Question 1

Which of the following best describes a thermochemical equation?

Accepted Answer

An equation that includes the enthalpy change $\Delta H$ along with the balanced chemical equation.

Question 2

Given the reaction $2	ext{Mg}(s) + 	ext{O}_2(g) ightarrow 2	ext{MgO}(s) \quad \Delta H = -1204\ 	ext{kJ}$, how much heat is released when 4.00 g of Mg reacts completely?

Accepted Answer

$-99.2\ 	ext{kJ}$

Question 3

In a thermochemical equation, what does a negative value for $\Delta H$ indicate?

Accepted Answer

The reaction is exothermic and releases heat.

Question 4

If 125 g of nitromethane ($	ext{CH}_3	ext{NO}_2$) is burned according to $4	ext{CH}_3	ext{NO}_2(l) + 7	ext{O}_2(g) ightarrow 4	ext{CO}_2(g) + 6	ext{H}_2	ext{O}(l) + 2	ext{N}_2(g)\quad \Delta H = -2414\ 	ext{kJ}$, how much heat is released? (Molar mass $= 61.04\ 	ext{g/mol}$)

Accepted Answer

$-1237\ 	ext{kJ}$

Question 5

Which step is necessary when converting from grams of a known substance to grams of an unknown substance in a chemical reaction?

Accepted Answer

Convert grams of known to moles of known, use the mole ratio, then convert to grams of unknown.

Question 6

In the reaction $2	ext{C}_6	ext{H}_6(l) + 15	ext{O}_2(g) ightarrow 12	ext{CO}_2(g) + 6	ext{H}_2	ext{O}(l)\quad \Delta H = -6278\ 	ext{kJ}$, what volume of benzene ($	ext{C}_6	ext{H}_6$) is needed to release $5.1	imes10^3\ 	ext{kJ}$ of heat? (Density $= 0.880\ 	ext{g/mL}$, Molar mass $= 78.1\ 	ext{g/mol}$)

Accepted Answer

$72.1\ 	ext{mL}$

Question 7

Which of the following is the correct sequence for solving a stoichiometry problem involving heat change?

Accepted Answer

Convert grams to moles, use the mole ratio, then use $\Delta H$ to find heat.

Question 8

The creation of liquid methanol is accomplished by the hydrogenation of carbon monoxide: $	ext{CO}(g) + 2	ext{H}_2(g) ightarrow 	ext{CH}_3	ext{OH}(l)\quad \Delta H = -91\ 	ext{kJ}$. How much heat ($	ext{kJ}$) is released when $125\ 	ext{g}$ CO reacts?

Accepted Answer

$-572\ 	ext{kJ}$

Question 9

Why is it important to use coefficients from a balanced chemical equation in stoichiometric calculations?

Accepted Answer

They show the relative number of moles of each substance involved in the reaction.

Question 10

A student wants to calculate the amount of heat released when 10.0 g of Mg reacts with excess $	ext{O}_2$ according to $2	ext{Mg}(s) + 	ext{O}_2(g) ightarrow 2	ext{MgO}(s)\quad \Delta H = -1204\ 	ext{kJ}$. What is the first step?

Accepted Answer

Convert 10.0 g Mg to moles Mg.

Question 11

Which of the following best explains why the enthalpy change ($\Delta H$) is proportional to the amount of substance reacted?

Accepted Answer

Because $\Delta H$ is defined per mole of reaction as written in the balanced equation.

Question 12

If a reaction has $\Delta H = +500\ 	ext{kJ}$, which statement is true?

Accepted Answer

The reaction absorbs 500 kJ of heat per the balanced equation.

Question 13

A chemical reaction releases $-6278\ 	ext{kJ}$ of heat for every 2 moles of benzene combusted. How much heat is released per mole of benzene?

Accepted Answer

$-3139\ 	ext{kJ}$

Question 14

Which of the following is NOT a step in solving a stoichiometry problem involving heat?

Accepted Answer

Multiply the grams of known directly by $\Delta H$ without converting to moles.

Stoichiometry and Thermochemical Equations: Study Notes

Study Guide - Practice Questions

Study Guide - Flashcards