Module 4.1 Energy Changes in Chemical Reactions

Equipment to determine the molar heat of combustion for methanol was set up.

After the results were obtained, the equipment was modified by removing the windshield and wooden block, before the investigation was repeated.

Which of the following predicts results for this investigation?

• The values obtained for both investigations would be the same.

• The value obtained for the first investigation would be higher than the value for the second one.

• The value obtained for the first investigation would be lower than the value obtained for the second one.

• There is no way of predicting the expected difference in the results obtained.

Substance X has a molar mass of 120 g mol^-1. When 14.3 grams of X is dissolved in 250 grams of water, the temperature of the water decreases by 8.6^oC.

Which of the values below is closest to the molar enthalpy of solution X?

• -75 kJ mol^-1

• -9 kJ mol^-1

• +75 kJ mol^-1

• +9 kJ mol^-1

The enthalpy changes in two different chemical reactions are graphed. The scales on the axes are the same.

Which of the following statements is true?

• Reaction 1 is exothermic and has a higher activation energy than Reaction 2.

• Reaction 1 is endothermic and has a higher activation energy than Reaction 2.

• Reaction 2 is exothermic and has a higher activation energy than Reaction 1.

• Reaction 2 is endothermic and has a higher activation energy than Reaction 1.

When one mole of calcium chloride (CaCl₂) dissolves in water, 81.0 kJ of heat energy is quickly released. A sample of calcium chloride is dissolved in 150 grams of water. The temperature of the water increases by 4.5^oC.

What mass of calcium chloride was dissolved?

• 0.387 g

• 3.87 g

• 3.19 kg

• 3.87 kg

When one mole of calcium chloride (CaCl₂) dissolves in water, 81.0 kJ of heat energy is quickly released. A sample of calcium chloride is dissolved in 150 grams of water. The temperature of the water increases by 4.5^oC.

Which of the following energy profile diagrams is most likely to match the dissolving of calcium chloride in water?

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• 
• 
• 

In the energy profile diagram, substance R can react to form product P₁ or product P₂. The reaction profiles for both substances are shown on the graph.

Compared to the reaction R → P₁, the reaction R → P₂ is which of the following?

• Slower and less endothermic

• Slower and more endothermic

• Faster and less endothermic

• Faster and more endothermic

Which of the following characteristics apply to the use of most catalysts in a chemical reaction?

• Slows a reaction by lowering the activation energy, making the enthalpy change higher.

• Speeds up a reaction by lowering the activation energy, making the enthalpy change lower.

• Speeds up a reaction with no effect on the activation energy, making the enthalpy change lower.

• Speeds up a reaction by lowering the activation energy, with no effect on the enthalpy change.

Examine the table of specific heats of four common substances.

Which of the following substances would need the smallest quantity of heat to change the temperature of 5 g of the substance by 10°C?

• I₂(s)

• H₂O(l)

• Au(s)

• He(g)

A reaction has the energy profile diagram shown.

Which of the following represents the energy profile of the REVERSE reaction?

• Final product energy = 40 kJ mol^-1

  ΔH = +10 kJ mol^-1

  Activation energy = 50 kJ

• Final product energy = 50 kJ mol^-1

  ΔH = +10 kJ mol^-1

  Activation energy = 60 kJ

• Final product energy = 50 kJ mol^-1

  ΔH = -10 kJ mol^-1

  Activation energy = 60 kJ

• Final product energy = 40 kJ mol^-1

  ΔH = -10 kJ mol^-1

  Activation energy = 50 kJ

Examine the energy profile for a chemical reaction, where I, II and III represent enthalpy changes during the reaction.

Which of the following statements is correct about this profile?

• The activation energy for the reverse reaction is III minus II.

• The energy required to break the reactant bonds is represented by II.

• The net energy released for the forward reaction is represented by II.

• The energy released by the formation of new bonds in the forward reaction is represented by I.