Exactly 1.5 g of a fuel burns under conditions of constant pressure and then again under conditions of constant volume. In measurement A the reaction produces 25.9 kJ of heat, and in measurement B the reaction produces 23.3 kJ of heat. Which measurement (A or B) corresponds to conditions of constant pressure? Explain.

Video transcript

Hi everyone for this problem, we're told to consider the two sets of experiments where 50 g of gasoline was burned and 59.6 killed jules and 31.5 kg joules of heat were measured. We're told that experiment one was done at constant pressure and experiment too was done at constant volume. We need to determine which experiment produce 31.5 kg joules of heat. So which of these two experiments produced less heat. Okay, so let's go ahead and get started. So for constant pressure, heat at constant pressure, that is our standard entropy change for the reaction. And so this is heat at constant pressure. And whenever we have a combustion combustion czar always exo thermic. And so this value is always a negative value for heat at constant volume. This is our change and internal energy for the reaction. So this is our heat at constant volume. So combustion are also they also involve expansion and expansions do work, which makes it a negative value as well. So this is always negative. But this is going to be more negative because expansions also do work. And so this means that our change in internal energy is more negative than our standard entropy change. And so that means that experiment one is going to be less negative and experiment two is going to be more negative. And so that means in terms of which experiment produced less heat, it's going to be experiment one, This is the one that produced 31.5 kill the jewels of heat. Okay, that is the end of this problem. I hope this was helpful.