Thermochemical Equations Answers

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The enthalpy of formation for C6 H6 (I) is 49.0 kJ/mol. Consider the following reaction.Is the reaction endothermic or exothermic, and what is the enthalpy of reaction? Use .

exothermic; Hrxn = 49.0 kJ

exothermic; Hrxn = –49.0 kJ

endothermic; Hrxn = 49.0 kJ

endothermic; Hrxn = –49.0 kJ

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Hydrogen reacts with chlorine to form hydrogen chloride (HCl (g), Hf = –92.3 kJ/mol) according to the reaction below.Which statement is correct? Use .

The enthalpy of the reaction is –184.6 kJ, and the reaction is exothermic.

The enthalpy of the reaction is –184.6 kJ, and the reaction is endothermic.

The enthalpy of the reaction is 184.6 kJ, and the reaction is endothermic.

The enthalpy of the reaction is 184.6 kJ, and the reaction is exothermic.

Which statement is true for most chemical reactions?

An energy change occurs during the breaking and forming of bonds.

The internal energy of the system increases during a reaction.

Energy is released during the formation of reactants.

The enthalpy of the products is higher than the enthalpy of the reactants.

How is enthalpy used to predict whether a reaction is endothermic or exothermic?

When the enthalpy of the reactants is higher than the enthalpy of the products, the reaction is endothermic.

When the enthalpy of the products is higher than the enthalpy of the reactants, the reaction is exothermic.

When the enthalpy change of the reaction is positive, the reaction is exothermic.

When the enthalpy change of the reaction is positive, the reaction is endothermic.

Consider the reaction.Standard Enthalpies of FormationSubstanceDHf (kJ/mol)C2 H2 (g)226.73CaCO3 (s)–1206.92CaO (s)–635.09CO (g)–110.525CO2 (g)–393.509H 2 O (I)–285.8H 2 O (g)–241.818C (s), diamond1.895C (s), graphite0.0Based on the equation and the information in the table, what is the enthalpy of the reaction? Use .

–453.46 kJ

–226.73 kJ

226.73 kJ

453.46 kJ

Which statement about Hf is true?

It is positive when the bonds of the product store more energy than those of the reactants.

It is zero for any pure substance in its standard state.

It is negative when a compound forms from elements in their standard states.

It is zero for any compound that is in the liquid state.

Which graph represents an exothermic reaction?

A graph has Reaction progression on the horizontal axis and energy on the vertical axis. A line starts about halfway up the vertical axis and travels a short distance parallel to the horizontal axis; this segment is A plus B. The line then curves up sharply, peaks, and falls sharply to run parallel to and near the horizontal axis; the final segment is C. A double-headed arrow extends from A plus B to C.

A graph has Reaction progression on the horizontal axis and energy on the vertical axis. A line segment labeled A plus B starts low and runs parallel to the horizontal axis. The line then curves up sharply, peaks, and falls a short distance to run parallel to the horizontal axis; the final segment is labeled C. A double-headed arrow extends from A plus B to C.

A graph has Reaction progression on the horizontal axis and energy on the vertical axis. A line segment labeled A plus B starts halfway up the vertical axis and runs parallel to the horizontal axis for a short distance. The line then peaks and returns to the same level as A plus B and continues parallel to the horizontal axis; this segment is labeled C.

A graph has Reaction progression on the horizontal axis and energy on the vertical axis. A line segment labeled A plus B starts low on the vertical axis and runs parallel to the horizontal axis for a short distance. The line then rises sharply and levels off; this level section is labeled C. A double-headed arrow extends from the level of A plus B to that of C.

Which statement is true if the Hrxn of a reaction is positive?

The reaction is endothermic.

The reaction is exothermic.

There is more reactant than product.

There is more product than reactant.

Butane C4 H10 (g),(Hf = –125.6), combusts in the presence of oxygen to form CO2 (g) (Hf = –393.5 kJ/mol), and H2 O(g) (Hf = –241.82) in the reaction: What is the enthalpy of combustion, per mole, of butane? Use .

–5,315 kJ/mol

–2,657.5 kJ/mol

2,657.4 kJ/mol

5,314.8 kJ/mol

The enthalpies of formation of the compounds in the combustion of methane, ,are CH4 (g): Hf = –74.6 kJ/mol; CO2 (g): Hf = –393.5 kJ/mol; and H2 O(g): Hf = –241.82 kJ/mol.How much heat is released by the combustion of 2 mol of methane? Use .

80.3 kJ

802.5 kJ

1,605.1 kJ

6,420.3 kJ

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