Q: A liquid propane tank contains 14.0 kg of propane ($$\mathrm{C_3H_8}). $$How much heat (in kJ) is liberated if all the propane combusts? ($$\mathrm{Molar\ mass = 44.09\ g/mol}$$, $$\Delta H_{rxn} = -2044\ \mathrm{kJ/mol})$$

$$6.49 \times 10^5$\ \mathrm{kJ}$$

Q: Which equation is used to calculate the standard 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 statements best describes an exothermic reaction?

Accepted Answer

A reaction in which heat is released to the surroundings.

Question 2

Given the reaction $$\mathrm{C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g)}$$ with $$\Delta H_{rxn} = -2044\ \mathrm{kJ/mol}$$, what does the negative sign of $$\Delta H_{rxn}$$ indicate?

Accepted Answer

The reaction is exothermic and releases heat.

Question 3

A liquid propane tank contains 14.0 kg of propane ($$\mathrm{C_3H_8}). $$How much heat (in kJ) is liberated if all the propane combusts? ($$\mathrm{Molar\ mass = 44.09\ g/mol}$$, $$\Delta H_{rxn} = -2044\ \mathrm{kJ/mol})$$

Accepted Answer

$$6.49 	imes 10^5$\ \mathrm{kJ}$$

Question 4

Which formula correctly relates the heat ($$q) $$absorbed or released by a substance to its mass, specific heat, and temperature change?

Accepted Answer

$$q = mC\Delta T$$

Question 5

A 1.0 g sample of Fe(s) and a 1.0 g sample of H$$_2O(l) $$both absorb 10.0 J of heat. Which sample experiences the greatest temperature change? (Specific heat of Fe = 0.45 J/g·K, H$$_2O = 4.18 J/g$$·K)

Accepted Answer

Fe(s), because it has a lower specific heat.

Question 6

How much heat energy is required to raise the temperature of 4.00 g of water by 8.00 °C? (Specific heat of water = 4.184 J/g·°C)

Accepted Answer

$$133\ \mathrm{J}$$

Question 7

In a coffee cup calorimeter, 50.0 mL of 1.00 M HCl reacts with 50.0 mL of 1.00 M NaOH. The temperature of the solution increases. What does this indicate about the reaction?

Accepted Answer

The reaction is exothermic.

Question 8

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

Accepted Answer

It is zero for elements in their most stable form at 1 atm and 25°C.

Question 9

Which equation is used to calculate the standard 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 10

If the enthalpy of fusion for water is $$+6.01\ \mathrm{kJ/mol}$$ and the enthalpy of vaporization is $$+44.0\ \mathrm{kJ/mol}$$, what is the enthalpy change for subliming 1 mole of ice?

Accepted Answer

$$+50.0\ \mathrm{kJ/mol}$$

Question 11

Which of the following changes will reverse the sign of $$\Delta H$$ for a reaction?

Accepted Answer

Reversing the reaction direction.

Question 12

Using Hess's Law, what is the enthalpy change for the reverse of the reaction $$\mathrm{H_2O(l) \rightarrow H_2O(g)} if$$ $$\Delta H = +44.0\ \mathrm{kJ/mol}$$ for vaporization?

Accepted Answer

$$-44.0\ \mathrm{kJ/mol}$$

Question 13

Which statement about bond enthalpy is correct?

Accepted Answer

Breaking bonds requires energy; forming bonds releases energy.

Question 14

Estimate the enthalpy change ($$\Delta H) $$for the reaction $$\mathrm{H_2$(g) + Cl_2$(g) \rightarrow 2HCl(g)}$$ using the following average bond enthalpies: $$\mathrm{H-H} = 436\ \mathrm{kJ/mol}$$, $$\mathrm{Cl-Cl} = 243\ \mathrm{kJ/mol}$$, $$\mathrm{H-Cl} = 431\ \mathrm{kJ/mol}.$$

Accepted Answer

$$-242\ \mathrm{kJ}$$

Question 15

Which of the following is an intensive property?

Accepted Answer

Specific heat capacity

Question 16

A reaction has $$\Delta H = +601\ \mathrm{kJ/mol}. $$Which term best describes this process?

Accepted Answer

Endothermic

Question 17

If $$5.60 	imes 10^3$ kJ of heat is liberated, what mass (g) of CO_2$ is released into the atmosphere? (Molar mass = 44.01 g/mol, $$\Delta H_{rxn} = -2044$$ kJ/mol for propane combustion)

Accepted Answer

$$3610\ \mathrm{g}$$

Question 18

Which of the following is the correct chemical equation for the standard formation of HCl(g) from its elements?

Accepted Answer

$$\mathrm{\frac{1}{2}H_2$(g) + \frac{1}{2}Cl_2$(g) \rightarrow HCl(g)}$$

Thermochemistry: Enthalpy, Calorimetry, Hess's Law, and Bond Enthalpy

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