Q: Given the reaction: $$2 \text{Mg} (s) + \text{O}_2 (g) \rightarrow 2 \text{MgO} (s)$$ with $$\Delta H = -1204\ \text{kJ}$$, what will be the enthalpy change if the reaction is reversed?

$$\Delta H = +1204\ \text{kJ}$$

Q: If the thermochemical equation $$2 \text{SO}_2 (g) + \text{O}_2 (g) \rightarrow 2 \text{SO}_3 (g)$$ has $$\Delta H = -198.4\ \text{kJ}$$, what is the $$\Delta H$$ for $$\text{SO}_2 (g) + \frac{1}{2} \text{O}_2 (g) \rightarrow \text{SO}_3 (g)$$?

$$\Delta H = -99.2\ \text{kJ}$$

Q: A reaction is given: $$4 \text{B} (s) + 3 \text{O}_2 (g) \rightarrow 2 \text{B}_2\text{O}_3 (s)$$, $$\Delta H_{rxn} = -2547\ \text{kJ}. $$What is $$\Delta H$$ for $$8 \text{B}_2\text{O}_3 (s) \rightarrow 8 \text{B} (s) + 6 \text{O}_2 (g)$$?

$$\Delta H = +10188\ \text{kJ}$$

Q: Given the following reactions: $$\text{Cl}_2 (g) + \text{F}_2 (g) \rightarrow 2 \text{ClF} (g)\ \Delta H = -167.4\ \text{kJ}$$ $$4 \text{ClF}_3 (g) + 4 \text{O}_2 (g) \rightarrow 2 \text{Cl}_2 (g) + 6 \text{F}_2 (g) + 6 \text{F}_2\text{O} (g)\ \Delta H = 682.8\ \text{kJ}$$ $$2 \text{F}_2 (g) + \text{O}_2 (g) \rightarrow 2 \text{F}_2\text{O} (g)\ \Delta H = -181.7\ \text{kJ}$$ What is the $$\Delta H$$ for $$\text{ClF} (g) + \text{F}_2 (g) \rightarrow \text{ClF}_3 (g)$$?

$$\Delta H = -123.5\ \text{kJ}$$

Q: Given the following reactions: $$\frac{1}{2} \text{S} (s) + \frac{1}{2} \text{O}_2 (g) \rightarrow \frac{1}{2} \text{SO}_2 (g)\ \Delta H = -296.8\ \text{kJ}$$ $$2 \text{SO}_2 (g) + \text{O}_2 (g) \rightarrow 2 \text{SO}_3 (g)\ \Delta H = -198.4\ \text{kJ}$$ What is the $$\Delta H$$ for $$\text{S} (s) + \frac{3}{2} \text{O}_2 (g) \rightarrow \text{SO}_3 (g)$$?

$$\Delta H = -395.2\ \text{kJ}$$

Question 1

Which of the following statements best describes Hess's Law?

Accepted Answer

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

Question 2

Given the reaction: $$2 	ext{Mg} (s) + 	ext{O}_2 (g) ightarrow 2 	ext{MgO} (s)$$ with $$\Delta H = -1204\ 	ext{kJ}$$, what will be the enthalpy change if the reaction is reversed?

Accepted Answer

$$\Delta H = +1204\ 	ext{kJ}$$

Question 3

If the thermochemical equation $$2 	ext{SO}_2 (g) + 	ext{O}_2 (g) ightarrow 2 	ext{SO}_3 (g)$$ has $$\Delta H = -198.4\ 	ext{kJ}$$, what is the $$\Delta H$$ for $$	ext{SO}_2 (g) + \frac{1}{2} 	ext{O}_2 (g) ightarrow 	ext{SO}_3 (g)$$?

Accepted Answer

$$\Delta H = -99.2\ 	ext{kJ}$$

Question 4

A reaction is given: $$4 	ext{B} (s) + 3 	ext{O}_2 (g) ightarrow 2 	ext{B}_2	ext{O}_3 (s)$$, $$\Delta H_{rxn} = -2547\ 	ext{kJ}. $$What is $$\Delta H$$ for $$8 	ext{B}_2	ext{O}_3 (s) ightarrow 8 	ext{B} (s) + 6 	ext{O}_2 (g)$$?

Accepted Answer

$$\Delta H = +10188\ 	ext{kJ}$$

Question 5

Which of the following is a correct application of rearranging thermochemical equations?

Accepted Answer

Multiplying the coefficients of a reaction by 2 will double the $$\Delta H$$ value.

Question 6

Given the following reactions:
$$	ext{Cl}_2 (g) + 	ext{F}_2 (g) ightarrow 2 	ext{ClF} (g)\ \Delta H = -167.4\ 	ext{kJ}$$
$$4 	ext{ClF}_3 (g) + 4 	ext{O}_2 (g) ightarrow 2 	ext{Cl}_2 (g) + 6 	ext{F}_2 (g) + 6 	ext{F}_2	ext{O} (g)\ \Delta H = 682.8\ 	ext{kJ}$$
$$2 	ext{F}_2 (g) + 	ext{O}_2 (g) ightarrow 2 	ext{F}_2	ext{O} (g)\ \Delta H = -181.7\ 	ext{kJ}$$
What is the $$\Delta H$$ for $$	ext{ClF} (g) + 	ext{F}_2 (g) ightarrow 	ext{ClF}_3 (g)$$?

Accepted Answer

$$\Delta H = -123.5\ 	ext{kJ}$$

Question 7

If a thermochemical equation is multiplied by a factor of $$n$$, how is the enthalpy change $$\Delta H$$ affected?

Accepted Answer

$$\Delta H is $$multiplied by $$n$$

Question 8

Which of the following best describes a reaction intermediate in Hess's Law problems?

Accepted Answer

A compound that appears in the steps but not in the overall reaction.

Question 9

Given the following reactions:
$$\frac{1}{2} 	ext{S} (s) + \frac{1}{2} 	ext{O}_2 (g) ightarrow \frac{1}{2} 	ext{SO}_2 (g)\ \Delta H = -296.8\ 	ext{kJ}$$
$$2 	ext{SO}_2 (g) + 	ext{O}_2 (g) ightarrow 2 	ext{SO}_3 (g)\ \Delta H = -198.4\ 	ext{kJ}$$
What is the $$\Delta H$$ for $$	ext{S} (s) + \frac{3}{2} 	ext{O}_2 (g) ightarrow 	ext{SO}_3 (g)$$?

Accepted Answer

$$\Delta H = -395.2\ 	ext{kJ}$$

Question 10

A student reverses a thermochemical equation and forgets to change the sign of $$\Delta H. $$What is the most likely consequence?

Accepted Answer

The calculated enthalpy change will be incorrect and may lead to wrong predictions about the reaction's energy flow.

Question 11

Which of the following is true about the enthalpy change ($$\Delta H) of a $$reaction?

Accepted Answer

$$\Delta H is $$directly proportional to the amount of substance involved in the reaction.

Question 12

In a multi-step reaction, what is the correct procedure to find the overall $$\Delta H$$?

Accepted Answer

Add the $$\Delta H$$ values of each step, making sure to adjust for any reversals or multiplications of the equations.

Question 13

Given the reaction: $$	ext{XeF}_2 (s) + 	ext{F}_2 (g) ightarrow 	ext{XeF}_4 (s)$$, and the following data:
$$	ext{Xe} (g) + 	ext{F}_2 (g) ightarrow 	ext{XeF}_2 (s)\ \Delta H = -262\ 	ext{kJ}$$
$$	ext{Xe} (g) + 2 	ext{F}_2 (g) ightarrow 	ext{XeF}_4 (s)\ \Delta H = -401\ 	ext{kJ}$$
What is the $$\Delta H$$ for $$	ext{XeF}_2 (s) + 	ext{F}_2 (g) ightarrow 	ext{XeF}_4 (s)$$?

Accepted Answer

$$\Delta H = -139\ 	ext{kJ}$$

Question 14

A chemical reaction is written as $$A + B ightarrow C$$, $$\Delta H = -200\ 	ext{kJ}. If $$the reaction is rewritten as $$2A + 2B ightarrow 2C$$, what is the new $$\Delta H$$?

Accepted Answer

$$\Delta H = -400\ 	ext{kJ}$$

Question 15

Which of the following best explains why the enthalpy change of a reaction is considered a state function?

Accepted Answer

It depends only on the initial and final states, not on the path taken.

Thermochemical Equations and Hess’s Law: Rearrangement and Calculation of Enthalpy Changes

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