[rexi]

Hi,
So I recently did a prac in which I heated up 100ml of water by 30 degrees Celcius each with two different alcohols- pentan-1-ol and ethanol -in spirit burners.
I found out that...
-The average mass of pentan-1-ol consumed was 0.66g
-The average mass of ethanol consumed was 1g.

I have a few questions. (Hoping someone can help me out with them, please?)

1. So I need to determine the energy absorbed by the water when the alcohol was burnt.
Would I just use delta T * m* c??

So for pentan-1-ol: 30 * 100 * 4.18 = 12 540 J or 12.540 KJ

(just want to double check as I haven't done these calculations for a while)


2. When calculating the energy released per gram of alcohol burnt, would I go...
12540 (KJ) / 0.66 =19000
19 * 100 = 1900 KJ
 ? or is that completely wrong. I'm a little bit confused about what to do here.

3. For the next part, I need to calculate the heat of combustion for each alcohol in units of KJ mol^-1

So again with penta-1-ol, I'd need to find the moles with n=m/M
So would I use n= (0.66/88.15) to get 0.00748... then divide by 0.66 and times by 100

I feel like I'm doing everything totally wrong and I'd be really grateful if anyone could help me to understand this

Thank you.

[Borek]

You are OK with 1 and 2, you did some strange things in 3. That is - you correctly calculated number of moles. You have number of moles, you have the energy released - start with these numbers.

[Enthalpy]

The implicit assumption, that all combustion heat goes in the water, seems overoptimistic. It can be slightly inaccurate (in a carefully optimised metrological setup) to off by several magnitudes (burner under glassware).

To compute a heat of combustion, unless the setup is meant for metrology, I'd rely on the tabulated heats of formation. This depends on how the question is formulated.
-277.6kJ/mol ethanol
-351.6kJ/mol 1-pentanol
don't forget that the produced water can be gaseous or liquid.

Sidenote: if really you want to write 4 or 5 digits in 12.540 KJ, then you need a more accurate density for water. 1000kg/m³ is at +4°C only.