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# A-LEVEL Mistake in June 2010 chem unit mark scheme ?

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1. A-LEVEL Mistake in June 2010 chem unit mark scheme ?
A student carried out a simple laboratory experiment to measure the enthalpy change for Process 3. The student showed that the temperature of 200 g of water increased by 8.0°C when 0.46 g of pure ethanol was burned in air and the heat produced was used to warm the water.

Use these results to calculate the value, in kJ mol–1, obtained by the student for this enthalpy change. (The specific heat capacity of water is 4.18 JK–1 g–1)

Process 3 Complete combustion of ethanol:
CH3CH2OH + 3O2 ---> 2CO2 + 3H2O

The mark scheme forgot to change 8°C to Kelvin, or the paper mistakenly said 8.0°C when it meant 8K ? ?... Can someone confirm this please, here is the mark scheme method;

M1 q = m c ΔT (this mark for correct mathematical formula)
M2 = 6688 (J) OR 6.688 (kJ) OR 6.69 (kJ) OR 6.7 (kJ)
M3 0.46g is 0.01 mol
therefore ΔH = – 669 kJmol-1 OR – 670 kJmol-1 OR – 668.8 kJmol-1
2. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
(Original post by iAre Teh Lejend)
A student carried out a simple laboratory experiment to measure the enthalpy change for Process 3. The student showed that the temperature of 200 g of water increased by 8.0°C when 0.46 g of pure ethanol was burned in air and the heat produced was used to warm the water.

Use these results to calculate the value, in kJ mol–1, obtained by the student for this enthalpy change. (The specific heat capacity of water is 4.18 JK–1 g–1)

Process 3 Complete combustion of ethanol:
CH3CH2OH + 3O2 ---> 2CO2 + 3H2O

The mark scheme forgot to change 8°C to Kelvin, or the paper mistakenly said 8.0°C when it meant 8K ? ?... Can someone confirm this please, here is the mark scheme method;

M1 q = m c ΔT (this mark for correct mathematical formula)
M2 = 6688 (J) OR 6.688 (kJ) OR 6.69 (kJ) OR 6.7 (kJ)
M3 0.46g is 0.01 mol
therefore ΔH = – 669 kJmol-1 OR – 670 kJmol-1 OR – 668.8 kJmol-1
No, the MS is correct.
3. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
It's not meant to be 8K...it's 8 degrees Celsius or 8 joules.
4. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
(Original post by Tullia)
No, the MS is correct.
Could you show me how the MS is correct ?
5. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
(Original post by iAre Teh Lejend)
Could you show me how the MS is correct ?
You don't need to change from degrees Celsius as you thought you did. Therefore, the MS is correct.
6. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
(Original post by Tullia)
You don't need to change from degrees Celsius as you thought you did. Therefore, the MS is correct.
Why not ?...
7. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
(Original post by iAre Teh Lejend)
Why not ?...
They're proportional.
8. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
(Original post by Tullia)
They're proportional.
I don't understand, in the book they always converted degrees Celcius to Kelvin. :/
9. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
(Original post by iAre Teh Lejend)
I don't understand, in the book they always converted degrees Celcius to Kelvin. :/
You can convert, but it makes no difference, as they're proportional.
10. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
You only need the temperature change. It doesn't matter whether it is in degrees or kelvin, the temperature change will still be 8.
11. Re: A-LEVEL Mistake in June 2010 chem unit mark scheme ?
You're only looking at the temperature change, 8 degrees or 8 kelvin is the same in the temperature change.

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