Question 1

Which of the following statements best describes an exothermic reaction?

Accepted Answer

The enthalpy change $$\Delta H is $$negative.

Question 2

A 45.0 mL sample of water is heated from 15.0°C to 35.0°C. How many joules of energy have been absorbed by the water? (Assume the density of water is 1.00 g/mL and $$c_{water} = 4.184 \ \mathrm{J} \cdot \mathrm{g}^{-1} \cdot ^\circ\mathrm{C}^{-1})$$

Accepted Answer

$$1883 \ \mathrm{J}$$

Question 3

If $$5430 \ \mathrm{J} of $$energy is used to heat $$1.25 \ \mathrm{L} of $$water at $$23.0^\circ\mathrm{C}$$, what is the final temperature of the water? ($$c_{water} = 4.184 \ \mathrm{J} \cdot \mathrm{g}^{-1} \cdot ^\circ\mathrm{C}^{-1}$$, density = $$1.00 \ \mathrm{g}/\mathrm{mL})$$

Accepted Answer

$$24.0^\circ\mathrm{C}$$

Question 4

A $$100.0 \ \mathrm{g}$$ sample of aluminum ($$c_{Al} = 0.900 \ \mathrm{J} \cdot \mathrm{g}^{-1} \cdot ^\circ\mathrm{C}^{-1}) at $$boiling water temperature is added to an insulated cup containing $$50.0 \ \mathrm{g} of $$water at $$5.0^\circ\mathrm{C}. $$What will be the final temperature of the mixture? ($$c_{water} = 4.184 \ \mathrm{J} \cdot \mathrm{g}^{-1} \cdot ^\circ\mathrm{C}^{-1})$$

Accepted Answer

$$25.1^\circ\mathrm{C}$$

Question 5

How much energy (in kJ) is absorbed by $$45.0 \ \mathrm{g} of $$ice as it melts? (Latent heat of fusion $$= 334 \ \mathrm{J}/\mathrm{g})$$

Accepted Answer

$$15.03 \ \mathrm{kJ}$$

Question 6

How much energy is needed to heat $$45 \ \mathrm{g} of $$water from $$10^\circ\mathrm{C} to$$ $$105^\circ\mathrm{C}$$? ($$c_{water} = 4.184 \ \mathrm{J} \cdot \mathrm{g}^{-1} \cdot ^\circ\mathrm{C}^{-1}$$, Latent heat of vaporization $$= 2260 \ \mathrm{J}/\mathrm{g})$$

Accepted Answer

$$101,280 \ \mathrm{J}$$

Question 7

When $$10.0 \ \mathrm{g} of$$ $$C_6H_{12} is $$burned, $$453 \ \mathrm{kJ} of $$energy is released. What is the $$\Delta H_{combustion}$$ for $$C_6H_{12}$$? (Molar mass $$= 84.0 \ \mathrm{g}/\mathrm{mol})$$

Accepted Answer

$$-3807 \ \mathrm{kJ}/\mathrm{mol}$$

Question 8

When $$10.0 \ \mathrm{g} of $$aluminum melts, $$3929 \ \mathrm{J} of $$energy is required. What is the $$\Delta H_{fus} of Al$$? (Molar mass $$= 27.0 \ \mathrm{g}/\mathrm{mol})$$

Accepted Answer

$$10.6 \ \mathrm{kJ}/\mathrm{mol}$$

Question 9

Given the following reactions and their enthalpy changes:
(1) $$3Fe_2O_3(s) + CO(g) \rightarrow 2Fe_3O_4(s) + CO_2$(g)$$, $$\Delta H = -47 \ \mathrm{kJ}
(2$$) $$Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g)$$, $$\Delta H = -25 \ \mathrm{kJ}
(3$$) $$Fe_3O_4(s) + CO(g) \rightarrow 3FeO(s) + CO_2$(g)$$, $$\Delta H = 19 \ \mathrm{kJ}$$
Calculate the standard enthalpy change for $$FeO(s) + CO(g) \rightarrow Fe(s) + CO_2$(g).$$

Accepted Answer

$$-1 \ \mathrm{kJ}$$

Question 10

Given the following standard enthalpies of formation:
$$CO(g): -393.5 \ \mathrm{kJ}/\mathrm{mol}$$
$$H_2O(g): -241.8 \ \mathrm{kJ}/\mathrm{mol}$$
$$C_5H_{12}(l): -173.1 \ \mathrm{kJ}/\mathrm{mol}$$
$$C_2H_5OH(l): -277.6 \ \mathrm{kJ}/\mathrm{mol}$$
Calculate the $$\Delta H_{combustion} of $$ethyl alcohol, $$C_2H_5OH(l).$$

Accepted Answer

$$-1367.8 \ \mathrm{kJ}/\mathrm{mol}$$

Question 11

Knowing that the $$\Delta H_{combustion} of $$octane, $$C_8H_{18}$$, is $$-5508.9 \ \mathrm{kJ}/\mathrm{mol}$$, calculate the $$\Delta H_f of $$octane given the following enthalpies of formation:
$$CO_2(g$$): -393.5 \ \mathrm{kJ}/\mathrm{mol}$$
H_2O(g$$): -241.8 \ \mathrm{kJ}/\mathrm{mol}$$

Accepted Answer

$$-249.9 \ \mathrm{kJ}/\mathrm{mol}$$

Question 12

A student mixes $$100.0 \ \mathrm{mL} of$$ $$1.0 \ \mathrm{M}$$ HCl and $$100.0 \ \mathrm{mL} of$$ $$0.500 \ \mathrm{M} Ca(OH$$)$$_2. It is $$determined that $$0.870 \ \mathrm{kJ} of $$energy is absorbed. Is the reaction endothermic or exothermic?

Accepted Answer

Endothermic, because energy is absorbed.

Question 13

In a calorimetry experiment, $$0.25 \ \mathrm{g} of $$MgO is added to $$100.0 \ \mathrm{mL} of$$ $$1.0 \ \mathrm{M}$$ HCl. The temperature of the solution increases from $$25.0^\circ\mathrm{C} to$$ $$29.1^\circ\mathrm{C}. $$What is the thermal energy change $$q$$ for the solution? ($$c_{solution} = 4.184 \ \mathrm{J} \cdot \mathrm{g}^{-1} \cdot ^\circ\mathrm{C}^{-1}$$, density $$= 1.00 \ \mathrm{g}/\mathrm{mL})$$

Accepted Answer

$$1714 \ \mathrm{J}$$

Question 14

Determine the experimental value of $$\Delta H^\circ$$ for the reaction between MgO and HCl in units of kJ/mol, given $$q = 1714 \ \mathrm{J}$$ and $$0.25 \ \mathrm{g}$$ MgO used. (Molar mass of MgO $$= 40.3 \ \mathrm{g}/\mathrm{mol})$$

Accepted Answer

$$-275 \ \mathrm{kJ}/\mathrm{mol}$$

Question 15

Which of the following processes is an example of an endothermic change?

Accepted Answer

Melting of ice

Question 16

A reaction has a positive $$\Delta H. $$Which of the following is true?

Accepted Answer

The reaction is endothermic and absorbs heat.

Question 17

Which statement about the potential energy diagram of an exothermic reaction is correct?

Accepted Answer

The diagram is downhill, with reactants higher than products.

Question 18

In a real-world scenario, why is the combustion of octane in car engines considered exothermic?

Accepted Answer

Because the reaction releases energy, heating the surroundings.

Energy and Chemical Reactions: Study Notes for General Chemistry

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