9.2 measuring enthalpy changes

Question 1

how many celsius is equivalent to 0 K?
what temperature does ice melt at (0 °C) in the Kelvin scale?
what temperature does water boil at (100 °C) in the Kelvin scale?

Answer 1

= -273 °C
= 273 K
= 373 K

Question 2

calculating an energy change formula

Answer 2

q = mcΔT
q = heat energy (J)
m = mass of the surroundings (g)
c = specific heat capacity (4.18 J g-1K-1)
ΔT = temperature change of the surroundings (K)

Question 3

specific heat capacity

Answer 3

is the energy required to raise the temperature of 1g of a substance by 1K

Question 4

to convert from Kelvin and Celsius:
room temp in Kelvin:

Answer 4

= add 273
= 298 K

Question 5

determination of an enthalpy change of combustion ΔcH using spirit burners.

the equation for the combustion of methanol is shown below:
CH3OH(l) + 1.5O2(g) –> CO2(g) + 2H2O(l)

Answer 5

1. measure out 150 cm3 of water and pour into a beaker. record initial temp to nearest 0.5°C
2. add methanol to the spirit burner, weigh the spirit burner containing methanol
3. place spirit burner under the beaker (which is attached to a clamp stand). light the burner and stir the water using the thermometer
4. extinguish the flame after 3 mins. immediately record the maximum temperature reached by the water
5. re-weigh the spirit burner, assume the wick has not been burnt

Question 6

determination of an enthalpy change of combustion ΔcH using spirit burners.
analysis part:
mass of spirit burner and methanol before burning = 196.97g
mass of spirit burner and methanol after burning = 195.37g
mass of fuel burnt = 1.60g
initial temp of water = 21.5°C
final temp of water = 62.5°C
ΔT = 41.0°C
for water, density = 1.00 gcm-3, c = 4.18 Jg-1K-1

Answer 6

calculation:
1.calculate the energy change of the water in kJ
q = 150 x 4.18 x 41.0
q = 25707 J
q = 25.707 kJ
2. calculate the amount, in mol, of CH3OH burnt
1.60g ÷ (12+3+16+1) = 1.60 ÷ 32.0
= 0.0500 mol
3. calculate ΔcH in kJmol-1
in this experiment, 0.05 mol transfers 25.707 kJ of energy to the water
1 mol CH3OH has lost 25.707 ÷ 0.0500 = 514.14 kJmol-1

= 514.14 kJ mol-1

Question 7

how accurate is the experimental ΔcH value?
4 reasons

Answer 7

1. heat loss to the surroundings other than the water
2. incomplete combustion of substance (ie. methanol).
3. evaporation of methanol from the wick
4. non standard conditions
   ALL BUT THE LAST ONE OF THESE REASONS WOULD LEAD TO A VALUE FOR ΔcH THAT IS LESS EXOTHERMIC THAN EXPECTED.

Question 8

how accurate is the experimental ΔcH value?
1. heat loss to the surroundings other than the water

Answer 8

this includes the beaker but mainly the air surrounding the flame

Question 9

how accurate is the experimental ΔcH value?
2. incomplete combustion of substance (ie. methanol).

Answer 9

there may be some incomplete combustion, with CO and C being produced instead of CO2. you would see carbon as a black layer of soot on the beaker

Question 10

how accurate is the experimental ΔcH value?
3. evaporation of methanol from the wick

Answer 10

the burner must be weighed as soon as possible after extinguishing the flame, otherwise some methanol may have evaporated from the wick.
spirit burners usually have a cover to reduce this error

Question 11

how accurate is the experimental ΔcH value?
4. non standard conditions

Answer 11

the date book value is a standard value. the conditions for this experiment are unlikely to be identical to standard conditions

Question 12

how to minimise errors from heat loss and incomplete combustion?

Answer 12

the use of draught screens and an input of oxygen gas

Question 13

determination of an enthalpy change of reaction, ΔrH

Answer 13

• many reactions take place between 2 solutions, or between a solid and a solution.
• the ΔH can be determined using plastic cups made of polystyrene foam
• polystyrene foam is cheap, waterproof, light weight, and offer some insulation against heat loss
• when carrying out reactions between aqueous solutions, the solution is the immediate surroundings, the energy transfer is between the chemical particles and water molecules in the solution

Question 14

determination of ΔrH for a solid and a solution:

an XS of zinc powder is added to 50.0 cm3 of 1.00 mol dm-3 copper(II) sulphate. the mixture is stirred until a maximum temp is obtained

find ΔrH for Zn(s) + CuSO4(aq) –> Cu(s) + ZnSO4(aq)

results:
initial temp of solution = 22.5 °C
final temp of solution = 60.5°C
temp change = 38.0°C

Answer 14

1. calculate the energy change in the solution in kJ
   q = 50.0 x 4.18 x 38.0
   = 7942 J
   = 7.942 kJ
2. calculate the amount in mol of CuSO4 that reacted
   1.00 x (50.0 ÷ 1000) = 0.0500 mol
3. calculate ΔrH in kJ mol-1
   1 mol of CuSO4 has lost
   7.942 ÷ 0.0500 = 159 kJ of energy to the solution

= -159 kJ mol-1

Question 15

determination of an enthalpy change of neutralisation, ΔneutH

a student measures out and mixes 35.0 cm3 of 2.40 mol dm-3 NaOH and 35.0 cm3 of 2.40 mol dm-3 HCl. the temp rises by 16.5 °C

calculate the enthalpy change of neutralisation, in kJ mol-1

Answer 15

1. calc the ΔH in the solution
   total vol = 70.0 cm3
   q = 70.0 x 4.18 x 16.5 = 4827.9 J
   = 4.8279 kJ
2. calc the amount of NaOH and HCl reacted
   n(NaOH) = n(HCl)
   = 2.40 x (35.0 ÷ 1000)
   = 0.0840 mol
3. calc ΔneutH
   formation of 1 mol H2O loses (4.8279 ÷ 0.0840) 57.5 kJ of energy to the solution

= -57.5 kJ mol-1