Endothermic and Exothermic Reactions
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MCAT Physical › 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?
Increase the temperature
Decrease the temperature
Add product to the reaction
Remove reactant from the reaction
Explanation
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.
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 warm container from an exothermic reaction
A cold container from an exothermic reaction
A warm container from an endothermic reaction
A cold container from an endothermic reaction
The products of this reaction have an equal energy level to the reactants
Explanation
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.
When a solution is formed, which of the following is true?
Solution formation that is exothermic results in stronger intermolecular bonds, compared to the bonds in the pure substances
The bonds in the solute molecules are broken in an exothermic process
The bonds between the solvent molecules are broken in an exothermic process
Formation of a solution causes a decrease in entropy
None of these statements are true
Explanation
When dealing with the enthalpy of a solution, there are three specific steps, each having their own enthalpy.
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The breaking of solute-solute
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The breaking of solvent-solvent bonds
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The formation of solute-solvent bonds
Remember that breaking bonds always requires an input of energy, and is thus considered endothermic. The formation of new bonds is an exothermic process. When an overall solution is exothermic, it means that the new intermolecular bonds are more stable. A system with less energy is considered more stable.
Is the reaction above endothermic or exothermic?
Exothermic because an increase in temperture would cause the reaction to shift to the left
Exothermic because an increase in temperture would cause the reaction to shift to the right
Endothermic because an increase in temperture would cause the reaction to shift to the left
Endothermic because an increase in temperture would cause the reaction to shift to the right
Explanation
The reaction is an exothermic reaction, since the heat is added on the products side. An endothermic reaction would have the heat on the reactants side. An increase in heat in this reaction would cause the Keq to decrease, and there will be a shift to the left of the reaction (toward the reactants).
Remember that Keq is dependent on temperature, and can be affected by changes in heat. Keep in mind, also, that the reverse reaction that occurs during the leftward shift will be endothermic, using the additional heat as a reactant rather than a product.
Why does a chemical 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 releases heat into 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 exothermic reaction; this pulls heat from the surroundings, making the ice pack feel cold.
Explanation
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.
Which of the following is true in the reaction above?
Explanation
The process of bond formation in chemistry is exothermic. By definition, exothermic reactions have a negative enthalpy change; therefore 
Reducing moles of gas will decrease entropy, meaning that 
Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils.
Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here.
In this law, 
A scientist is creating a solution to study vapor pressure. When she adds the solute, intermolecular bonds in the solute break to allow the solution to form. The formation of the solution is exothermic. Which of the following is true?
Breaking solute-solute bonds absorbs energy
Breaking solvent-solvent bonds releases energy
Breaking solute-solute bonds releases energy
Forming solute-solvent bonds absorbs energy
Forming solute-solute bonds absorbs energy
Explanation
Even though the dissolution process is exothermic, breaking bonds ALWAYS requires energy input. In the case of an exothermic dissolution, more energy is released when solute-solvent bonds are formed, than was absorbed when solute-solute bonds were broken. The initial step requires an input of activation energy, but the final product allows for a net energy release.
Based on the energy diagram, which chemical processes are exothermic?
Explanation
An exothermic process, by definition, involves a reaction in which the products are lower in energy than the reactants. The reduction in chemical energy results in a release of heat from the reaction.
In the diagram, the path between points is irrelevant. We are simply looking for any instances in which the product point is below the reactant point. Point C has less energy than point B, and point D has less energy than A, B, or C. Transitions from B to C, C to D, or A to D will all result in a reduction of chemical energy, and a release of heat.
5.6g of manganese reacts with 650mL of 6.0M hydrochloric acid to form manganese (V) chloride and hydrogen gas. Along with the products, a large amount of heat is evolved.
This reaction is classified as __________.
Exothermic, with a negative
Endothermic, with a negative
Exothermic, with a positive
Endothermic, with a positive
Explanation
Any reaction in which heat is evolved (released) is classified as exothermic. The change in heat content (
The formation of nitrous oxide is a 2-step process.
The overall enthalpy of the reaction is +68kJ.
Given the above information, which of the following statements must be true?
The forward overall reaction has a higher activation energy than the reverse overall reaction.
Both of the steps are endothermic.
The overall reaction releases heat to the surroundings.
Both of the steps are exothermic.
Explanation
When a reaction has a positive enthalpy change, it is said to be endothermic. This means that it requires an addition of heat into the system. Because the forward reaction requires more energy than the reverse reaction in order to take place, we see that the activation energy of the endothermic reaction is greater than the activation energy of the reverse reaction.
Essentially, the forward reaction requires a net input of energy, while the reverse reaction results in a net release of energy.