enthalpy

Question 1

how to do enthalpy of combustion experiment

Answer 1

1. set up the apparatus: A calorimeter with a thermometer and 100cm3 water in it, on a clamp and stand above a spirit burner with ethanol in it
2. Use a balance to measure the starting mass of the spirit burner and the fuel (ethanol)
3. Use a thermometer to measure the start temperature of the water
4. Remove the cap and immediately (So fuel doesn’t evaporate) light the wick
5. Stir the water with thermometer to make sure the thermal energy is distributed
6. after 5 mins replace the cap and use the thermometer to read the final temperature of the water
7. Use the balance to measure the final mass of the spirit burner and fuel
8. Subtract final mass from start mass to calculate mass of fuel combusted

Question 2

How to calculate standard enthalpy change of combustion

Answer 2

To calculate thermal energy transferred into the water use q=mc/\T and divide by 1000 to get the answer in KJ
find the number of moles of ethanol that reacted = mass/RFM
then /\cH = q/moles and make the answer a negative as combustion is exothermic

Question 3

How to calculate standard enthalpy change of reaction in KJ mol-1

Answer 3

To calculate thermal energy transferred into the solution use q=mc/\T where m = vol and divide by 1000 to get the answer in KJ
find the number of moles of solid that reacted = mass/RFM
then /\rH = q/moles and make the answer a negative as its exothermic

Question 4

how to do enthalpy of reaction experiment

Answer 4

1. pour 100cm3 of CuSO4 solution in a polystyrene cup in a beaker with a thermometer
2. to make sure the solution is at room temp (standard state), measure the solution every 30s until we reach a constant
3. add the solid and stir with thermometer
4. take temp readings every 30secs for 5min and draw the graph extending the cooling line till we reach the time we added the solid.
5. work out change in temp and calculate the standard enthalpy change of reaction

Question 5

what is the importance of a polystyrene cup

Answer 5

Polystyrene is an insulator so it reduces heat lost to surroundings so we can measure the temperature more accurately

Question 6

What is the result if we use two times the mass/volume of solution?

Answer 6

The temperature change would be double but change in entropy is the same because twice the energy would be used
The ratio of energy produced to the number of moles is the same