required practical 2

Question 1

what is the aim of this practical?

Answer 1

to measure enthalpy changes

Question 2

what is the method for this practical (combustion of alcohols)?

Answer 2

1. measure out 100cm3 of water with a measuring cylinder and pour into a calorimeter (copper can)
2. measure the initial mass of the spirit burner containing the first alcohol with a mass balance
3. position the calorimeter directly above the spirit burner with a clamp and boss
4. measure the initial temperature of the water with a thermometer
5. remove the cap of the spirit burner and immediately light the wick - heat until the water temperature increases by 30 degrees
6. after replacing the cap on the spirit burner, immediately measure the new temperature of the water
7. measure the new mass of the spirit burner
8. calculate the temperature increase of the water and mass of fuel which was burnt
9. repeat for the other alcohols
10. calculate the enthalpy changes

Question 3

how do you calculate the enthalpy change of combustion for each alcohol?

Answer 3

first do q = mc ∆ T (all values in terms of water) to find the change in heat energy
then calculate how many moles of alcohol burnt using the value for mass that burnt
finally find the enthalpy change by doing ∆ H = q/ moles x 1000

Question 4

what is the mass of water used the same as?

Answer 4

the volume of water used

Question 5

what sources of error may cause your calculated enthalpy change to be lower than the actual standard values?

Answer 5

1. while the spirit burner is uncapped but not lit, fuel will evaporate (seems like more fuel burnt than actually did)
2. not all the heat energy released by the fuel is transferred to the water
3. the fuel doesn’t undergo complete combustion (less energy released in incomplete)
4. experiment not done under standard conditions

Question 6

what is the method for this practical (enthalpy of neutralisation of sodium hydrogencarbonate)?

Answer 6

1. use a measuring cylinder to transfer 30cm^3 of HCl to a polystyrene cup, and place the cup in a beaker
2. place a thermometer thorough the lid of the cup
3. use a balance to weigh 2g of sodium hydrogencarbonate into a weighing boat
4. measure the temperature of the HCl to 1dp
5. stir the HCl, measured and record the temperature and start a timer
6. continue stirring, measuring and recording the temperature at the first, second and third minutes
7. at the fourth minute, add the sodium hydrogencarbonate - stir gently but don’t measure the temperature
8. stir gently and continuously for the rest of the experiment, measuring the temperature at the fifth, sixth, seventh, eight, ninth and tenth minutes

Question 7

what is the method for this practical (thermal decomposition of sodium hydrgoencarbonate)?

Answer 7

1. weigh a clean, dry crucible and record the mass
2. measure and add the sodium hydrogencarbonate and record the total mass of the crucible and its contents
3. set up a tripod and bunsen burner on a heatproof mat, and place a pipe-clay triangle on the heatproof mat
4. heat the crucible and contents for 10 mins, then leave to cool for 5 mins
5. use tongs to transfer the crucible and contents to a balance and record the total mass
6. heat the crucible for another 5 mins, then leave to cool for 5 mins, and measure the total mass again
7. repeat step 6 and record the final man of the crucible and contents