AP Chemistry



AP Chemistry Name ____________________________

Unit 4, Test Review Date ________________ Hour ____

The heating curve for a sample of pure ethanol is provided above. The temperature was recorded as a 50.0 g sample of solid ethanol was heated at a constant rate.

1. Which of the following explains why the slope of segment T is greater than the slope of segment R?

A) The specific heat capacity of gaseous ethanol is greater than the specific heat capacity of liquid ethanol.

B) The specific heat capacity of liquid ethanol is greater than the specific heat capacity of gaseous ethanol.

C) The heat of vaporization for ethanol is greater than the heat of fusion for ethanol.

D) The heat of fusion for ethanol is greater than the heat of vaporization for ethanol.

2. Which of the following explains why segment S is longer segment Q?

A) The specific heat capacity of gaseous ethanol is greater than the specific heat capacity of liquid ethanol.

B) The specific heat capacity of liquid ethanol is greater than the specific heat capacity of gaseous ethanol.

C) The heat of vaporization for ethanol is greater than the heat of fusion for ethanol.

D) The heat of fusion for ethanol is greater than the heat of vaporization for ethanol.

3. Aluminum metal can be recycled from scrap metal by melting the metal to evaporate impurities.

a. Calculate the amount of heat needed to purify 1.00 mole of Al originally at 298 K by

melting it. The melting point of Al is 933 K. The specific heat capacity is 0.89 J/g∙K, and the

heat of fusion of Al is 10.7 kJ/mol.

b. The equation for the overall process of extracting Al for Al2O3 is shown below. Which

requires less energy, recycling existing Al or extracting Al from Al2O3? Justify your answer

with a calculation.

Al2O3 (s) ( 2 Al (s) + 3/2 O2 (g) ΔH = 1675 kJ/molrxn

4. Use the reactions below to determine ΔH for this reaction: 3 C (s) + 4 H2 (g) ( C3H8 (g)

C (s) + O2 (g) ( CO2 (g) ΔH = -394 kJ

C3H8 (g) + 5 O2 (g) ( 3 CO2 (g) + 4 H2O (l) ΔH = -2220 kJ

H2 (g) + ½ O2 (g) ( H2O (l) ΔH = -286 kJ

5. Look at the overall equation that you solved for in problem 4.

a. What kind of reaction does this represent? How do you know?

b. Determine the mass of carbon that must react to release 1750 kJ of energy.

c. Determine the amount of energy released when 16.7 grams of hydrogen gas react.

6. The specific heat of lead is 0.129 J/g∙°C. Determine the amount of heat needed to raise the temperature of 4.55 grams of lead from 12.5°C to 26.8°C.

7. The specific heat of copper is 0.0385 J/g∙°C. Determine the mass of a sample of copper if it takes 250. J of energy to raise the temperature from 14.8°C to 29.2°C.

8. The following table lists the ∆Hf for some iron oxides.

|Compound |∆Hf |

|FeO(s) |- 272.0 kJ/mol |

|Fe2O3(s) |- 824.2 kJ/mol |

|Fe3O4(s) |- 1118.4 kJ/mol |

FeO(s) + Fe2O3(s) ( Fe3O4(s)

a. Use the formation enthalpies to determine ΔH for the above reaction.

b. What does the sign of ΔH indicate about the relative strengths of the bonds of the reactants compared to the bonds of the products?

9. 30.0 grams of aluminum that is 365 K is placed into 30.0 grams of water that is 298 K. Will the resulting temperature be closer to 298 K, closer to 365 K, or right in the middle? Explain.

10. A combustion reaction is performed in a calorimeter containing 75.0 mL of distilled water. If the combustion of 0.155 grams of ethane, C2H6, causes the water to increase in temperature from 23.5°C to 49.2°C, determine the ΔH for the combustion reaction in kJ/molethane. (Note: The reaction doesn’t happen IN the water. The calorimeter allows energy from the combustion to be transferred to the water. Your mass of surroundings is the mass of just the water.)

11. Use bond energies to estimate ΔH for this reaction: H2 (g) + Cl2 (g) ( 2 HCl (g)

|Bond Energies in kJ mol-1 |

|H – H 436 |

|Cl – Cl 243 |

|H – Cl 431 |

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