 22

Specific heat capacity of polystyrene calorimeter is not zero
There is loss of heat to the surroundings
The densities of the acid and base are not exactly equal to the density of water

Question 23

Give the procedure

Answer 23

Use two separate graduated cylinders, add HCl to one and place into one polystyrene cup and do the same with sodium hydroxide in a separate cup
Measure the temp of the HCL solution
Using a second thermometer measure temp of sodium hydroxide
When both solutions are at same temperature, quickly add base to the acid, stirring well and take care to avoid any loss of liquid
Place lid on polystyrene cup and with continuous swirling record the max temperature reached

Question 24

State one precaution the student should take when using these solutions

Answer 24

Avoid contact with eyes and skin
Use protective clothing [gloves, lab coat]
Use eye protection [glasses]

Question 25

If the HCL and NaOH solutions had been stored at slightly different temperatures, explain how the initial temperature could have been obtained

Answer 25

• get average temperature of both solutions

- wait until both are at same temperature [room temp]

Question 26

List three precautions which should have been taken in order to obtain an accurate value for the highest temperature reached by the reaction mixture

Answer 26

• thermometer to 0.2 oC
• add without splashing
• replace cover quickly (immediately)
• stir constantly

Question 27

What was the advantage of mixing the solutions in a polystyrene foam cup rather than a glass beaker or in a metal calorimeter

Answer 27

Good insulator -> doesn’t lose heat easily to environment

Question 28

Taking the total heat capacity of the reaction mixture to be 420JK, calculate the heat released if the temp rise was recorded to be 6.7 and hence calculate the heat of reaction

Answer 28

420 x 6.7 = 2814

2814 / no.moles [.05] = -56280

Question 29

Name the piece of apparatus used in industry to accurately measure the heats of combustion of foods and fuels

Answer 29

Bomb Calorimetre