How to Calculate Enthalpy of Reaction - AP Chem Unit 6, Topic 6a

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[@crish.2823]

Hi great video! Could you briefly explain why you include the mass of the water in the q=mc(delta T) equation? And if the answer is that water is part of the solution, why would the process be considered exothermic (if the water is part of the solution and the solution gets warmer, when is energy being released? Or if the reason why it is an exothermic reaction is because the water is getting warmer, that means the water is part of the surroundings, but if that’s the case, how is it also part of the solution and why is its mass included in the equation?)? Sorry for such a long question with so many parts!

[@JeremyKrug]

Excellent questions! The short answer is that the water is part of the solution, but not part of the system. Somewhat longer answer: We can't directly measure the heat change of the individual particles that participate in the reaction, so we have to measure the temperature change of the entire solution (the particles of which are almost completely water -- there are very few particles of reactants and products, compared to the number of water molecules in the solution). Of course, we can't measure the ΔT of all the surroundings (the entire universe), so we isolate a very small part of the surroundings in an insulated/isolated container (the calorimeter). Since the water is part of the surroundings, we have to change the sign when we move from calculating Q to calculating ΔH of the actual system. I hope this helps!

[@crish.2823]

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[@pracheenai]

can you please explain why the last problem is exothermic and is -7461? why is it not endo

[@JeremyKrug]

When a reaction has a negative ΔH, it means the system is losing heat to the surroundings. So if you put your hand next to the system, you will feel some of that heat being emanated from it. That means the reaction must be exothermic. I hope this helps!

[@leilaammi009]

tysm

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[@VivaanGupta-c2y]

Great stuff in the video, but google says that the enthalpy for the decomposition of Calcium Chloride is roughly 81.3 kJ/mol and if you don't count the mass of CaCl2 in the equation, you get something similar. Could you please explain more about why we should count the mass of CaCl2 in the equation?

[@JeremyKrug]

Good question! Since we are calculating the enthalpy of SOLUTION of calcium chloride, we need to consider the mass of the SOLUTION (not just the solvent) when calculating Q, which includes the mass of CaCl2 as well as the mass of the water. For further information, I'll refer you to the 2023 AP Chemistry Exam, FRQ Question 3, part (g), which is a very similar question. College Board also expects the total mass of solution to be included when calculating Q. Thanks for watching! apcentral.collegeboard.org/media/pdf/ap23-sg-chemistry.pdf