Question 1

Which statement best describes Hess’s law?

Accepted Answer

The enthalpy change of a process is the same whether the process occurs in one step or several steps.

Question 2

If you reverse a chemical reaction, what happens to the sign and magnitude of $$\Delta H$$?

Accepted Answer

The sign of $$\Delta H$$ reverses, but the magnitude stays the same.

Question 3

A chemist wants to calculate the enthalpy change for the reaction $$\mathrm{C_{graphite}(s) \rightarrow C_{diamond}(s)}. $$Given the following data:

$$\mathrm{C_{graphite}(s) + O_2$(g) \rightarrow CO_2$(g)}$$ $$\Delta H = -394\ \mathrm{kJ}$$

$$\mathrm{C_{diamond}(s) + O_2$(g) \rightarrow CO_2$(g)}$$ $$\Delta H = -396\ \mathrm{kJ}$$

What is the enthalpy change for the conversion of graphite to diamond?

Accepted Answer

$$\Delta H = +2\ \mathrm{kJ}$$

Question 4

Which of the following best explains why Hess’s law is consistent with the law of conservation of energy?

Accepted Answer

The total energy change is independent of the path taken, so energy is conserved overall.

Question 5

Given the following thermochemical equations:

$$\mathrm{C_2H_6(g) + 3.5\ O_2$(g) \rightarrow 2\ CO_2$(g) + 3\ H_2O(l)}$$ $$\Delta H = -1559\ \mathrm{kJ}$$

$$\mathrm{C_2H_4(g) + 3\ O_2$(g) \rightarrow 2\ CO_2$(g) + 2\ H_2O(l)}$$ $$\Delta H = -1411\ \mathrm{kJ}$$

$$\mathrm{2\ H_2$(g) + O_2$(g) \rightarrow 2\ H_2O(l)}$$ $$\Delta H = -572\ \mathrm{kJ}$$

What is $$\Delta H$$ for $$\mathrm{C_2H_6(g) \rightarrow C_2H_4(g) + H_2$(g)}$$?

Accepted Answer

$$\Delta H = +138\ \mathrm{kJ}$$

Question 6

If the coefficients in a balanced chemical equation are multiplied by a factor of 2, what happens to $$\Delta H$$?

Accepted Answer

$$\Delta H is $$doubled.

Question 7

Which characteristic of enthalpy change is the basis of Hess’s law?

Accepted Answer

Enthalpy change is independent of the path taken.

Question 8

A reaction occurs too quickly or is too dangerous to measure directly by calorimetry. Which method can be used to determine its enthalpy change?

Accepted Answer

Use Hess’s law to combine known reactions.

Question 9

Consider the following reactions:

$$\mathrm{Xe(g) + 2\ F_2$(g) \rightarrow $$XeF_4$$(s)}$$ $$\Delta H = -251\ \mathrm{kJ}$$

What is $$\Delta H$$ for the reverse reaction $$\mathrm{$$XeF_4$$(s) \rightarrow Xe(g) + 2\ F_2$(g)}$$?

Accepted Answer

$$\Delta H = +251\ \mathrm{kJ}$$

Question 10

A student is asked to calculate the enthalpy change for the reaction $$\mathrm{Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g)}$$ using the following data:

$$\mathrm{C(graphite) + \frac{1}{2}O_2$ \rightarrow CO}$$ $$\Delta H = -110.5\ \mathrm{kJ}$$

$$\mathrm{C(graphite) + O_2$(g) \rightarrow CO_2$(g)}$$ $$\Delta H = -393.5\ \mathrm{kJ}$$

$$\mathrm{2Fe(s) + \frac{3}{2}O_2$(g) \rightarrow Fe_2O_3(s)}$$ $$\Delta H = -824.2\ \mathrm{kJ}$$

What is the enthalpy change for the reaction?

Accepted Answer

$$\Delta H = -24.8\ \mathrm{kJ}$$

Question 11

Which real-world scenario best illustrates the concept of Hess’s law?

Accepted Answer

Climbing a mountain by different routes but reaching the same elevation.

Question 12

Why is Hess’s law particularly useful for reactions that are too fast or dangerous to measure directly?

Accepted Answer

It allows calculation of enthalpy changes using known reactions.

Question 13

Which equation correctly represents the enthalpy change for the formation of nitrogen dioxide gas from nitrogen and oxygen, given:

$$\mathrm{N_2$(g) + O_2$(g) \rightarrow 2NO(g)}$$ $$\Delta H = +180\ \mathrm{kJ}$$

$$\mathrm{2NO(g) + O_2$(g) \rightarrow 2NO_2(g)}$$ $$\Delta H = -112\ \mathrm{kJ}$$

What is $$\Delta H$$ for $$\mathrm{N_2$(g) + 2O_2(g) \rightarrow 2NO_2(g)}$$?

Accepted Answer

$$\Delta H = +68\ \mathrm{kJ}$$

Question 14

A chemist combines two reactions:

$$\mathrm{A \rightarrow B}$$ $$\Delta H = +50\ \mathrm{kJ}$$

$$\mathrm{B \rightarrow C}$$ $$\Delta H = -30\ \mathrm{kJ}$$

What is $$\Delta H$$ for $$\mathrm{A \rightarrow C}$$?

Accepted Answer

$$\Delta H = +20\ \mathrm{kJ}$$

Question 15

Which of the following is NOT a rule for manipulating enthalpy changes according to Hess’s law?

Accepted Answer

If you reverse a reaction, keep the sign of $$\Delta H$$ unchanged.

Question 16

Which of the following best describes an exothermic reaction?

Accepted Answer

A reaction where $$\Delta H is $$negative and heat is released.

Question 17

A reaction has $$\Delta H = -100\ \mathrm{kJ}$$ for 1 mol of reactant. What is $$\Delta H if 2 $$mol of reactant are used?

Accepted Answer

$$\Delta H = -200\ \mathrm{kJ}$$

Question 18

Which of the following is a practical application of Hess’s law in industry?

Accepted Answer

Calculating the enthalpy change for the production of ethene from ethane.

Lesson 5.4: Hess’s Law and Enthalpy Calculations in Thermochemistry

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