Q: Given the thermochemical equation: $$C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g) \quad \Delta H = -2044 \ kJ$$, how much heat (in kJ) is released when $$13.2 \ kg of $$propane is completely combusted?

$$13,200 \ g \times \frac{1 \ mol}{44.1 \ g} \times \frac{-2044 \ kJ}{1 \ mol} = -611,948 \ kJ$$

Q: Which of the following is the correct formula for calculating the heat absorbed or released by a substance?

$$q = m \times C_s \times \Delta T$$

Question 1

Which of the following best describes the law of conservation of energy?

Accepted Answer

Energy cannot be created or destroyed, only transferred or transformed.

Question 2

A 53.0 g silicon wafer is heated from $$23.5^\circ C to$$ $$41.0^\circ C. If $$the specific heat of silicon is $$0.712 \ \frac{J}{g \cdot ^\circ C}$$, how much heat is absorbed by the wafer?

Accepted Answer

$$q = (53.0 \ g)(0.712 \ \frac{J}{g \cdot ^\circ C})(17.5^\circ C) = 660 \ J$$

Question 3

Which equation correctly expresses the relationship for pressure-volume work done by a system at constant external pressure?

Accepted Answer

$$w = -P \Delta V$$

Question 4

A chemical reaction occurs in a bomb calorimeter (constant volume). Which of the following statements is true?

Accepted Answer

The heat measured equals the internal energy change $$\Delta E of $$the reaction.

Question 5

Which of the following best describes an exothermic reaction?

Accepted Answer

The system releases heat to the surroundings and $$\Delta H is $$negative.

Question 6

A balloon is inflated from $$0.100 \ L to$$ $$1.75 \ L$$ against an external pressure of $$1.00 \ atm. $$How much work (in J) is done by the system?

Accepted Answer

$$w = -P \Delta V = -(1.00 \ atm)(1.75 \ L - 0.100 \ L) = -1.65 \ L \cdot atm = -167 \ J$$

Question 7

Which of the following is true for an endothermic reaction occurring in a flask?

Accepted Answer

The temperature of the flask falls.

Question 8

Given the thermochemical equation: $$C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g) \quad \Delta H = -2044 \ kJ$$, how much heat (in kJ) is released when $$13.2 \ kg of $$propane is completely combusted?

Accepted Answer

$$13,200 \ g 	imes \frac{1 \ mol}{44.1 \ g} 	imes \frac{-2044 \ kJ}{1 \ mol} = -611,948 \ kJ$$

Question 9

In a coffee-cup calorimeter, $$0.158 \ g of Mg $$reacts with excess HCl in $$100.0 \ mL of $$solution. The temperature rises from $$25.6^\circ C to$$ $$32.8^\circ C. If $$the density is $$1.00 \ g/mL$$ and $$C_s = 4.18 \ J/g^\circ C$$, what is the heat absorbed by the solution?

Accepted Answer

$$q = (100.0 \ g)(4.18 \ J/g^\circ C)(7.2^\circ C) = 3,009.6 \ J$$

Question 10

Which of the following statements about enthalpy ($$\Delta H) is $$correct?

Accepted Answer

$$\Delta H is $$the heat exchanged at constant pressure.

Question 11

A 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.

Question 12

Which of the following is the correct formula for calculating the heat absorbed or released by a substance?

Accepted Answer

$$q = m 	imes C_s 	imes \Delta T$$

Question 13

If a system does $$250 \ J of $$work on the surroundings and absorbs $$100 \ J of $$heat, what is the change in internal energy ($$\Delta E) of $$the system?

Accepted Answer

$$\Delta E = q + w = 100 \ J + (-250 \ J) = -150 \ J$$

Question 14

Which of the following best describes the difference between $$\Delta H$$ and $$\Delta E$$ for a reaction that produces a large amount of gas?

Accepted Answer

$$\Delta H$$ includes the pressure-volume work, while $$\Delta E$$ does not.

Question 15

A cold pack used for athletic injuries contains chemicals that, when mixed, absorb heat from the surroundings. Which type of reaction is this?

Accepted Answer

Endothermic

Question 16

In a constant pressure calorimeter, the heat measured is equal to:

Accepted Answer

The change in enthalpy ($$\Delta H) of $$the reaction.

Question 17

Which of the following is a correct stoichiometric relationship for the reaction $$C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g) \quad \Delta H = -2044 \ kJ$$?

Accepted Answer

$$1 \ mol \ C_3H_8$$ releases $$2044 \ kJ of $$heat.

Question 18

A student dissolves $$5.00 \ g of $$NaCl in $$100.0 \ mL of $$water in a coffee-cup calorimeter. The temperature drops from $$24.11^\circ C to$$ $$23.98^\circ C. If $$the density is $$1.000 \ g/mL$$ and $$C_s = 4.184 \ J/g^\circ C$$, what is the heat change for the solution?

Accepted Answer

$$q = (100.0 \ g)(4.184 \ J/g^\circ C)(-0.13^\circ C) = -54.4 \ J$$

Thermochemistry - Energy, Work, and Calorimetry

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