Q: A student heats 100.0 g of water from 20.0 °C to 70.0 °C. Calculate the thermal energy absorbed by the water. Use $$c = 4.18\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C}).$$

$$q = 100.0\ \mathrm{g} \times 4.18\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C}) \times (70.0 - 20.0)^\circ\mathrm{C} = 20,900\ \mathrm{J}$$

Q: A 4.0 kg sample of ethylene glycol ($$c = 3.5\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C})) $$absorbs 250 kJ of energy. What is the temperature change, $$\Delta T$$?

$$\Delta T = \frac{250,000\ \mathrm{J}}{4000\ \mathrm{g} \times 3.5\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C})} = 18^\circ\mathrm{C}$$

Question 1

Which property of Nomex makes it suitable for use in firefighter suits, as shown in the image of firefighters near a blaze?

Accepted Answer

It has a high specific heat capacity, allowing it to absorb large amounts of thermal energy without a significant temperature increase.

Question 2

A student heats 100.0 g of water from 20.0 °C to 70.0 °C. Calculate the thermal energy absorbed by the water. Use $$c = 4.18\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C}).$$

Accepted Answer

$$q = 100.0\ \mathrm{g} 	imes 4.18\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C}) 	imes (70.0 - 20.0)^\circ\mathrm{C} = 20,900\ \mathrm{J}$$

Question 3

Which device, as shown in the second image, is best suited for measuring the energy change of a combustion reaction involving gases?

Accepted Answer

Bomb calorimeter

Question 4

A 50.0 mL sample of water at 25.0 °C absorbs 418 J of energy. What is the final temperature of the water? ($$c = 4.18\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C})$$, $$d = 1.00\ \mathrm{g}/\mathrm{mL})$$

Accepted Answer

$$\Delta T = \frac{q}{mc} = \frac{418\ \mathrm{J}}{50.0\ \mathrm{g} 	imes 4.18\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C})} = 2.0^\circ\mathrm{C}$$; Final $$T = 27.0^\circ\mathrm{C}$$

Question 5

Which of the following statements about exothermic and endothermic reactions is correct?

Accepted Answer

Exothermic reactions have a negative $$\Delta H$$ and release energy to the surroundings.

Question 6

A chef flambés a dish using ethanol, as shown in the third image. The combustion of ethanol is represented by $$\mathrm{C_2H_5OH}(l) + 3\ \mathrm{O_2$}(g) \rightarrow 2\ \mathrm{CO_2$}(g) + 3\ \mathrm{H_2O}(l) + 1367\ \mathrm{kJ}. $$What does the energy term indicate?

Accepted Answer

The reaction is exothermic and releases 1367 kJ per mole of ethanol.

Question 7

A 4.0 kg sample of ethylene glycol ($$c = 3.5\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C})) $$absorbs 250 kJ of energy. What is the temperature change, $$\Delta T$$?

Accepted Answer

$$\Delta T = \frac{250,000\ \mathrm{J}}{4000\ \mathrm{g} 	imes 3.5\ \mathrm{J}/(\mathrm{g}\cdot{}^\circ\mathrm{C})} = 18^\circ\mathrm{C}$$

Question 8

Which equation correctly relates the enthalpy change of a reaction to the molar enthalpy change and the amount of substance?

Accepted Answer

$$\Delta H = n\Delta H_r$$

Question 9

A student dissolves 10.5 g of KBr in 125 g of water, causing the temperature to drop from 24.2 °C to 21.1 °C. What is the sign of $$\Delta H_{sol}$$ for this process?

Accepted Answer

Positive, because the system absorbs energy.

Question 10

Which of the following best describes the difference between a coffee-cup calorimeter and a bomb calorimeter?

Accepted Answer

A bomb calorimeter is used for reactions involving gases and operates at constant volume, while a coffee-cup calorimeter is used for reactions in solution and operates at constant pressure.

Question 11

Which of the following is the correct thermochemical equation for the vaporization of 1 mol of water if $$\Delta H_{vap} = 44.0\ \mathrm{kJ/mol}$$?

Accepted Answer

$$\mathrm{H_2O}(l) \rightarrow \mathrm{H_2O}(g)\ \Delta H = 44.0\ \mathrm{kJ}$$

Question 12

A reaction in a calorimeter causes the temperature of the water to decrease. Which of the following statements is true?

Accepted Answer

The reaction is endothermic and $$q is $$positive.

Question 13

Which of the following best explains why sand heats up faster than water under the sun?

Accepted Answer

Sand has a lower specific heat capacity than water.

Question 14

If the enthalpy of the products is less than that of the reactants, what can be said about the reaction?

Accepted Answer

The reaction is exothermic and $$\Delta H is $$negative.

Question 15

A student mixes 50.0 mL of 1.0 mol/L HCl and 50.0 mL of 1.0 mol/L NaOH in a calorimeter. The temperature rises from 25.0 °C to 31.9 °C. What is the sign of $$q$$ for the reaction?

Accepted Answer

Negative, because the system releases energy.

Question 16

Which of the following is the correct formula for calculating the quantity of thermal energy transferred in a calorimetry experiment?

Accepted Answer

$$q = mc\Delta T$$

Lesson 5.2: Calorimetry and Enthalpy: Measuring and Understanding Thermal Energy Changes

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