Consider the following information. Ethanol burns in excess air according to the following equation. C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) ΔH = -1364 kJ mol⁻¹ The cooking pot is made from aluminium and has a mass of 150 g. The specific heat capacity of aluminium is 9.000 J g⁻¹ °C⁻¹. The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹. i. Calculate the minimum amount of energy, in kJ, required to heat 550 g of water and the pot from 18.5°C to 100.0°C. ii. Calculate the mass, in g, of ethanol that needs to be completely burnt to provide this energy. iii. Only 35% of the energy released by the combustion of ethanol is transferred to the cooking pot and contents. Calculate the mass, in g, of ethanol that needs to be burnt in practice to heat the water and the pot from 18.5°C to 100.0°C. b. Other camping stoves use butane (C4H10) as fuel. Given that, on complete combustion, 6.00 g of butane releases the same amount of energy as 10.0 g of ethanol, calculate the magnitude of ΔH, in kJ mol⁻¹ for the reaction. 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)

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Consider the following information - VCE - SSCE Chemistry - Question 6 - 2007 - Paper 1

Question 6

Consider the following information. Ethanol burns in excess air according to the following equation. C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) ΔH = -1364 kJ mol⁻¹ T... show full transcript

Worked Solution & Example Answer:Consider the following information - VCE - SSCE Chemistry - Question 6 - 2007 - Paper 1

Step 1

i. Calculate the minimum amount of energy, in kJ, required to heat 550 g of water and the pot from 18.5°C to 100.0°C.

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Answer

To find the total energy required, we can use the formula:

$Q = mcΔT$

Where:

$Q$ = energy (in J)
$m$ = mass (in g)
$c$ = specific heat capacity (in J g⁻¹ °C⁻¹)
$ΔT$ = change in temperature (in °C)

First, calculate the energy required to heat the water:

For the water:
- $m_{water} = 550 g$
- $c_{water} = 4.18 J g⁻¹ °C⁻¹$
- $ΔT_{water} = 100.0 - 18.5 = 81.5 °C$
Thus,

$Q_{water} = 550 imes 4.18 imes 81.5 = 198,198 ext{ J}$
For the pot (aluminium):
- $m_{pot} = 150 g$
- $c_{pot} = 9.000 J g⁻¹ °C⁻¹$
- $ΔT_{pot} = 100.0 - 18.5 = 81.5 °C$
Thus,

$Q_{pot} = 150 imes 9.000 imes 81.5 = 1,096.25 ext{ J}$

Total energy required:

$Q_{total} = Q_{water} + Q_{pot} = 198,198 + 1,096.25 = 199,294.25 ext{ J} ext{ or } 199.29 ext{ kJ}$

Step 2

ii. Calculate the mass, in g, of ethanol that needs to be completely burnt to provide this energy.

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Answer

Using the given enthalpy change for the combustion of ethanol:

$ΔH = -1364 ext{ kJ mol}^{-1}$

To find the number of moles of ethanol required:

Convert the energy to be provided into kJ:
$Q_{required} = 199.29 ext{ kJ}$
Calculate moles required:

$ext{Moles of ethanol} = rac{Q_{required}}{-ΔH} = rac{199.29}{1364} = 0.146 ext{ mol}$
Now, calculate the mass:
- Molar mass of ethanol, $C_2H_5OH = 2(12) + 6(1) + 16 = 46 ext{ g mol}^{-1}$
$ext{Mass of ethanol} = 0.146 ext{ mol} imes 46 ext{ g mol}^{-1} = 6.70 ext{ g}$

Step 3

iii. Only 35% of the energy released by the combustion of ethanol is transferred to the cooking pot and contents. Calculate the mass, in g, of ethanol that needs to be burnt in practice to heat the water and the pot from 18.5°C to 100.0°C.

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Answer

Since only 35% of the energy is useful,

Calculate the energy that actually contributes:

$Q_{effective} = 0.35 imes 199.29 ext{ kJ} = 69.75 ext{ kJ}$
Now, calculate the moles of ethanol needed:

$ext{Moles needed} = rac{Q_{effective}}{-ΔH} = rac{69.75}{1364} = 0.0512 ext{ mol}$
Finally, calculate the mass of ethanol:

$ext{Mass of ethanol} = 0.0512 ext{ mol} imes 46 ext{ g mol}^{-1} = 2.36 ext{ g}$

Consider the following information - VCE - SSCE Chemistry - Question 6 - 2007 - Paper 1

Worked Solution & Example Answer:Consider the following information - VCE - SSCE Chemistry - Question 6 - 2007 - Paper 1

i. Calculate the minimum amount of energy, in kJ, required to heat 550 g of water and the pot from 18.5°C to 100.0°C.

ii. Calculate the mass, in g, of ethanol that needs to be completely burnt to provide this energy.

iii. Only 35% of the energy released by the combustion of ethanol is transferred to the cooking pot and contents. Calculate the mass, in g, of ethanol that needs to be burnt in practice to heat the water and the pot from 18.5°C to 100.0°C.

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