AP Chemistry : Thermodynamics

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #1 : Calorimetry, Specific Heat, And Calculations

The following is a list of specific heat capacities for a few metals.

\displaystyle \small C_{copper} = 0.385

\displaystyle \small C_{iron} = 0.444

\displaystyle \small C_{silver} = 0.240

\displaystyle \small C_{aluminum} = 0.900

A 50g sample of an unknown metal is heated with 800 joules. If the temperature of the metal increases by 41.6oC, what is the identity of the unknown metal?

Possible Answers:

Iron

Silver

Copper

Aluminum

Correct answer:

Copper

Explanation:

We need to find the specific heat of the unknown sample of metal in order to locate it on the list. We can do this by using the equation that allows us to determine the specific heat capacity of an element.

\displaystyle \small q = mC\Delta T

Since we know the change in temperature, we can simply plug in the values and solve for the value of \displaystyle \small C.

\displaystyle \small 800J = (50g)C(41.6^oC)

\displaystyle \small C = 0.385

Going back to the list, we see that this is the specific heat capacity for copper, so we confirm that the unknown metal is copper. 

Example Question #1 : Calorimetry, Specific Heat, And Calculations

In which instance would a bomb calorimeter be more useful than a coffee-cup calorimeter?

Possible Answers:

When measuring the heat capacity of a solid

When measuring the specific heat of water vapor

When measuring the mass of a substance

None of the above

Correct answer:

When measuring the specific heat of water vapor

Explanation:

Bomb calorimeters are most useful when dealing with a gas, because they can operate well at high pressures. Coffee-cup calorimeters are not useful when water begins to boil, producing vapor.

Example Question #1 : Thermodynamics

How much heat does it take to heat 100g ice at 0C to boiling point?

Cice= 2.1 J/goC

Cwater= 4.2 J/goC

ΔHvap= 2260 J/g

ΔHfus=334 J/g

Possible Answers:

33.4 kJ

75.4 kJ

300.5 kJ

42 kJ

55.4 kJ

Correct answer:

75.4 kJ

Explanation:

You need heat for the phase change, using the enthalpy of fusion (100g*334 J/g = 33400 J). Add to this the heat to get to boiling point using the specific heat of water (100g*100C*4.2 J/goC = 42000 J). Totalling 75400 J (75.4 kJ)

Example Question #1 : Thermodynamics

The specific heat capacity of an unknown liquid is 0.32\frac{J}{kg\cdot K}\displaystyle 0.32\frac{J}{kg\cdot K}. The density of the liquid is 0.0321 \frac{g}{mL}\displaystyle 0.0321 \frac{g}{mL} If a chemist applies 243 J of heat to 300 mL of this liquid starting at 27.1^{\circ}C\displaystyle 27.1^{\circ}C, what is the final temperature?

Possible Answers:

78882\hspace{1 mm}K\displaystyle 78882\hspace{1 mm}K

None of the available answers

270.1^{\circ}C\displaystyle 270.1^{\circ}C

2700.1^{\circ}C\displaystyle 2700.1^{\circ}C

78882^{\circ}C\displaystyle 78882^{\circ}C

Correct answer:

78882^{\circ}C\displaystyle 78882^{\circ}C

Explanation:

First we will determine the mass of the liquid:

300\hspace{1 mm}mL\times\frac{0.0321\hspace{1 mm}g}{1\hspace{1 mm}mL}=9.63\hspace{1 mm}g\displaystyle 300\hspace{1 mm}mL\times\frac{0.0321\hspace{1 mm}g}{1\hspace{1 mm}mL}=9.63\hspace{1 mm}g

Now we will examine the relationship between heat and specific heat capacity:

Q=cm\Delta T\displaystyle Q=cm\Delta T

Where Q\displaystyle Q is heat in Joules, c\displaystyle c is the specific heat capacity, m\displaystyle m is the mass and \Delta T\displaystyle \Delta T is the change in temperature. We can rearrange this

\Delta T=\frac{Q}{cm}\displaystyle \Delta T=\frac{Q}{cm}

\Delta T=\frac{243\hspace{1 mm}J}{0.32J\cdot kg^{-1}\cdot K^{-1}0.00963kg}=78855\hspace{1 mm}K\displaystyle \Delta T=\frac{243\hspace{1 mm}J}{0.32J\cdot kg^{-1}\cdot K^{-1}0.00963kg}=78855\hspace{1 mm}K

If we begin at 27.1^{\circ}C\displaystyle 27.1^{\circ}C, we will end at 78882^{\circ}C\displaystyle 78882^{\circ}C

 

Example Question #1 : Thermodynamics

A 50g sample of a metal was heated to \displaystyle 95^oC then quickly transferred to an insulated container containing 50g of \displaystyle H_2O at \displaystyle 25^oC.  The final temperature of the \displaystyle H_2O was \displaystyle 30^oC.

Which of the following can be concluded?

Possible Answers:

The specific heat of the metal is greater than that of the water

The water gained an amount of thermal energy that was more than the amount of thermal energy lost by the metal

The metal lost an amount of thermal energy that was more than the amount of thermal energy gained by the water

The specific heat of the water is greater than that of the metal

None of the other answers

Correct answer:

The specific heat of the water is greater than that of the metal

Explanation:

When the heated metal is placed in the container of the cooler water there will be a transfer of thermal energy from the metal to the water.  This transfer will occur towards an equilibrium of thermal energy in the water and in the metal. Thus we can conclude that the amount of thermal energy lost by the metal will equal the amount of thermal energy gained by the water.  However we notice that the water increases by only 5oC  and the metal decreases by 65oC.  This is becasue of the difference of the specific heats of these substances.  The specific heat capacity of a substance is the heat required to increase the temperature of 1g of a substance by 1oC. The metal can be conluded to have a smaller specific heat than the water because the same amount of energy transfer led to a much larger change in termperature for the metal as compared to the water.  

Example Question #1 : Calorimetry, Specific Heat, And Calculations

Which of the following is the correct molar specific heat of water used when making calculations involving a calorimeter?

Possible Answers:

418.4 J/gK

4.184 J/gK

41.84 J/gK

0.4184 J/gK

Correct answer:

4.184 J/gK

Explanation:

4.184 J/gK is the cited value for the specific heat of water and should be memorized. This is used during calorimeter calculations, specifically when using the equation q= mc delta(T).

Example Question #2 : Thermodynamics

\displaystyle \small C_{ice} = 0.5 \frac{cal}{g^{\circ}C}

\displaystyle \small C_{water} = 1.0\frac{cal}{g^{\circ}C}

\displaystyle \small \Delta H_{fusion} = 80\frac{cal}{g}

How much energy is needed to raise the temperature of five grams of ice from \displaystyle \small -10^oC to \displaystyle \small 35^oC?

Possible Answers:

\displaystyle \small 200cal

\displaystyle \small 600 cal

\displaystyle \small 120cal

\displaystyle \small 280cal

Correct answer:

\displaystyle \small 600 cal

Explanation:

This question involves the total energy needed for three different processes: the temperature raise from \displaystyle \small -10^oC to \displaystyle \small 0^oC, the melting of the ice, and the temperature raise from \displaystyle \small 0^oC to \displaystyle \small 35^oC. For the first and third transitions we will use the equation \displaystyle \small q=mC\Delta T. For the melting of ice, we will use the equation \displaystyle q=m\Delta H_{fusion}.

1. \displaystyle \small \small q =m_{ice}C_{ice}\Delta T = (5g)(0.5\frac{cal}{g^{\circ}C})(10^{\circ}C) = 25 cal

2. \displaystyle \small q = m\Delta H_{fusion} = (5g)(80\frac{cal}{g}) = 400 cal

3. \displaystyle \small q = m_{water}C_{water}\Delta T = (5g)(1.0\frac{cal}{g^{\circ}C})(35^{\circ}C) = 175 cal

Finally, we will need to sum the energy required for each step to find the total energy.

\displaystyle 25cal+400cal+175cal=600cal

Example Question #1 : Calorimetry, Specific Heat, And Calculations

You want to prepare a cup of tea. To do so, you pour \displaystyle 100\,mL of tap water at \displaystyle 20^{0}C in a cup that does not absorb microwave radiation and heat it in a microwave oven at \displaystyle 700\,W  of power. If you assume a density of  \displaystyle 1.00\, \frac{g}{mL}  for the water and know that its specific heat capacity is \displaystyle 4.184\, \frac{J}{g^{o}C} , what time do you need to set in the microwave oven to heat the water to \displaystyle 60^{o}C?

Possible Answers:

\displaystyle 24s

\displaystyle 700W of power is not enough to heat that a amount of water to the desired temperature

\displaystyle 23.9s

 \displaystyle 40s

Correct answer:

\displaystyle 23.9s

Explanation:

\displaystyle Q=m\cdot C\cdot\Delta T

Since the density of water is \displaystyle 1.00\, \frac{g}{mL}, the mass of \displaystyle 100\,mL is \displaystyle 100\,g. Plug in known values to the equation and solve.

\displaystyle 100g\cdot\frac{4.184J}{g^{\text o}C}\cdot \left ( 60 - 20 \right )^{\text o}C=16736J

Use the formula below to find the time needed to heat up the sample of water in the microwave:

\displaystyle P=\frac{\Delta W}{\Delta t}

\displaystyle \Delta t=\frac{16736J}{700W}=23.9s

Our answer must contain three significant figures.

Example Question #1 : Thermodynamics

How much heat is needed to raise \displaystyle 5 grams of aluminum by \displaystyle 20^{o}\:C?

\displaystyle C_{aluminum}=0.900\: \frac{J}{g\cdot^{o}\textrm{C}}.

Possible Answers:

\displaystyle 45\:J

\displaystyle 90\:J

\displaystyle 9\:J

\displaystyle 120\:J

Correct answer:

\displaystyle 90\:J

Explanation:

To find the amount of heat needed to change the temperature of a given material by a certain amount, we'll need to use the equation for specific heat. The specific heat capacity of a compound represents the amount of energy necessary to raise \displaystyle 1 gram of that substance by \displaystyle 1\: ^{o}C.

\displaystyle q=mC\Delta T

\displaystyle q=(5\: grams)(0.900\: \frac{J}{g\cdot ^{o}C})(20\: ^{o}C)=90\:J

Example Question #2 : Thermodynamics

A 20g sample of iron at a temperature of \displaystyle 120^oC is placed into a container of water. There are 300 milliliters of water in the container at a temperature of \displaystyle 30^oC

\displaystyle \small C_{iron} = 0.444\frac{J}{g^\circ C}

\displaystyle \small C_{water} = 4.184\frac{J}{g^\circ C}

\displaystyle \small \rho_{water} = 1\frac{g}{mL}

What is the final temperature of the water?

Possible Answers:

\displaystyle \small 30.63^{\circ}C

\displaystyle \small 114.0^{\circ}C

\displaystyle 32.71^oC

\displaystyle \small 34.87^{\circ}C

\displaystyle \small 36.0^{\circ}C

Correct answer:

\displaystyle \small 30.63^{\circ}C

Explanation:

There are two things to note before solving for the final temperature.

1. The density of water allows us to say that 300 milliliters of water is the same thing as 300 grams of water.

\displaystyle 300mL*\frac{1g}{1mL}=300g

2. Since the heat from the iron is being transferred to the water, we can say that the heat transfer is equal between both compounds. Since the heat is conserved in the system, we can set the two equations equal to one another.

\displaystyle q=mC\Delta T

\displaystyle q_i=q_w\rightarrow m_iC_i\Delta T=m_wC_w\Delta T

\displaystyle \small m_{i}C_{i}(T_{initial}-T_{final}) = m_{w}C_{w}(T_{final}-T_{initial})

Notice how the change in temperature for iron has been flipped in order to avoid a negative number.

\displaystyle \small (20g)(0.444\frac{J}{g^{\circ}C})(120^{\circ}C-T_{f}) = (300g)(4.184\frac{J}{g^{\circ}C})(T_{f}-30^{\circ}C)

\displaystyle \small T_{f} = 30.63^{\circ}C

Because water has a much higher heat capacity compared to iron, the temperature of the water is not changed significantly.

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