Measuring enthalpy Flashcards
A typical CALORIMETER ( A plastic cup made of polystyrene)
Reaction ~ Zinc added to CuSO4(aq)
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
STEPS for calculating:
MOLAR ENTHALPY CHANGE OF REACTION
- Write EQUATION for the reaction
- Calculate HEAT TRANSFER of the :
- surroundings
- system
heat lost from SYSTEM = heat gained by surroundings
( q surr = NEGATIVE q sys) - Calculate the MOLES of the reagent that is NOT in excess.
- Find the MOLAR ENTHALPY CHANGE OF REACTION using equation:
Heat transferred from SYSTEM/ number of moles of reagent NOT in excess
HEAT TRANSFER equation
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
ASSUMPTIONS made when calculating heat transfer
- 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)
Measuring Enthalpy changes taking place in a solution
- 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.
Calculating Molar enthalpy change of COMBUSTION
- 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
Why is the EXPERIMENTAL enthalpy change of combustion is LESS EXOTHERMIC than the ACTUAL value from data book?
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
Why is a BOMB CALORIMETER more accurate at measuring enthalpy change:
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
Standard enthalpy change of NEUTRALISATION
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)
VALUE for standard enthalpy change of neutralisation
-57.5 kJ mol-1
Which piece of APPARATUS is required to work out the enthalpy change of NEUTALISATION
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
How to CALCULATE the molar enthalpy change of NEUTRALISATION
1) Write an EQUATION for the reaction
2) Calculate the HEAT TRANSFER to the SURROUNDINGS
3) Calculate heat lost from SYSTEM ( qsurr = qsys)
4) Deduce the MOLES of WATER produced in the reaction
5) Calculate the molar enthalpy change of neutralisation using:
q sys / moles of water
