MCAT Physical : Endothermic and Exothermic Reactions
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Example Question #11 : Endothermic And Exothermic Reactions
A student is performing a reaction with unknown compounds in his chemistry lab. The only information the student knows about the reaction is that it is endothermic and reversible. Using this knowledge alone, how can the student increase the yield of his product?
Remove reactant from the reaction
Increase the temperature
Add product to the reaction
Decrease the temperature
Increase the temperature
To answer this question, we need to have a solid understanding of Le Chatelier's principle.
In an endothermic reaction, heat is needed to facilitate the reaction. To increase the products, we want to shift the reaction to the right.
We should already know that adding product or removing reactant shifts the equilibrium to the left, and yields more starting material rather than product. In an endothermic reaction, we can consider heat as a reactant; thus, adding heat (increasing temperature) would allow us to shift the reaction to the right.
Decreasing the temperature, removing reactant, or adding product would all increase the yield of the starting materials.
Example Question #11 : Endothermic And Exothermic Reactions
For any given chemical reaction, one can draw an energy diagram. Energy diagrams depict the energy levels of the different steps in a reaction, while also indicating the net change in energy and giving clues to relative reaction rate.
Below, a reaction diagram is shown for a reaction that a scientist is studying in a lab. A student began the reaction the evening before, but the scientist is unsure as to the type of the reaction. He cannot find the student’s notes, except for the reaction diagram below.
Growing frustrated by his inability to decipher what chemical reaction the student had started, the scientist decides to measure the reaction vessel's temperature. Based only on the above reaction diagram, what is he most likely to find if the reaction is ongoing?
(Assume that the reaction vessel is defined as the system, and that the entropy of the system decreases)
A cold container from an exothermic reaction
A warm container from an endothermic reaction
A cold container from an endothermic reaction
A warm container from an exothermic reaction
The products of this reaction have an equal energy level to the reactants
A warm container from an exothermic reaction
Point 5 is the energy level of the products of the reaction, while point 1 is the energy level of the reactants. Point 5 is lower than is point 1, indicating that the products of this reaction contain lower overall energy than do the reactants. This energy must be released in some form, likely as heat, characteristic of an exothermic reaction.
The question further specifies that there is a local decrease in entropy of the system, thus, the only way that entropy of the universe can increase is to release heat and increase the entropy of the surroundings.
Example Question #12 : Endothermic And Exothermic Reactions
Why does a chemical ice pack feel cold?
The chemicals in the ice pack undergo an exothermic reaction; this pulls heat from the surroundings, making the ice pack feel cold.
The chemicals in the ice pack undergo an exothermic reaction; this releases heat into the surroundings, making the ice pack feel cold.
The chemicals in the ice pack undergo an endothermic reaction; this releases heat into the surroundings, making the ice pack feel cold.
The chemicals in the ice pack undergo an endothermic reaction; this pulls heat from its surroundings for the reaction, making the ice pack feel cold.
The chemicals in the ice pack undergo an endothermic reaction; this pulls heat from its surroundings for the reaction, making the ice pack feel cold.
Endothermic reactions by definition, require heat as a reactant. By drawing in heat from the surrounding, the surrounding will have a lower temperature compared to air temperature, which is why the ice pack feels cold. If it were an exothermic reaction, the pack would release heat, making the pack feel warm.
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