Heat Given Off During a Reaction

[Samantha Melin 2E]

In lecture, Dr. Lavelle gave an example of an exothermic reaction and asked how much heat was released during the reaction. The answer we ended up with was negative. Dr. Lavelle stated that the answer was negative because the heat given off by the reaction was equal to the negative of the heat absorbed by the solution. Can someone elaborate on this?

[Alena Zhu 2I]

There are two parts to a chemical system, the system and the surrounding. When we say the heat released was negative, it means that the system lost heat to the surroundings, which would be the solution that is absorbing the heat and reflecting the increase in temperature. The system gave off that heat to the surrounding which is why the value is negative, because the system lot it but the surroundings gained it.

It's a little confusing but hopefully that was helpful!

[Shivani Sakthi 1l]

Hello!
Think of this in terms of a calorimeter with a reaction taking a place inside of it. Let us say that the reaction is exothermic, meaning that the reaction releases heat to its surroundings. To elaborate, a system is the region of focus, which would be the reaction in this case, while the surroundings is the basin in which the system/reaction is immersed in. So, when the reaction releases heat to its surroundings, the calorimeter in this case, the temperature of the calorimeter will rise. Therefore, we can be certain that if there is an increase in temperature in the surroundings/calorimeter, the surroundings is taking away heat from the system/reaction. Therefore, even though we are discussing an increase in temperature, we would know that the reaction is losing heat. So, it is very important to distinguish whether the temperature change is occurring in a system or its surroundings. Then, you can use the statement that the "the heat that is system gives off is equal in opposite to its surroundings" , to determine the direction of heat transfer in the system and its surroundings. So, if the surroundings/calorimeter gains heat marked by a change in temperature, we know that the heat that the system gives off will be equal in opposite. Therefore, the heat transfer in the system will be marked by a loss of heat and a decrease in temp. Hope this helped!

[SofiaMammaro-1K]

Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light. In an exothermic reaction, energy is released because the total energy of the products is less than the total energy of the reactants. For this reason, the change in enthalpy, ΔH, for an exothermic reaction will always be negative. In the presence of water, a strong acid will dissociate quickly and release heat, so it is an exothermic reaction.
Endothermic reactions are reactions that require external energy, usually in the form of heat, for the reaction to proceed. Since endothermic reactions draw in heat from their surroundings, they tend to cause their environments to cool down. They are also generally non-spontaneous, since endothermic reactions yield products that are higher in energy than the reactants. As such, the change in enthalpy for an endothermic reaction is always positive. In order to melt the ice cube, heat is required, so the process is endothermic.