Q: Which formula is used to calculate the heat absorbed or released by a substance when its temperature changes but no phase change occurs?

$$q = (\text{specific heat}) \times (\text{mass}) \times \Delta T$$

Question 1

A 2 kg object is moving at a speed of 1 m/s. What is its kinetic energy, and what is the correct unit for this energy?

Accepted Answer

$$E_k = \frac{1}{2}mv^2$ = 1\ \mathrm{J}$$

Question 2

Which of the following best describes the sign conventions for heat ($$q) $$and work ($$w) in $$the first law of thermodynamics?

Accepted Answer

Heat absorbed by the system is positive, and work done by the system is negative.

Question 3

A gas expands against a constant external pressure, doing 150 J of work on the surroundings and absorbing 200 J of heat from the surroundings. What is the change in internal energy ($$\Delta E) of $$the system?

Accepted Answer

$$\Delta E = q + w = 200\ \mathrm{J} + (-150\ \mathrm{J}) = 50\ \mathrm{J}$$

Question 4

Which equation correctly expresses the relationship between enthalpy ($$H$$), internal energy ($$E$$), pressure ($$P$$), and volume ($$V$$)?

Accepted Answer

$$H = E + PV$$

Question 5

During a chemical reaction in a bomb calorimeter, the temperature of the water increases. Which of the following statements is true about the sign of $$q$$ and the type of reaction?

Accepted Answer

$$q \u003c 0$$; the reaction is exothermic.

Question 6

A reaction has $$\Delta H \u003e 0. $$Which of the following is true?

Accepted Answer

The reaction is endothermic and absorbs heat from the surroundings.

Question 7

Which formula is used to calculate the heat absorbed or released by a substance when its temperature changes but no phase change occurs?

Accepted Answer

$$q = (	ext{specific heat}) 	imes (	ext{mass}) 	imes \Delta T$$

Question 8

A chemical reaction in a coffee-cup calorimeter causes the temperature of the solution to decrease. What can you conclude about the reaction?

Accepted Answer

The reaction is endothermic; $$q \u003e 0$$ for the system.

Question 9

Which of the following correctly expresses the work done by a gas during expansion at constant pressure?

Accepted Answer

$$w = -P \Delta V$$

Question 10

Given the following enthalpies of formation: $$\Delta H_f^\circ(CO$_2(g)) = -393.5$ kJ/mol, $$\Delta H_f^\circ$$(H_2$O(l)) = -285.8 kJ/$$mol, $$\Delta H_f^\circ(C$_2H$_6(g)) = -84.7$ kJ/mol. Calculate the enthalpy change for the combustion of ethane: $$\mathrm{C_2H_6(g) + \frac{7}{2}$$O_2(g$$) \rightarrow 2CO_2(g) + 3H_2O(l)}$

Accepted Answer

$$\Delta H = [2(-393.5) + 3(-285.8)] - [(-84.7)] = -1559.9\ \mathrm{kJ}$$

Question 11

Which of the following best describes the energy changes in a rechargeable battery as it is used to power a fan?

Accepted Answer

The battery loses energy as both heat and work, with $$\Delta E \u003c 0.$$

Question 12

A sample of gas is compressed by the surroundings, and the temperature of the gas increases. Which of the following is true?

Accepted Answer

Work is done on the system ($$w \u003e 0$$), and the internal energy increases.

Question 13

Which of the following is the correct expression for the change in enthalpy ($$\Delta H) of a $$reaction?

Accepted Answer

$$\Delta H = H_{	ext{products}} - H_{	ext{reactants}}$$

Question 14

A chemical reaction occurs in a closed, rigid container (constant volume). Which of the following is true about the work done and the relationship between $$\Delta E$$ and $$q$$?

Accepted Answer

No work is done ($$w = 0$$), so $$\Delta E = q.$$

Question 15

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

Accepted Answer

Exothermic reactions release heat to the surroundings; endothermic reactions absorb heat from the surroundings.

Question 16

A 100 g sample of water is heated from 25°C to 75°C. The specific heat of water is $$4.18\ \mathrm{J/g\cdot K}. $$How much heat is absorbed by the water?

Accepted Answer

$$q = (4.18\ \mathrm{J/g\cdot K})(100\ \mathrm{g})(75-25) = 20,900\ \mathrm{J}$$

Question 17

Which of the following best describes the energy transformation when gasoline combusts in a car engine?

Accepted Answer

Chemical energy is converted to heat and work.

Question 18

If a reaction has $$\Delta H = -2220\ \mathrm{kJ}$$, what does this indicate about the reaction?

Accepted Answer

The reaction is exothermic and releases 2220 kJ of heat.

Question 19

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

Accepted Answer

The enthalpy change for an overall reaction is the sum of the enthalpy changes for the individual steps.

Energy, Work, and Heat in Chemical Systems: Principles and Applications

Guida di studio - Domande di pratica

Study guide - Flashcard