Reaction Shift

[samantha eberle 2L]

I am a little confused over this practice midterm problems given by one of the UAs.

An undergraduate student is investigating the decomposition of dinitrogen tetroxide gas, N₂O₄ (g), into nitrogen dioxide gas, NO₂ (g), as represented below.


8. Which of the following would shift the reaction to the reactant side?
A.) Introduce a metallic catalyst, such as Ni(s).
B.) Decrease the temperature by 87 degrees Celsius
C.) Add an inert gas, such as Ne (g)
D.) Triple the concentration of N₂O₄(g)

I can understand why options A, C, and D are incorrect, but I am struggling to understand how we know that this reaction is endothermic, since that would justify a decrease in hea favoring reactants. Thanks for any help!

[Emily La 2B]

The reaction is endothermic because there is a breaking of bonds when converting N2O4 to NO2, and breaking bonds requires energy, which means that the reaction requires heat to happen. Since endothermic reactions require heat, decreasing the temperature would cause the system to shift to the reactant side because there wouldn't be enough heat for the reaction to progress.

[Keanu Ngo 3B]

Why is shifting equilibrium to the reactant side is not the same as raising the K constant of a reaction?

[Lauren Hsieh 2I]

In regards to the comment above, raising the K constant of a reaction requires changing the temperature. Shifting an equilibrium to the reactant side results from adding more products or reducing the reaction concentration. The reaction eventually reaches equilibrium again in order to maintain the ratio of products and reactants as defined by the unchanging K constant of the reaction.

[105764441]

The reaction is an endothermic reaction. We know this because the problem says DECOMPOSITION, which signals that bonds are being broken. If bonds are being broken, you need an energy INPUT. I like to remember this by thinking of the fact that in Karate, you need energy to break a board, just as you need the energy to break a bond. Anyhow, if you need an energy input to break bonds, then in the reverse direction of the reaction, energy will be released as the bonds are built back up. So, in order to build back more reactant, we would need a decrease in energy so that the forward reaction is not favored and thus the reverse reaction would be favored.

[Shania Hao 3L]

I agree with the above answers!
The breaking of bonds requires energy to occur. In terms of an endothermic reaction, which is the case here, a decrease in temperature would favor the reverse reaction since it would require heat in order to favor the forward reaction, and without that heat present, it would favor the reactants since there would not be enough heat to break the bonds.

[Hope_Hayashida_3E]

We know this is endothermic because bonds are being broken. You can tell because it is a decomposition reaction. Because it is endothermic, that means that delta H is positive in the forward direction (products). Therefore, decreasing the temperature would favor the reactants.

[Mark Torres 2I]

To do this problem I just ruled out the other answers. A is wrong because introducing a solid, assuming it isn't aqueous, doesn't shift the reaction at all. C is wrong because adding an inert gas doesn't shift a reaction. And D is wrong because adding more reactant only shifts the reaction to the products side. In terms of B, I agree with the comments above!

[Yu_Jia_Xu_2I]

I think when a bond is being broken, energy is required. And a reaction that requires an energy input is most likely endothermic. In contrast, when a bond is created, it releases energy, thus making the reaction exothermic. When the reaction is endothermic, reducing the temperature would produce more reactant. When the reaction is exothermic, reducing the temperature would produce more product, vice versa.

[Gabrielle Linden 1C]

The key part of this question was that the gas decomposed! When a substance decomposes, chemical bonds are broken and simpler substances are formed. We also know that breaking a bond requires an input of energy, so it would be an endothermic reaction. Since endothermic reactions require heat to proceed, lowering the temperature would favor the reverse reaction in the direction of the reactants, which would release heat.

[Emily H 1F]

B is correct because decomposition reactions require energy (endothermic) to progress, thus decreasing the temperature would make the reaction occur more slowly, indicating a shift to the reactants side of the equation

[Emily H 1F]

B is correct because decomposition reactions require energy (endothermic) to progress, thus decreasing the temperature would make the reaction occur more slowly, indicating a shift to the reactants side of the equation

[Kayla Nguyen 2F]

Bonds are being broken (decomposition) which requires the input of energy so it is endothermic. Since it is endothermic, reducing the temp means there is less heat to for the forward reaction, shifting the reaction towards the reactants.
the answer is B