Energy changes
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We are currently studying enthalpy changes in solutions. When we work this out, we use three stages:
1) 'delta' H = mass in grams of the liquid x the specific heat capacity (4.2) x temp. change
2) the number of moles we have of the substance that will be used up first.
3) the energy from 1) divided by the mole value from 2).
What I don't understand is why we always use the specific heat capacity value of 4.2 kJ kg-1 K-1 when the liquid isn't always water. Also, when we are given 50 cm3 of a liquid such as HCl at 2M, the teacher says that this is 50 g of HCl - when clearly it isn't. If I work out the moles of HCl by multiplying the volume over 1000 by the concentration, then working out the mass by multiplying the moles by the Mr of HCl, the answer I get is 3.65 g. Is this right? Why, if the mass of HCl that I have is 3.65 g, does my teacher tell me it is 50 g?
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Corrie writes.....
You are correct, except I would use 4.2 J g-1 deg-1 because you are working in g in the lab. Otherwise you have to remember to convert your mass to kg - but now you must remember to convert your energy to kJ later on!
We use the value 4.2 as an approximation: the actual value for HCl solution instead of water won't be exactly 4.2 and the mass of the solution won't be exactly 50 g (pure water), but this is considered good enough for school level work.
If you are asked about the approximations in an exam questions, then you can explain the two above, or include it in your write-up. They contribute to the overall error but are not usually treated quantitatively. More important are experimental errors such as heat loss, errors in temperatue measurement (esp. if the temp. change is only small) etc.
One other piece of advice: your formula
mass in grams of the liquid x the specific heat capacity x temp. change
works out the amount of energy involved in your experiment. I would not call it 'delta' H or give it a sign at this stage - just call it 'amount of heat or energy'. Work everything out as you have done until you have an answer in kJ mol-1, then decide whether the reaction is endo- or exothermic BASED ON the temp. change given (down or up). Now give your answer the appropriate sign and call it 'delta H'.
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updated: 12 February 2008
