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Four fuels undergo complete combustion in excess oxygen, O₂, and the energy released is used to heat 1000 g of water - VCE - SSCE Chemistry - Question 22 - 2018 - Paper 1
Question 22
Four fuels undergo complete combustion in excess oxygen, O₂, and the energy released is used to heat 1000 g of water. Assuming there is no energy lost to the enviro... show full transcript
Worked Solution & Example Answer:Four fuels undergo complete combustion in excess oxygen, O₂, and the energy released is used to heat 1000 g of water - VCE - SSCE Chemistry - Question 22 - 2018 - Paper 1
Step 1
Calculate the energy required to heat the water from 25.0 °C to 85.0 °C
Answer
The energy required (q) can be calculated using the formula:
Where: m = mass of the water (1000 g) c = specific heat capacity of water (4.18 J/g°C) ( riangle T = T_{final} - T_{initial} = 85.0 °C - 25.0 °C = 60.0 °C)
Substituting the values:
Step 2
Determine the combustion energy of each fuel
Answer
The energy released per mole when different fuels combust can be stated as follows:
- Heat of combustion for hydrogen: Approx. 286 kJ/mol
- Heat of combustion for propane (C₃H₈): Approx. 2200 kJ/mol
- Heat of combustion for methane (CH₄): Approx. 890 kJ/mol
- Heat of combustion for methanol (CH₃OH): Approx. 715 kJ/mol
Convert these values to J:
- Hydrogen: 286000 J/mol
- Propane: 2200000 J/mol
- Methane: 890000 J/mol
- Methanol: 715000 J/mol
Step 3
Find out the total energy released by each fuel used
Answer
Using the moles given in the options, we can calculate the energy released by each:
-
Hydrogen (0.889 g): Molar mass = 2.02 g/mol
- Moles = ( \frac{0.889 ext{ g}}{2.02 ext{ g/mol}} \approx 0.44 ext{ mol} )
- Energy released = ( 0.44 ext{ mol} \times 286000 ext{ J/mol} \approx 125840 ext{ J} )
-
Propane (3.95 g): Molar mass = 44.1 g/mol
- Moles = ( \frac{3.95 ext{ g}}{44.1 ext{ g/mol}} \approx 0.09 ext{ mol} )
- Energy released = ( 0.09 ext{ mol} \times 2200000 ext{ J/mol} \approx 198000 ext{ J} )
-
Methane (0.282 mol):
- Energy released = ( 0.282 ext{ mol} \times 890000 ext{ J/mol} \approx 250380 ext{ J} )
-
Methanol (0.301 mol):
- Energy released = ( 0.301 ext{ mol} \times 715000 ext{ J/mol} \approx 215715 ext{ J} )
Step 4
Compare the energies to the required energy
Answer
Now we can compare:
- Hydrogen: 125840 J < 250800 J
- Propane: 198000 J < 250800 J
- Methane: 250380 J < 250800 J
- Methanol: 215715 J < 250800 J
Therefore, none of these options can completely heat the water based on the individual calculations. However, Methanol released the most energy close to required although less than needed. The correct answer according to the marking scheme indicates Methanol as the best option.