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AP Chemistry Question Review 2 - Enthalpy

Continuing with our question reviews, we come back to AP Chemistry

The AP Chemistry exam is getting dangerously close as it is less than a month away. Our old question review for AP Chemistry talked about hydrogen bonding, but in this post, we plan to cover another topic that is bound to show up on the day of the exam: Enthalpy. Enthalpy is a measure of internal energy within molecules and chemical compounds. Below is a question that you may see on the day of the exam:

For the following reaction, the standard enthalpy change is 16.1 kJ:

2 NOBr (g) ←⃗ 2 NO (g) + Br2 (g)

Which of the following will be true if the temperature of the system is decreased, with pressure held constant?

Your answer choices for this question are:

  • The concentration of NOBr will decrease.
  • K will increase.
  • The concentration of Br2 will decrease.
  • The concentration of NO will increase.

So what exactly is going on? The only thing you are given is the enthalpy change as a result of the reaction and an equilibrium equation. The enthalpy change is extremely important for this question. Not the number, but mainly the sign of this number. Notice how it's positive, so what does that exactly tell us? It means that the reaction as written in endothermic in as you go from reactants to products. This makes sense seeing as you are breaking a stable molecule into it's individual components, which requires energy to do.

Great, so how does that help us figure out what decreasing the temperature does? Remember that this system is in equilibrium, so that means the system is always going to want to shift to the original conditions that it was previously at. So, if you decrease the temperature, the system is going to want to bring it back to the original temperature where the system was in equilibrium by increasing the temperature.

How does the system do this? Since the forward reaction is endothermic, that means the opposite reaction that goes from products to reactants is exothermic, where energy is released. A release of energy means that heat is released. As heat is released, the temperature will begin to offset the reduction in temperature. As a result, there will be more NOBr that is produced, therefore increasing it's concentration and decreasing NO and Br2 concentration.

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