# GRE Subject Test: Chemistry : General Thermodynamics

## Example Questions

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### Example Question #1 : General Thermodynamics

Which law of thermodynamics states that a crystal's entropy at a temperature of absolute zero is equal to 0?

Second law

First law

Third law

Zeroth law

Third law

Explanation:

There are four laws of thermodynamics:

Zeroth law of thermodynamics: If two independent systems are each in thermodynamic equilibrium with a thrid independent system, then the first two systems must be in thermodyanmic equilibrium with each other.

First law of thermodynamics: Energy is neither created nor destroyed.

Second law of thermodynamics: The entropy of the universe increases following every reaction.

Third law of termodynamics: At a temperature of 0 Kelvin (absolute zero), the entropy of the crystal is 0.

### Example Question #1 : Enthalpy

Propane gas combusts according the following chemical equation:

Given the following standard enthalpy values, what is the enthalpy of the reaction for the combustion of one mole of propane?

Explanation:

Given the molar enthalpy values for reactants and products, we can solve for the enthalpy of the reaction using the equation:

Keep in mind that the molar enthalpy values must be multiplied by the coefficients that are present in the balanced reaction:

Oxygen is omitted because its enthalpy value is zero.

### Example Question #2 : Physical Chemistry

Which is not characteristic of an endothermic reaction?

Absorption of energy from surroundings

Releasing of energy to surroundings

Breaking of a chemical bond

The net change of enthalpy is positive

A chemical reaction that feels cold

Releasing of energy to surroundings

Explanation:

An endothermic reaction absorbs energy in the form of heat. An endothermic reaction involves breaking of chemical bonds because it involves the absorption of energy. Endothermic reactions tend to feel cold because it is taking heat away from your skin. Since endothermic reactions involve absorbing energy, often in the form of heat, the change in enthalpy is positive.

Therefore, the answer is "releasing of energy to surroundings" which is not a characteristic of an endothermic process. Instead, it is a characteristic of an exothermic reaction which is the direct opposite of an endothermic reaction.

### Example Question #2 : Physical Chemistry

Given the enthalpies of formation, what is the enthalpy of combustion of octane in the reaction:

Explanation:

The equation for enthalpy of reaction is:

Given our chemical reaction and the enthalpies of formation, we can find the enthalpy of reaction.

First, find the total enthalpy for the products.

Then, find the total enthalpy for the reactants.

Since the oxygen is elemental, its heat of formation is zero.

Return to the original equation to calculate the final enthalpy of reaction.

### Example Question #1 : Thermodynamics And Phases

Which of the following reactions has a positive value for change in entropy ?

A salt precipitates out of a saturated solution

Water evaporates into water vapor

Water freezing into ice

Water evaporates into water vapor

Explanation:

Entropy is a measure of the disorder of a system. In other words, when the components of a system become more uniform and spread out, the entropy of the system has increased. For example, when a liquid becomes gaseous, the molecules separate from one another, increasing the disorder of the system. As a result, the evaporation of water results in an increase in entropy.

All other options result in a more ordered system, which decreases entropy.

### Example Question #4 : Physical Chemistry

At , a reaction has a Gibb's free energy change of . If the enthalpy change of the reaction is , what is the entropy change of the reaction?

Explanation:

We can relate enthalpy, entropy, and Gibb's free energy using the Gibb's free energy equation:

Keep in mind that temperature must be in Kelvin. Since we know enthalpy and Gibb's free energy for the reaction, we can solve for the entropy change:

### Example Question #2 : Gibbs Free Energy

Which of the following options indicates that a chemical reaction is unfavorable?

All of these

Explanation:

The sign of  indicates the direction of a chemical reaction and determines if a reaction is spontaneous or not. A chemical reaction in which  is considered to be a spontaneous reaction because it will proceed without requiring any outside energy. To approach this problem, we must consider the equation for the Gibbs free energy of a reaction, . A reaction is considered to be favorable when  is negative. The second law of thermodynamics indicates that the entropy of the universe is spontaneously increasing. This corresponds to a positive .

### Example Question #351 : Gre Subject Test: Chemistry

Use the following values for water as needed.

If burning wood releases  of heat energy per gram of wood consumed, what mass of wood must be consumed to heat  of water from  to , and then to convert it to water vapor?

Explanation:

There are two processes requiring added heat in this problem:

1. Raising the temperature of the liquid water from  to  (use )

2. Boiling the water at a constant temperature of  (use )

To use either of these equation, we need to find the mass of the water using the relation between mass, density, and volume.

Use this mass with the given specific heat and temperatures to find the heat for part 1 of the process.

Then, use the mass with the given heat of vaporization to find the energy needed to convert the water to water vapor.

Sum the energies for step 1 and step 2.

This is the total amount of energy needed from the burning wood. Use stoichiometry to find the grams of wood needed to produce this amount of energy.

### Example Question #1 : General Thermodynamics

What was the final temperature of water if a  sample of water absorbs  of heat energy and heats up from ? (specific heat of water is )

Explanation:

Specific heat is the amount of heat needed to raise 1 gram of a substance by 1oC. Calorimeters used for these types of experiments because they are designed to be well-insulated, so no heat is gained from or lost to the surroundings.

### Example Question #1 : General Thermodynamics

An  metal at  was put into  of water at . The final temp of the water and metal was . Assuming heat was not lost to the surroundings find the specific heat of the metal? The specific heat of water is .