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

It is exothermic, with both negative enthalpy and entropy changes.

It is endothermic, with both positive enthalpy and entropy changes.

It is exothermic, with a negative enthalpy and positive entropy change.

It is endothermic, with a positive enthalpy and negative entropy change.


2. If the reaction between two solutions has a positive enthalpy change, which of the following best describes the reaction? (2 points)

It is exothermic and heat is added to the system.

It is endothermic and heat is added to the system.

It is exothermic and heat is lost to the surroundings.


3) Which statement below best describes the relationship of the reaction below? (2 points)
3A + B yields 2AB

The rate of formation of AB is twice the disappearance of A.

For every two AB produced, the reaction requires three A.

The rate of disappearance of A is two times the formation of AB.

4) Describe how you would use the information depicted on the potential energy diagram below to determine the amount of required activation energy and the enthalpy change of the reaction. (2 points)
(see chart)

Which of the following must be true about a reaction if it is only spontaneous at high temperatures It is exothermic with both negative enthalpy and entropy cha class=

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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.

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