Original post by ChemistryOCRIf we think of the definition of enthalpy change of combustion, it is the energy change which occurs when one mole of a compound is burnt completely with oxygen. Changing the mass of the water cannot change the value of this enthalpy change, because the energy is being given off. What does change, is how much the water is heated up by. More water is heated up, so the temperature change will be less, and without numbers it's hard to disprove, but mass increases, delta T decreases, so the answer should be the same. Do you understand? The same amount of energy is heating a higher volume of water, and because of the specific heat capacity, 4.18 J of energy is required to raise the temperature of 1 g of water by 1 degrees, so you can see that more mass will equal a smaller temperature change.
Regarding your second question, initial temperature does not really matter. It is the temperature change that is required. If the temperature rises by 20 degrees, it will always increase by 20 degrees no matter what the initial temperature is, because the amount of energy given off was enough to raise that volume of water by 20 degrees. Again look back at the definition of specific heat capacity, understanding the definition makes this clearer. If the mass of fuel being burnt is constant, the energy heating the water is constant, and so the temperature will raise by the same amount, regardless of initial temperature. Only when mass of water changes does it get more complicated as you can probably see in my first answer for you, but you are unlikely to be asked that, I never encountered it and working out the answer is a mathematical skill, not chemistry as such, because it's a ratio question. Mass goes up, change in temp goes down, Q should be the same, because the same amount of heat is being given off, to the best of my knowledge this is all correct, but due to lack of data, real questions and the fact that I've never encountered it before makes it a slightly dodgy area, but I'm sure, if they were asking what would happen, you know the same amount of energy is being given off, so no the enthalpy change cannot differ. How you record the enthalpy change however, will.