Question

Which of the following must be true about a reaction if it is only spontaneous at high temperatures?

Answer 1

1) Answer is: It is endothermic, with both positive enthalpy and entropy changes.

Endothermic reactions (ΔH>0) that increase the entropy of the system (ΔS>0) are spontaneous at high temperatures.

The change in Gibbs free energy (ΔG), at constant temperature and pressure, is: ΔG=ΔH−TΔS.

ΔH is the change in enthalpy.

ΔS is change in entropy.

T is temperature of the system.

When ΔG is negative, a reaction (occurs without the addition of external energy) will be spontaneous (exergonic).

2) Answer is: It is endothermic and heat is added to the system.

There are two types of reaction:

1) endothermic reaction (chemical reaction that absorbs more energy than it releases, ΔH>0).

2) exothermic reaction (chemical reaction that releases more energy than it absorbs).

For example, the breakdown of ozone is an endothermic process. Ozone has lower energy than molecular oxygen (O₂) and oxygen atom, so ozone need energy to break bond between oxygen atoms.

3) Answer is: For every two AB produced, the reaction requires three A.

Balanced chemical reaction: 3A + B → 2AB.

From balanced chemical reaction: n(A) : n(AB) = 3 : 2.

n(A) = 3 · n(AB) ÷ 2.

A and B are reactants and AB is product of balanced chemical reaction.

For every two AB produced, the reaction requires one B.

4) Answer is:

the amount of required activation energy = potential energy of the B - potential energy of the reactants A.

the enthalpy change of the reaction = potential energy of the products C - potential energy of the reactants A.

For all chemical reaction some energy is required and that energy is called activation energy (energy that needs to be absorbed for a chemical reaction to start).

This is endothermic reaction.