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 $$5.1\$$times10^3$$\ 	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

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