Q: A 50.0 g block of copper ($$C_s = 0.385 \ J \ g$$^{-1}$$ \ ^\circ C$$^{-1}$$) is heated from 25.0$$ ^\circ C$$ to 60.0$$ ^\circ C$$. How much heat (q$) is absorbed by the copper?

$$q = 50.0 \times 0.385 \times (60.0 - 25.0) = 673 \ J$$

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

$$\Delta H^\circ_{rxn} = \sum v_p \Delta H^\circ_f \text{(products)} - \sum v_r \Delta H^\circ_f \text{(reactants)}$$

Q: A sample of water (mass = 100 g, $$C_s = 4.184 \ J \ g$$^{-1}$$ \ ^\circ C$$^{-1}$$) is heated from 20.0$$ ^\circ C$$ to 30.0$$ ^\circ C$$. Calculate the heat absorbed by the water.

$$q = 100 \times 4.184 \times (30.0 - 20.0) = 4184 \ J$$

Question 1

Which of the following statements best describes the First Law of Thermodynamics?

Accepted Answer

Energy is neither created nor destroyed, but can be transferred between system and surroundings.

Question 2

A chemical hand warmer contains iron that reacts with oxygen to produce heat. Which type of reaction is this, and what is the sign of $$\Delta H$$?

Accepted Answer

Exothermic; $$\Delta H \u003c 0$$

Question 3

Which equation correctly relates the change in internal energy ($$\Delta U) to $$heat ($$q) $$and work ($$w$$)?

Accepted Answer

$$\Delta U = q + w$$

Question 4

A 50.0 g block of copper ($$C_s = 0.385 \ J \ g$$^{-1}$$ \ ^\circ C$$^{-1}$$) is heated from 25.0$$ ^\circ C$$ to 60.0$$ ^\circ C$$. How much heat (q$) is absorbed by the copper?

Accepted Answer

$$q = 50.0 	imes 0.385 	imes (60.0 - 25.0) = 673 \ J$$

Question 5

Which of the following is an example of a state function?

Accepted Answer

Internal energy ($$U) of a $$system

Question 6

In a bomb calorimeter, a sample is combusted at constant volume. Which equation is used to relate the heat measured to the change in internal energy?

Accepted Answer

$$\Delta U = q_{v,r} = -q_{v,cal}$$

Question 7

Which of the following best describes the difference between heat capacity and specific heat capacity?

Accepted Answer

Heat capacity depends on the amount of substance, while specific heat capacity is per unit mass.

Question 8

A reaction is performed in a coffee-cup calorimeter at constant pressure. Which equation is used to calculate the enthalpy change ($$\Delta H) $$for the reaction?

Accepted Answer

$$\Delta H = q_{p,r} = -C_{p,cal} \Delta T$$

Question 9

For the reaction $$C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l) at 298 K$$, what is the value of $$\Delta n ($$change in moles of gas) and the sign of the pressure-volume work ($$w$$)?

Accepted Answer

$$\Delta n = -3$$; $$w is $$negative

Question 10

Which of the following correctly expresses the standard enthalpy change of a reaction using enthalpies of formation?

Accepted Answer

$$\Delta H^\circ_{rxn} = \sum v_p \Delta H^\circ_f 	ext{(products)} - \sum v_r \Delta H^\circ_f 	ext{(reactants)}$$

Question 11

If a chemical equation is reversed, what happens to the sign of the enthalpy change ($$\Delta H$$)?

Accepted Answer

It is reversed (changes sign).

Question 12

Which of the following is true about standard enthalpy of formation ($$\Delta H^\circ_f) $$for elements in their standard states?

Accepted Answer

It is defined as zero.

Question 13

A reaction absorbs heat from the surroundings and causes the temperature of the surroundings to decrease. Which type of reaction is this?

Accepted Answer

Endothermic

Question 14

Which of the following equations is used to calculate the pressure-volume work done by a gas during expansion at constant pressure?

Accepted Answer

$$w = -P \Delta V$$

Question 15

A sample of water (mass = 100 g, $$C_s = 4.184 \ J \ g$$^{-1}$$ \ ^\circ C$$^{-1}$$) is heated from 20.0$$ ^\circ C$$ to 30.0$$ ^\circ C$$. Calculate the heat absorbed by the water.

Accepted Answer

$$q = 100 	imes 4.184 	imes (30.0 - 20.0) = 4184 \ J$$

Question 16

Which of the following statements about calorimetry is correct?

Accepted Answer

Coffee-cup calorimetry measures $$\Delta H at $$constant pressure.

Question 17

Which of the following is NOT a correct unit for specific heat capacity?

Accepted Answer

$$J \ $$mol^{-1}$$ \ K$$^{-1}$$

Question 18

A chemical reaction is performed at constant volume and releases 500 J of heat to the calorimeter. What is the value of $$\Delta U$$ for the reaction?

Accepted Answer

$$\Delta U = -500 \ J$$

Question 19

Which of the following best explains why enthalpy and internal energy are state functions?

Accepted Answer

They depend only on the difference between the final and initial states, not the path.

Chapter 6: Thermochemistry – Study Notes

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