College Chemistry : Thermochemistry and Changes of State

Study concepts, example questions & explanations for College Chemistry

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Example Questions

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Example Question #1 : Solutions, States Of Matter, And Thermochemistry

Which of the following is true of a closed system? 

Possible Answers:

A closed system allows for the exchange of energy between the system and its surroundings, but does not allow the exchange of matter.  

A closed system does not allow for the exchange of matter or energy between the system and its surroundings. 

A closed system allows for the exchange of matter between the system and its surroundings, but does not allow the exchange of energy.  

A closed system allows for the exchange of matter and energy between the system and its surroundings. 

A closed system contains no energy or matter and does not allow for the exchange of matter or energy between the system and its surroundings.  

Correct answer:

A closed system allows for the exchange of energy between the system and its surroundings, but does not allow the exchange of matter.  

Explanation:

A closed system allows for the exchange of energy between the system and its surroundings, but does not allow the exchange of matter. This is the definition of a closed system. An open system allows for the exchange of both matter and energy between the system and its surroundings. An isolated system on the other hand does not allow the exchange of either matter or energy between the system and its surroundings.

Example Question #1 : Solutions, States Of Matter, And Thermochemistry

When  of liquid octane  undergoes combustion in a bomb calorimeter, the temperature increases from  degrees Celsius to  degrees Celsius. The heat capacity for the bomb calorimeter is . Find the  for the combustion of octane in .

Possible Answers:

Correct answer:

Explanation:

Recall the following equation:

Now, since the bomb calorimeter keeps the volume constant, we know the following relationship:

Thus, we can then write the following equation for :

Start by finding :

From this, we know that 

Now, find .

Your answer must have  significant figures.

Example Question #1 : Thermochemistry And Changes Of State

How much heat energy is needed to raise the temperature of  of copper from  to ? The specific heat capacity of copper is .

Possible Answers:

Correct answer:

Explanation:

In this question, we're given the mass of copper, along with its specific heat capacity, and we're asked to determine the amount of heat energy necessary to increase its temperature by a given amount.

To solve this problem, we'll need to make use of the following equation.

Since we know what the values are for the mass and specific heat, we'll need to figure out what the temperature will be. Since the Kelvin and Celsius temperature scales both change by the same amount and only differ at their zero point, we can take the difference of the temperatures in degrees Celsius and use that value (since it will be equivalent to the change in the Kelvin temperature as well).

Plugging this information into the first expression, we can solve for the amount of heat energy that will bring this mass of copper to the desired temperature.

Example Question #3 : Solutions, States Of Matter, And Thermochemistry

Use average bond enthalpies to estimate the enthalpy change, in kilojoules, of the combustion of one mole of hexane  to form liquid water and carbon dioxide gas.

Type of Bond Average bond enthalpy 
Carbon-hydrogen single bond 414
Carbon-oxygen double bond 736
Oxygen-oxygen double bond 498
Oxygen-hydrogen single bond 464
Carbon-carbon single bond 347
Possible Answers:

Correct answer:

Explanation:

Start by writing the balanced equation for the combustion of hexane:

Recall the following equation:

Remember that energy is required to break bonds, so  should be positive. Energy is also released when bonds are made so  should be negative.

Next, draw out the Lewis structures of each molecule to figure out the number of bonds made or broken.

For the hexane, there are   bonds that must be broken, and   bonds that must be broken.

1

For each oxygen, there is only 1 oxygen double bond to break. However, we will need to multiply this number by its stoichiometric coefficient.

3

For each carbon dioxide, there are  double bonds between carbon and oxygen to break. For the given equation, there are  total of these double bonds because we have  moles of carbon dioxide.

2

For each water, there are  single bonds between hydrogen and oxygen. For the given equation, there are a total of  of these bonds because we have  moles of water.

Now, use the given information regarding to average bond enthalpies to find the change in enthalpy for the reaction.

Example Question #1 : Specific Heat Capacity

 block of silver initially at  absorbs  of heat. If the specific heat capacity of silver is , in degrees Celsius, what is the final temperature of the block of silver?

Possible Answers:

Correct answer:

Explanation:

Recall the equation that gives the relationship between change in temperature and amount of heat:

where 

, and

Since the question asks for the final temperature, re-arrange the equation to solve for .

Substitute in the given values to solve for the final temperature.

Example Question #2 : Specific Heat Capacity

The specific heat capacity is defined as the amount of heat energy necessary to change a given amount of a substance by a certain temperature. Which of the following correctly expresses the units of specific heat capacity?

Possible Answers:

Correct answer:

Explanation:

For this question, we're given a definition for the specific heat capacity of a substance and we're asked to identify the correct units for this term.

We can also recall the equation that relates all of these terms.

Rearranging this expression to isolate the term for the specific heat capacity gives us the following.

Next, we can recall what units would be appropriate to use for each of the following terms in the above expression. The  term represents heat energy added to or removed from the system, so this value would be in units of joules. Next, the  term represents the mass of the substance, so grams can be used for this term. Finally, the  term represents the change in temperature of the substance, so we can use the absolute temperature in kelvins here.

Putting all this together gives us the following.

Example Question #1 : Thermochemistry And Changes Of State

How much heat is needed to raise the temperature of 10.0 g of water from 10.0 oC to 35.0 oC?

Specific heat capacity for water is 

Possible Answers:

Correct answer:

Explanation:

Recall the relationship between heat and specific heat capacity

Plug in known values and solve for Q

Example Question #1 : Solutions, States Of Matter, And Thermochemistry

An insulated container is filled with 50.0 g of water at 15.0 oC. 120.0 g of lead is heated to 100 oC and added to the insulated container. What is the final temperature of the system once it comes to equilibrium?

Specific heat of water is 

Specific heat of lead is 

Possible Answers:

Correct answer:

Explanation:

Since Pb starts at a higher temperature than the water, we know that energy (in the form of heat) will be transferred from Pb to water. Due to the law of conservation of energy, the exact same amount of energy lost by Pb must be gained by water.

Recall that

Combining the two equations, we have

Combine like terms then solve for final temperature

Example Question #1 : Solutions, States Of Matter, And Thermochemistry

How much heat does it require to make a  block of lead initially at  go to ? The specific heat capacity of lead is .

Possible Answers:

Correct answer:

Explanation:

Recall the equation that gives the relationship between the change in temperature and the amount of heat:

, where

,

, and

Substitute in the given values to find how much heat is required to increase the temperature of the block of lead the specified amount.

Make sure to round the answer to three significant figures.

Example Question #2 : Thermochemistry And Changes Of State

Consider the typical phase diagram of a compound given below.

Phase diagram

Which of the following lines or points on the diagram represents a situation in which the rate of vaporization of the compound is equal to its rate of condensation?

Possible Answers:

Line B

Point D

Line A

Line C

None of these

Correct answer:

Line C

Explanation:

In this question, we're presented with a phase diagram and are asked to determine where on the graph the rate of vaporization equals the rate of condensation.

First, it's important to realize that when the rate of vaporization and condensation are equal, we have an equilibrium of liquid and gas phases. In other words, for a given temperature and pressure, the rate at which the liquid evaporates into a gas is exactly equal to the rate at which the gas condenses into a liquid.

On a phase diagram, the area of the upper left portion of the diagram represents the solid state. The middle portion of the diagram represents the liquid state. The bottom and right most part of the diagram represents the gas phase.

Furthermore, each line on the diagram represents the specific combination of temperature and pressure in which a given compound will exist in equilibrium between two phases. The point where all three lines intersect, however, represents the triple point. This tells us the temperature and pressure in which the compound will exist in an equilibrium between all three states.

Because we are looking for the equilibrium line that represents equilibrium of vaporization and condensation, we want the line that separates the liquid portion of the diagram from the gas portion. Based on the identification of regions on the diagram discussed above, that would be line C as shown in the diagram. Line A represents equilibrium between solid and gas (sublimation rate = deposition rate). Line B represents equilibrium between solid and liquid (melting rate = freezing rate).

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