AP Chemistry - Unit 6 – Thermodynamics

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1. Which of the following is defined by the measure of the average kinetic energy?

• A. Potential Energy

• B. Velocity

• C. Heat

• D. Temperature

Answer explanation

Temperature is defined as the measure of the average kinetic energy of the sample.

Tags

NGSS.MS-PS1-4

NGSS.MS-PS3-4

2. What is the amount of energy is required to raise the temperature of 43 g of liquid water from 25°C to 55°C? Water's specific heat is 4.18 J/g°C.

• A. 5.39 kJ

• B. 5390 kJ

• C. 4.49 kJ

• D. 9.88 kJ

Answer explanation

Use the equation Q=mCΔT. The change in temperature (ΔT) is 30°C, the mass is 43 g, and the specific heat of liquid water is 4.18 J/g°C. There are 1000J for every kJ. Therefore, the hear is 5.39 kJ.

Tags

NGSS.HS-PS3-1

3. What is the amount of energy required to melt 7.2 g of water at 0°C? Water's enthalpy of fusion is 6.01 kJ/mol and water's enthalpy of vaporization is 40.7 kJ/mol.

• A. 16.3 kJ

• B. 43.3 kJ

• C. 293 kJ

• D. 2.40 kJ

Answer explanation

We must use water's enthalpy of fusion because that refers to the amount of energy required to melt 1 mol of water. The heat of vaporization refers to vaporizing 1 mol of water.

Tags

NGSS.HS-PS3-1

4. When this reaction occurs, 75.0 kJ of energy is released. What mass of copper is produced?

• A. 0.588 g

• B. 36.7 g

• C. 110 g

• D. 1.73 g

Answer explanation

1 mol of copper is produced for every 129.7 kJ of energy that is released by this reaction. Using this as a conversion factor, one finds that 0.588 mol of copper should be produced. Multiply by 63.55 g/mol, copper's molar mass, to find 36.7 g of copper is produced.

Tags

NGSS.HS-PS1-4

5. Which of the following statements is true regarding the following reaction?

• A. The reaction will cause the surrounding's temperature to increase because the reaction is exothermic.

• B. The reaction will cause the surrounding's temperature to decrease because the reaction is exothermic.

• C. The reaction will cause the surrounding's temperature to increase because the reaction is endothermic.

• D. The reaction will cause the surrounding's temperature to decrease because the reaction is endothermic.

Answer explanation

When enthalpy (ΔH) is negative, this indicates the reaction is exothermic. Therefore, the reaction will release energy to the surrounding causing the temperature to increase.

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-4

6. Determine the enthalpy of the reaction given the following data.

• A. 352.6 kJ/mol

• B. -352.6 kJ/mol

• C. 176.3 kJ/mol

• D. -176.3 kJ/mol

Answer explanation

Given the heats of formation we can use the "products minus reactants" equation. ΔH = [2x-268.6]-[2x-92.3]

Tags

NGSS.HS-PS1-4

7. Which of the following is an endothermic process?

• A. Combusting methane

• B. Freezing benzene

• C. Condensing water

• D. Melting Wax

Answer explanation

Consider the relative energy of the states of matter. Since solid has less energy than liquid, melting is the only endothermic process listed in the options.

8. What is the enthalpy of reaction depicted in the reaction below?

• A. -299.4 kJ/mol

• B. 149.7 kJ/mol

• C. -149.7 kJ/mol

• D. 299.4 kJ/mol

Answer explanation

In order to determine the enthalpy of reaction, one must use Hess's Law. Both reactions with given enthalpies must be halved. The second reaction also must be flipped. Once this is done, add up the enthalpies to determine the new one.

Tags

NGSS.HS-PS1-4

9. What is the final temperature of 15 g of nickel (C=0.44 J/g°C) that absorbs 240 J of energy at 15°C?

• A. 36°C

• B. -21°C

• C. 22°C

• D. 51°C

Answer explanation

Using Q=mCΔT, one finds the change of temperature to be 36°C. Add the initial temperature to find the final temperature of 51°C.

Tags

NGSS.HS-PS3-1