Q: A reaction occurs in a cylinder with a piston. 1150 J of heat is lost to the surroundings and the piston moves down, doing 480 J of work on the system. What is the change in internal energy ($$\Delta E) of $$the system?

$$\Delta E = -1150 \text{ J} + 480 \text{ J} = -670 \text{ J}$$

Q: Which of the following reactions is endothermic?

$$\text{H}_2\text{O}_{(l)} + 342 \text{ J} \rightarrow \text{H}_2\text{O}_{(g)}$$

Q: Which of the following is NOT a correct conversion for energy units?

$$1 \text{ calorie} = 100 \text{ J}$$

Question 1

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

Accepted Answer

Energy cannot be created or destroyed, only transferred from one form to another.

Question 2

A reaction occurs in a cylinder with a piston. 1150 J of heat is lost to the surroundings and the piston moves down, doing 480 J of work on the system. What is the change in internal energy ($$\Delta E) of $$the system?

Accepted Answer

$$\Delta E = -1150 	ext{ J} + 480 	ext{ J} = -670 	ext{ J}$$

Question 3

Which of the following reactions is endothermic?

Accepted Answer

$$	ext{H}_2	ext{O}_{(l)} + 342 	ext{ J} ightarrow 	ext{H}_2	ext{O}_{(g)}$$

Question 4

Which statement about enthalpy ($$\Delta H) is $$correct?

Accepted Answer

At constant pressure, $$\Delta H = q.$$

Question 5

Calculate the amount of heat released when 4.5 g of methane ($$	ext{CH}_4) is $$burned in a constant-pressure system. ($$\Delta H = -890 	ext{ kJ/mol}$$, molar mass $$= 16.04 	ext{ g/mol})$$

Accepted Answer

$$-250 	ext{ kJ}$$

Question 6

Which of the following is the correct SI unit for energy?

Accepted Answer

Joule (J)

Question 7

If 96.0 kJ of heat is released when magnesium is burned in air, how many grams of MgO are formed? ($$2	ext{Mg} + 	ext{O}_2 ightarrow 2	ext{MgO}$$, $$\Delta H = -2408 	ext{ kJ}$$, molar mass of MgO $$= 40.305 	ext{ g/mol})$$

Accepted Answer

1.94 g

Question 8

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

Accepted Answer

Exothermic reactions have $$\Delta H \u003c 0.$$

Question 9

For the reaction $$	ext{CH}_4 + 2	ext{O}_2 ightarrow 	ext{CO}_2 + 2	ext{H}_2	ext{O}$$, $$\Delta H = -890 	ext{ kJ}. If 3 $$moles of methane are burned, how much heat is released?

Accepted Answer

$$-2670 	ext{ kJ}$$

Question 10

Which property of enthalpy is demonstrated by the fact that $$\Delta H$$ for a reaction in the forward direction is equal in size but opposite in sign to $$\Delta H$$ for the reverse reaction?

Accepted Answer

Enthalpy is path-independent and reversible.

Question 11

A student claims that the enthalpy change for $$	ext{CH}_4 (g) + 2	ext{O}_2 (g) ightarrow 	ext{CO}_2 (g) + 2	ext{H}_2	ext{O} (l) is $$the same as for $$	ext{CH}_4 (g) + 2	ext{O}_2 (g) ightarrow 	ext{CO}_2 (g) + 2	ext{H}_2	ext{O} (g). Is $$this correct?

Accepted Answer

No, because enthalpy depends on the state of the products and reactants.

Question 12

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

Accepted Answer

$$\Delta E = q + W$$

Question 13

A chemical reaction occurs at constant pressure and releases 500 kJ of heat. What is the enthalpy change ($$\Delta H) $$for the reaction?

Accepted Answer

$$\Delta H = -500 	ext{ kJ}$$

Question 14

Which of the following best describes the surroundings in a chemical reaction?

Accepted Answer

Everything outside of the system being studied.

Question 15

If the enthalpy change for $$	ext{N}_2 + 2	ext{O}_2 ightarrow 2	ext{NO}_2 is$$ $$+180.5 	ext{ kJ}$$, what is $$\Delta H$$ for the reverse reaction?

Accepted Answer

$$-180.5 	ext{ kJ}$$

Question 16

Which of the following is NOT a correct conversion for energy units?

Accepted Answer

$$1 	ext{ calorie} = 100 	ext{ J}$$

Question 17

Which of the following is an example of work in the context of thermochemistry?

Accepted Answer

Energy used to cause an object with mass to move.

Question 18

A reaction absorbs heat from the surroundings. Which term best describes this process?

Accepted Answer

Endothermic

Question 19

Which of the following statements about energy in chemical reactions is correct?

Accepted Answer

Energy is transferred between the system and surroundings.

Chapter 5: Thermochemistry – Study Notes

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