Q: Which equation correctly relates the heat ($$q) $$absorbed or released by a substance to its mass ($$m$$), specific heat ($$C_s$$), and temperature change ($$\Delta T$$)?

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

Q: Which of the following equations is used to calculate the standard enthalpy change for a reaction using standard enthalpies of formation?

$$\Delta H_{rxn}^\circ = \sum n \Delta H_f^\circ (\text{products}) - \sum n \Delta H_f^\circ (\text{reactants})$$

Question 1

Which of the following statements about enthalpy ($$H) is $$CORRECT?

Accepted Answer

Enthalpy is a state function and is defined as $$H = E + PV.$$

Question 2

A chemical reaction is carried out in a coffee-cup calorimeter. Which of the following is measured directly in this experiment?

Accepted Answer

The enthalpy change ($$\Delta H) at $$constant pressure.

Question 3

Which equation correctly relates the heat ($$q) $$absorbed or released by a substance to its mass ($$m$$), specific heat ($$C_s$$), and temperature change ($$\Delta T$$)?

Accepted Answer

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

Question 4

A 25.0 g block of metal at 100.0 $$^\circC is $$placed in 100.0 g of water at 20.0 $$^\circC. If $$the final temperature is 21.1 $$^\circC$$, which of the following best explains the direction of heat flow?

Accepted Answer

Heat flows from the metal to the water until both reach 21.1 $$^\circC.$$

Question 5

Which of the following is TRUE about the enthalpy of formation ($$\Delta H_f^\circ) $$for elements in their standard state?

Accepted Answer

It is defined as zero.

Question 6

Given the reaction: $$2C_2H_6(g) + 7O_2(g) \rightarrow 4CO_2(g) + 6H_2O(\ell)$$, $$\Delta H = -3120 kJ. $$What is the enthalpy change when 24.0 g of ethane ($$C_2H_6) is $$burned?

Accepted Answer

$$-1.24 	imes 10^3$ kJ

Question 7

Which calorimeter is best suited for measuring the heat of combustion of a fuel?

Accepted Answer

Bomb calorimeter

Question 8

In a bomb calorimeter, the reaction occurs at constant volume. What is actually measured in this experiment?

Accepted Answer

The change in internal energy ($$\Delta E)$$

Question 9

Which of the following statements about Hess’s Law is CORRECT?

Accepted Answer

The enthalpy change for a reaction is the sum of the enthalpy changes for any series of steps that leads from reactants to products.

Question 10

Calculate the standard enthalpy change ($$\Delta H_{rxn}^\circ) $$for the reaction: $$P_4O_{10}(s) + 6H_2O(g) \rightarrow 4H_3PO_4(s)$$, given the following data: $$\Delta H_f^\circ$$ [P$$_4O$_{10}(s$$)] = -1640 kJ/mol, $$\Delta H_f^\circ$$ [H$$_2O(g$$)] = -242 kJ/mol, $$\Delta H_f^\circ$$ [H$$_3PO$_4(s$$)] = -1279 kJ/mol.

Accepted Answer

$$-2024 kJ$$

Question 11

A 6.50 g sample of NaOH is dissolved in 100.0 g of water, causing the temperature to rise from 21.6 $$^\circC to 37.8$$ $$^\circC. $$What is the heat released by the reaction? ($$C_s$$ for water = 4.184 J/g·$$^\circC)$$

Accepted Answer

6.77 kJ

Question 12

Which of the following best describes an exothermic reaction?

Accepted Answer

A reaction in which $$\Delta H \u003c 0$$ and heat is released to the surroundings.

Question 13

Which of the following is a common error when calculating the enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

Forgetting to multiply the $$\Delta H_f^\circ of $$reactants by their stoichiometric coefficients.

Question 14

Which of the following best explains why the specific heat capacity of water is important in calorimetry experiments?

Accepted Answer

Water’s specific heat is well known and relatively high, allowing accurate measurement of heat changes.

Question 15

Which of the following equations is used to calculate the standard enthalpy change for a reaction using standard enthalpies of formation?

Accepted Answer

$$\Delta H_{rxn}^\circ = \sum n \Delta H_f^\circ (	ext{products}) - \sum n \Delta H_f^\circ (	ext{reactants})$$

Question 16

A reaction is carried out in a bomb calorimeter and the temperature of the water increases. What does this indicate about the reaction?

Accepted Answer

The reaction is exothermic.

Question 17

Which of the following best describes the relationship between $$q_{system}$$ and $$q_{surroundings} in a $$calorimetry experiment?

Accepted Answer

$$q_{system} = -q_{surroundings}$$

Question 18

Which of the following is NOT a property of enthalpy ($$H$$)?

Accepted Answer

It depends on the path taken between reactants and products.

Question 19

Which of the following best describes the use of Hess’s Law in real-world chemistry?

Accepted Answer

It allows chemists to calculate enthalpy changes for reactions that are difficult or dangerous to measure directly.

Thermochemistry and Calorimetry: Enthalpy, Calorimetric Methods, and Hess’s Law

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