Q: A student uses a coffee cup calorimeter (as shown in the image) to measure the heat of a reaction. Which equation should they use to calculate the heat absorbed by the solution?

$$q_{soln} = m_{soln} \times C_{s,soln} \times \Delta T$$

Q: Given the reaction: $$\mathrm{C_3H_8 (g) + 5 O_2$ (g) \rightarrow 3 CO_2$ (g) + 4 H_2O (g)}$$ with $$\Delta H_{rxn} = -2044\ \mathrm{kJ}$$, how much heat is released when 13.2 kg of propane ($$\mathrm{C_3H_8}) is $$combusted?

$$-6.12 \times 10^5$\ \mathrm{kJ}$$

Q: Which of the following equations correctly expresses the relationship for calculating the enthalpy change of a reaction using standard enthalpies of formation?

$$\Delta H_{rxn}^\circ = \sum n_p \Delta H_f^\circ (\text{products}) - \sum n_r \Delta H_f^\circ (\text{reactants})$$

Question 1

Which of the following best describes the process occurring in the image of the boiling pot of water?

Accepted Answer

An endothermic process, because heat is absorbed to convert liquid water to vapor.

Question 2

A student uses a coffee cup calorimeter (as shown in the image) to measure the heat of a reaction. Which equation should they use to calculate the heat absorbed by the solution?

Accepted Answer

$$q_{soln} = m_{soln} 	imes C_{s,soln} 	imes \Delta T$$

Question 3

Which of the following statements about Hess’s Law is correct?

Accepted Answer

The enthalpy change for a reaction is the sum of the enthalpy changes for each step in the process.

Question 4

Given the reaction: $$\mathrm{C_3H_8 (g) + 5 O_2$ (g) \rightarrow 3 CO_2$ (g) + 4 H_2O (g)}$$ with $$\Delta H_{rxn} = -2044\ \mathrm{kJ}$$, how much heat is released when 13.2 kg of propane ($$\mathrm{C_3H_8}) is $$combusted?

Accepted Answer

$$-6.12 	imes 10^5$\ \mathrm{kJ}$$

Question 5

Which of the following is true about the standard enthalpy of formation ($$\Delta H_f^\circ$$)?

Accepted Answer

It is zero for any element in its standard state.

Question 6

Which process is represented by the image of a burning fire?

Accepted Answer

Combustion, an exothermic reaction.

Question 7

If the reaction $$\mathrm{A + 2B \rightarrow C}$$ has $$\Delta H_1$, and $$\mathrm{C \rightarrow 2D}$$ has $$\Delta $$H_2$$, what is the enthalpy change for $$\mathrm{A + 2B \rightarrow 2D}$$?

Accepted Answer

$$\Delta H_1$ + \Delta H_2$

Question 8

Which of the following equations correctly expresses the relationship for calculating the enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

$$\Delta H_{rxn}^\circ = \sum n_p \Delta H_f^\circ (	ext{products}) - \sum n_r \Delta H_f^\circ (	ext{reactants})$$

Question 9

A reaction is reversed. What happens to the sign of $$\Delta H_{rxn}$$?

Accepted Answer

It changes sign.

Question 10

Which of the following is an example of an endothermic process?

Accepted Answer

Boiling water

Question 11

Given the reaction $$2 \mathrm{Al(s)} + \mathrm{Fe_2O_3(s)} \rightarrow \mathrm{Al_2O_3(s)} + 2 \mathrm{Fe(s)}$$, and the following standard enthalpies of formation: $$\Delta H_f^\circ (\mathrm{Al_2O_3}) = -1676.7\ \mathrm{kJ/mol}$$, $$\Delta H_f^\circ (\mathrm{Fe_2O_3}) = -826.0\ \mathrm{kJ/mol}$$, $$\Delta H_f^\circ (\mathrm{Al}) = 0.0$$, $$\Delta H_f^\circ (\mathrm{Fe}) = 0.0$$, what is $$\Delta H_{rxn}^\circ$$?

Accepted Answer

$$-850.7\ \mathrm{kJ}$$

Question 12

Which of the following best describes the process of dissolving salt in water (as shown in the image)?

Accepted Answer

It can be either endothermic or exothermic, depending on the salt.

Question 13

If a chemical equation is multiplied by a factor of 3, what happens to $$\Delta H_{rxn}$$?

Accepted Answer

$$\Delta H_{rxn} is $$multiplied by 3.

Question 14

Which of the following is the correct balanced equation for the standard heat of formation of glucose ($$\mathrm{C_6H_{12}O_6$}$$)?

Accepted Answer

$$6\ \mathrm{C(s, graphite)} + 6\ \mathrm{H_2$(g)} + 3\ \mathrm{O_2$(g)} \rightarrow \mathrm{C_6H_{12}O_6$(s)}$$

Question 15

A student calculates $$\Delta H_{rxn}$$ for a reaction using the following data: $$\Delta H_f^\circ (NH$_3(g$$)) = $$-45.9 kJ/$$mol, $$\Delta H_f^\circ (NO(g$$)) = $$+91.3 kJ/$$mol, $$\Delta H_f^\circ (H$_2O(g$$)) = $$-241.8 kJ/$$mol. For the reaction $$4\ \mathrm{NH_3$(g)} + 5\ \mathrm{O_2$(g)} \rightarrow 4\ \mathrm{NO(g)} + 6\ \mathrm{H_2O(g)}$$, what is $$\Delta H_{rxn}^\circ$$?

Accepted Answer

$$-902\ \mathrm{kJ}$$

Question 16

Which of the following best explains why the freezing of water is considered an exothermic process?

Accepted Answer

Energy is released to the surroundings as water molecules form stronger hydrogen bonds.

Question 17

The image of the thermite reaction (Al + Fe$$_2O$_3) is an $$example of:

Accepted Answer

A highly exothermic reaction used in welding and metal cutting.

Question 18

Which of the following is true about the enthalpy change ($$\Delta H) $$for a chemical reaction?

Accepted Answer

$$\Delta H is $$negative for exothermic reactions and positive for endothermic reactions.

lecture 9 (ns) Thermochemistry II: Hess’s Law and Heat of Formation

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