Q: A 19.0 g sample of water is heated from 21°C to 88°C. How much heat (in joules) is required? (Specific heat of water = $$4.184 \ \mathrm{J} / \mathrm{g} \cdot ^\circ \mathrm{C})$$

$$q = 19.0 \times 4.184 \times (88 - 21) = 19.0 \times 4.184 \times 67 = 5326 J$$

Q: Which formula is used to calculate the enthalpy change of a reaction using standard enthalpies of formation?

$$\Delta H^\circ = \sum \Delta H_f^\circ \text{(products)} - \sum \Delta H_f^\circ \text{(reactants)}$$

Q: A calorimeter has a heat capacity of $$20.7 \ \mathrm{kJ}/^\circ \mathrm{C}. If $$the temperature rises from 25.0°C to 25.9°C, how much heat was absorbed?

$$q = 20.7 \times (25.9 - 25.0) = 20.7 \times 0.9 = 18.63 kJ$$

Question 1

Which of the following statements best describes the Law of Conservation of Energy?

Accepted Answer

Energy is neither created nor destroyed, but can be converted from one form to another.

Question 2

A chemical reaction in a vessel causes the temperature of the vessel to increase. Which of the following is true about the reaction?

Accepted Answer

The reaction is exothermic and releases heat to the surroundings.

Question 3

Calculate the amount of energy in joules contained in 3.5 kilocalories (kcal).

Accepted Answer

$$3.5 	imes 4184 = 14644 J$$

Question 4

Which equation would you use to calculate the heat required to raise the temperature of 50.0 g of water from 20°C to 80°C, given the specific heat of water is $$4.184 \ \mathrm{J} / \mathrm{g} \cdot ^\circ \mathrm{C}$$?

Accepted Answer

$$q = s m \Delta T$$

Question 5

A reaction has $$\Delta H = -250 \ \mathrm{kJ}. $$What does this value indicate?

Accepted Answer

The reaction releases 250 kJ of heat to the surroundings.

Question 6

If the equation $$2 \mathrm{NH}_3 (g) \rightarrow \mathrm{N}_2 (g) + 3 \mathrm{H}_2 (g)$$ has $$\Delta H = +92 \ \mathrm{kJ}$$, what is $$\Delta H$$ for $$4 \mathrm{NH}_3 (g) \rightarrow 2 \mathrm{N}_2 (g) + 6 \mathrm{H}_2 (g)$$?

Accepted Answer

$$+184 kJ$$

Question 7

Which of the following best describes the difference between a system and its surroundings in thermochemistry?

Accepted Answer

The system is the part of the universe being studied, and the surroundings are everything else.

Question 8

A 19.0 g sample of water is heated from 21°C to 88°C. How much heat (in joules) is required? (Specific heat of water = $$4.184 \ \mathrm{J} / \mathrm{g} \cdot ^\circ \mathrm{C})$$

Accepted Answer

$$q = 19.0 	imes 4.184 	imes (88 - 21) = 19.0 	imes 4.184 	imes 67 = 5326 J$$

Question 9

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

Accepted Answer

Only the most stable form of an element at 25°C and 1 atm is assigned $$\Delta H_f^\circ = 0 kJ/$$mol.

Question 10

Which formula is used to calculate the enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

$$\Delta H^\circ = \sum \Delta H_f^\circ 	ext{(products)} - \sum \Delta H_f^\circ 	ext{(reactants)}$$

Question 11

A calorimeter has a heat capacity of $$20.7 \ \mathrm{kJ}/^\circ \mathrm{C}. If $$the temperature rises from 25.0°C to 25.9°C, how much heat was absorbed?

Accepted Answer

$$q = 20.7 	imes (25.9 - 25.0) = 20.7 	imes 0.9 = 18.63 kJ$$

Question 12

Which of the following reactions is endothermic?

Accepted Answer

$$\mathrm{NH}_4NO_3 (s) \rightarrow \mathrm{NH}_4^+ (aq) + \mathrm{NO}_3^- (aq) \quad q = +36 kJ$$

Question 13

If you reverse a chemical equation, what happens to the sign of $$\Delta H$$?

Accepted Answer

It changes sign.

Question 14

Which of the following is a correct application of Hess's Law?

Accepted Answer

The enthalpy change of a reaction is independent of the pathway taken.

Question 15

A sample of copper (Cu) weighing 104 g is heated from 17°C to 34°C. Given the specific heat of copper is $$0.384 \ \mathrm{J} / \mathrm{g} \cdot ^\circ \mathrm{C}$$, calculate the heat absorbed.

Accepted Answer

$$q = 104 	imes 0.384 	imes (34 - 17) = 104 	imes 0.384 	imes 17 = 679 J$$

Question 16

Which of the following is NOT a correct energy unit conversion?

Accepted Answer

1 cal = 1000 J

Question 17

In a calorimetry experiment, why is it important to use an insulated container?

Accepted Answer

To prevent heat loss to the surroundings and ensure accurate measurement of heat change.

Question 18

Which of the following best describes specific heat?

Accepted Answer

The amount of heat needed to raise 1 g of a substance by 1°C.

Thermochemistry: Energy, Enthalpy, and Calorimetry

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