Physical Chemistry : Thermochemistry

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Thermochemistry

What amount of heat must be added to a 20-gram sample of ice in order to raise the temperature from  to ?

Possible Answers:

Correct answer:

Explanation:

This question requires three steps:

1. We need to raise the temperature of the ice from  to  degrees Celsius.

2. We need to melt the ice.

3. We need to raise the temperature of the water from  to  degrees Celsius.

Steps one and three can use the equation:

Step 2 requires the enthalpy of fusion to be multiplied by the mass of the water sample.

Finding these three added heats will give us the total amount of heat needed to raise the temperature from  to .

Example Question #1 : Thermochemistry

What is the process of converting a solid directly to a gas?

Possible Answers:

Freezing

Condensation

Deposition

Sublimation

Correct answer:

Sublimation

Explanation:

Certain substances, such as carbon dioxide, are able to skip the liquid phase and go from a solid to a gas under standard conditions. This process is called sublimation.

Example Question #1 : Heat Capacity And Specific Heat

What is the specific heat capacity for a 50-gram sample of metal that increases in temperature by 10 degrees celsius when 2000 joules of energy is added?

Possible Answers:

Correct answer:

Explanation:

Use the equation:

We can calculate the specific heat capacity for the unknown metal. Since we know the added heat , the mass of the sample , and the change in temperature , we can solve for .

Example Question #2 : Heat Capacity And Specific Heat

A  sample of an unknown metal at  is immersed in water and is allowed to reach equilibrium. If final temperature of the system is  and  of energy are released, what is the identity of the metal?

Sp heat

Possible Answers:

It is impossible to determine

Correct answer:

Explanation:

Recall that the formula for specific heat is . Rearranging the equation for specific heat (c) yields 

Remark: keep in mind that the release of energy and the cooling of the metal will give negative values for both  and

Example Question #2 : Thermochemistry

The heat capacity of a bomb calorimeter assembly is  . What is the heat of combustion of  of sucrose in a bomb calorimeter that results in the temperature rising from  to ?

Possible Answers:

Correct answer:

Explanation:

A bomb calorimeter is a device used to measure the quantity of heat change for a process. The heat of a reaction which is denoted as , is the negative of the thermal energy gained by the calorimeter:

The heat capacity of a calorimeter is:

Plugging the values given into the equation gives:

Using the relation provided earlier:

Because we are dealing with 1.1 grams of sucrose, the heat of combustion of sucrose is:

Example Question #3 : Thermochemistry

The heat capacity of a bomb calorimeter assembly is  . What is the heat of combustion of  of caffeine in a bomb calorimeter that results in the temperature rising from  to ?

Possible Answers:

Correct answer:

Explanation:

A bomb calorimeter is a device used to measure the quantity of heat change for a process. The heat of a reaction which is denoted as q, is the negative of the thermal energy gained by the calorimeter:

The heat capacity of a calorimeter is:

Plugging the values given into the equation gives:

Using the relation provided earlier:

Because we are dealing with 1.65 grams of sucrose, the heat of combustion of sucrose is:

Example Question #1 : Endothermic And Exothermic Reactions

Which of the following are endothermic?

Possible Answers:

Melting

More than one of these are endothermic processes

Condensation

Freezing

Correct answer:

Melting

Explanation:

Endothermic processes involve a positive change in enthalpy. This means that the enthalpy of products is higher than the enthalpy of reactants and net heat energy is consumed. Phase changes that involve increasing the distance between particles (meaning conversion of solid to liquid (melting), liquid to gas (evaporation) and solid to gas (sublimation)) require an input of energy and are considered endothermic processes. On the other hand, phase changes that decrease the distance between particles (such as gas to liquid (condensation), liquid to solid (freezing), and gas to solid (deposition)) release energy and are considered exothermic processes.

Example Question #4 : Thermochemistry

Which of the following is true regarding an exothermic reaction?

I. Entropy always increases

II. It is always spontaneous

III. It can facilitate other energy-consuming reactions

Possible Answers:

I and III

III only

II only

I and II

Correct answer:

III only

Explanation:

Exothermic reactions involve release of heat energy. This means that the energy of the products is lower than the energy of the reactants. Entropy is the measure of disorder in a system. It does not depend on the enthalpy and can increase or decrease during an exothermic process. Spontaneity is determined by looking at the change in Gibbs free energy, . Negative  corresponds to a spontaneous process and positive  corresponds to a nonspontaneous process. While the equation for Gibbs free energy, , involves change in enthalpy, the spontaneity also depends on temperature and change in entropy. Exothermic reactions release energy in the form of heat. This heat energy can be used to power other processes that require energy; therefore, exothermic reactions can facilitate active processes that consume energy.

Example Question #1 : Endothermic And Exothermic Reactions

In an __________ reaction, the products are more stable than the reactants; in an __________ reaction the reactants are more stable than the products.

Possible Answers:

exergonic . . . exergonic

endergonic . . . endergonic

exergonic . . . endergonic

endergonic . . . exergonic

Correct answer:

exergonic . . . endergonic

Explanation:

Exergonic reactions release energy; therefore, the energy of products is lower than that of the reactants. Endergonic reactions consume energy; therefore, the energy of products is greater than that of the reactants. In other words, exergonic reactions are spontaneous, while endergonic reactions are nonspontaneous, and require the net input of energy to drive the reaction.

Example Question #5 : Thermochemistry

Consider the following spontaneous reaction:

What can you conclude about the enthalpy change in this reaction?

Possible Answers:

It is positive

It is zero

It is negative

Cannot be determined from the given information

Correct answer:

It is negative

Explanation:

We need to use the following equation to answer this question:

Above,  is change in Gibbs free energy,  is change in enthalpy,  is temperature, and  is change in entropy. The question states that the reaction is spontaneous; therefore,  is negative. We can also determine the  for this reaction by looking at the phases of the products and reactants. Recall that entropy is a measure of disorder. When it comes to phases, gases have the highest entropy and solids have the lowest entropy. This is because in gases the molecules are spread out and have more room for disorder while solids are compressed and well packaged, decreasing the disorder of the atoms/molecules. Liquids are intermediate in entropy. In this reaction, we are creating a liquid from two molecules of gas; therefore, we are decreasing the entropy of the system (going to a more ordered, liquid state). The change in entropy for this reaction is negative.

Rearranging and solving the equation above for  gives us:

Since both  and  are negative,  will always be negative (regardless of temperature). This is an exothermic reaction.

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