# How to judge if a reaction is exothermic or endothermic

Mix $\ce{HCl}$ and $\ce{NaOH}$ together (reactants are at the same temperature) in a calorimeter. The temperature increases, and the book says it's an exothermic reaction. I don't get it. The thermometer is in the solution which means it measures the temperature of the solution. It's the system. The temperature of the system increases, this means that heat flows in. How could this be an exothermic reaction?

• The purpose of the calorimeter is to isolate the system and prevent heat from entering or exiting the system. If the temperature increases, then the heat must come from what is inside the calorimeter. – LDC3 Apr 20 '14 at 18:06
• I just confuse about what the system is in this reaction. – user40003 Apr 20 '14 at 18:23
• if we just put a thermometer in a container with opening to carry out this reaction, still the number on thermometer will increase and the container, which is the surroundings will feel hot because the heat flowed from the system to surroundings. But what is the system? When we measure the temp of solution, it also increases. – user40003 Apr 20 '14 at 18:25
• Is the solution surroundings or system? – user40003 Apr 20 '14 at 18:26
• The system is the solution, colorimeter, thermometer and anything inside the container (colorimeter). If it is open on top, the heat can transfer to the outside. If the solution temperature increases above room temperature, the heat from the room could not have entered the system. The system must have produced the heat. – LDC3 Apr 20 '14 at 18:30

This is one of the most commonly misunderstood principles in thermochemistry - if you can wrap your mind around it, you are most of the way there!

The good news is, there is an easy way to do that. In any physics problem (thermochemistry is really physics) you have to be very careful with how you define the system and the surroundings. This is important for two reasons.

1. It lets you write energy balance equations across the system boundaries (so we can use math to solve the problem)
2. By choosing the right system boundaries, we can make the math much, much easier.

Since thermochemistry at this level always involves the same types of problems, chemists have done the hard part for you and already selected the best system boundaries.

The system is whatever is in the reaction equation.

That's it. The entire rest of the universe is the "surroundings."

If the reaction molecules are the only thing in the system, then that means the water, the calorimeter, the themometer, you, me, the room - everything else - counts as surroundings. Now, the water did heat up. We know this because the thermometer absorbed some heat and showed a temperature increase. Since they are all part of the surroundings, the heat must have come from the system. Since heat left the system, the sign of $q_{reaction}$ is negative, and the reaction is exothermic.
• Also I wouldn't say that $\ce{NaCl}$ was created unless we know we were mixing liquid $\ce{HCl}$ with molten $\ce{NaOH}$ or if we were otherwise somehow working with pure $\ce{HCl}$ and pure $\ce{NaOH}$. Throw water into the mix and what you get isn't $\ce{NaCl}$ but mostly solvated $\ce{Na^+}$ and $\ce{Cl^-}$ ions. – Dissenter May 20 '14 at 19:55