Energetics

Writing only "exothermic gives out heat" or "endothermic takes in heat" can be marked incomplete, and reversing the temperature direction is a frequent error. Tie the energy transfer to a measurable change: exothermic transfers energy TO the surroundings, so their temperature RISES; endothermic takes energy IN, so their temperature FALLS.

Defining a fuel as merely "a source of energy" scores zero. A creditworthy definition needs BOTH ideas: the substance burns / combusts, AND it releases heat (thermal) energy. "Source of energy" is too vague because it names neither the combustion nor the heat released — both are required for the mark.

For an exothermic reaction $\Delta H$ must carry a negative sign — energy leaves the chemicals. Writing $+882\ \text{kJ/mol}$ for a combustion loses the final mark because a positive value is endothermic. A correct magnitude with the wrong sign, or without the required significant figures, still drops marks — check both.

When explaining why the experimental $\Delta H$ is less exothermic than the true value, "energy lost" or "energy escaped" is too vague and scores zero. Name the pathway: heat lost to the surroundings. Also creditworthy are incomplete combustion of the fuel and heat absorbed by the metal can. "Human error" and "misread the thermometer" earn nothing.

To find the highest temperature, ADD the temperature rise to the starting temperature; some candidates wrongly subtract the two readings. When finding $\Delta T$ for an exothermic change it is highest reading minus start; for a cooling (endothermic) change it is start minus lowest reading. Get the direction right before substituting into $Q = mc\Delta T$.

In $Q = mc\Delta T$ the mass $m$ is the mass of the WATER being heated, never the mass of fuel burned. The fuel mass is used only later, to find the moles ($n = m/M_r$) for the $\Delta H$ step. Putting the fuel mass into $Q = mc\Delta T$ gives a tiny, wrong heat value and loses the accuracy mark.