Measuring enthalpy

Question 1

A typical CALORIMETER ( A plastic cup made of polystyrene)

Reaction ~ Zinc added to CuSO4(aq)

Answer 1

SURROUNDINGS ~ Water in the CuSO4(aq)

SYSTEM ~ Reacting particles ~ Zn(s) and Cu2+(aq)

• Heat is exchanged from the reacting particles and the solution.
• The temperature change is measured using a THERMOMETER
• HEAT changes are measured using a CALORIMETER

Question 2

STEPS for calculating:
MOLAR ENTHALPY CHANGE OF REACTION

Answer 2

1. Write EQUATION for the reaction
2. Calculate HEAT TRANSFER of the :
   - surroundings
   - system
   heat lost from SYSTEM = heat gained by surroundings
   ( q surr = NEGATIVE q sys)
3. Calculate the MOLES of the reagent that is NOT in excess.
4. Find the MOLAR ENTHALPY CHANGE OF REACTION using equation:

Heat transferred from SYSTEM/ number of moles of reagent NOT in excess

Question 3

HEAT TRANSFER equation

Answer 3

q = mc TRIANGLE T

q = heat transfer (J)

m = mass of solution (g)

c = specific heat capacity ( Jg-1K-1)

TRIANGLE T = change in temperature

Question 4

ASSUMPTIONS made when calculating heat transfer

Answer 4

• DENSITY of all solutions is 1gcm-3
  ( mass in grams = volume in cm3)
• SPECIFIC HEAT CAPACITY of all solutions is the same as that of water ( c= 4.18Jg-1K-1)

Question 5

Measuring Enthalpy changes taking place in a solution

Answer 5

• As soon as the reactants are MIXED , HEAT is RELEASED
• The maximum temperature reached is NOT the TRUE MAXIMUM
• Heat has been LOST before the reading is taken

COOLING CURVE CORRECTION:

STRENGTH ~ allows the temperature to be read at times when it was not possible to take measurements

WEAKNESS ~ the exact maximum temperature may be unknown as it may be between two readings.

Question 6

Calculating Molar enthalpy change of COMBUSTION

Answer 6

• Calculate the heat transferred to the SURROUNDINGS
• Calculate the heat transferred from the SYSTEM
• Calculate MOLES of FUEL
• Calculate the molar enthalpy change of COMBUSTION :

TRIANGLEcH = heat loss from system (J) / mole of fuel

Question 7

Why is the EXPERIMENTAL enthalpy change of combustion LESS EXOTHERMIC than the ACTUAL value from data book?

Answer 7

The apparatus using a SPIRIT BURNER leads to LESS heat transferred to the water than expected because:

HEAT LOSS TO THE SURROUNDINGS:
- Other than the water i.e the beaker, air around the flame and top of the water

INCOMPLETE COMBUSTION:
- Of ethanol where CO and C are produced instead of CO2
- A black layer of SOOT would be produced on the beaker if carbon is being produced.

EVAPORATION:
- Of ethanol from the wick
- The spirit burner must be weighed as soon as possible after extinguishing the flame
- Or the wick is immediately covered with a CAP

Question 8

Why is a BOMB CALORIMETER more accurate at measuring enthalpy change:

Answer 8

REDUCES HEAT LOSS to the surroundings because:

COMPLETE COMBUSTION:
- Fuel is burnt in pure oxygen

INSULATION:
- Insulating the water reduces heat loss to surroundings

USING A STIRRER:
- Allows heat to be distributed uniformly & quickly
- More accurate temperature readings can be taken

Question 9

Standard enthalpy change of NEUTRALISATION

Answer 9

The enthalpy change when ONE MOLE OF WATER (l) is formed …

When an AQUEOUS ACID is NEUTRALISED…

With an AQUEOUS BASE…

Under STANDARD CONDITIONS (298K and 100 KPa)

Question 10

VALUE for standard enthalpy change of neutralisation

Answer 10

-57.5 kJ mol-1

Question 11

Which piece of APPARATUS is required to work out the enthalpy change of NEUTRALISATION

Answer 11

CALORIMETER:

• A plastic cup made of polystyrene
• Heat is exchanged from the reacting particles and the solution
• The temperature change is measured using a thermometer

Question 12

How to CALCULATE the molar enthalpy change of NEUTRALISATION

Answer 12

1) Write an EQUATION for the reaction

2) Calculate the HEAT TRANSFER to the SURROUNDINGS

3) Calculate heat lost from SYSTEM ( qsurr = qsys)

4) Calculate MOLES of reagents

5) Deduce the MOLES of WATER produced in the reaction using MOLAR RATIO

6) Calculate the molar enthalpy change of neutralisation using:

         q sys / moles of water