Question 1

A chemical reaction is carried out at constant pressure and releases heat to the surroundings. Which of the following best describes this reaction?

Accepted Answer

It is exothermic, and ΔH is negative.

Question 2

Which of the following best describes Hess's Law?

Accepted Answer

The enthalpy change for a reaction is the same, regardless of the number of steps or the path taken, because enthalpy is a state function.

Question 3

Which of the following reactions has a standard enthalpy change (ΔH°rxn) equal to the standard enthalpy of formation (ΔHf°) of HBr(g)?

Accepted Answer

½ H2(g, 1 bar) + ½ Br2(l, 1 bar) → HBr(g, 1 bar)

Question 4

A student wants to determine the enthalpy change for a reaction that cannot be performed directly in the laboratory. Which approach should they use?

Accepted Answer

Apply Hess's Law by combining known enthalpy changes for related reactions.

Question 5

Which of the following statements about the temperature dependence of enthalpy is correct?

Accepted Answer

The enthalpy change of a reaction at a new temperature can be calculated using the heat capacity at constant pressure (Cp) and integrating over the temperature range.

Question 6

Consider the reaction: Fe2O3(s) + 3H2(g) → 2Fe(s) + 3H2O(l). Which of the following is the correct expression for ΔHrxn using standard enthalpies of formation?

Accepted Answer

ΔHrxn = [2ΔHf°(Fe) + 3ΔHf°(H2O)] - [ΔHf°(Fe2O3) + 3ΔHf°(H2)]

Question 7

Which of the following best describes a real-world application of Hess's Law?

Accepted Answer

Calculating the enthalpy change for the combustion of a new fuel by combining known enthalpy changes for related reactions.

Question 8

If the heat capacity at constant pressure (Cp) for a reaction is constant over a temperature range, which equation allows you to calculate the enthalpy change at a new temperature T2, given ΔH at T1?

Accepted Answer

ΔH(T2) = ΔH(T1) + ΔCp × (T2 - T1)

Question 9

Which of the following is NOT a correct statement about enthalpy (H)?

Accepted Answer

Enthalpy is measured on an absolute scale for all substances.

Question 10

A reaction has ΔH \u003e 0. Which of the following is true about this reaction?

Accepted Answer

It is endothermic and absorbs heat from the surroundings.

Question 11

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

Accepted Answer

Its value depends only on the current state of the system, not on the path taken to reach that state.

Question 12

Which of the following is the correct general formula for the enthalpy change of a reaction at constant pressure?

Accepted Answer

ΔHrxn = ΣνiΔHf°(products) - ΣνjΔHf°(reactants)

Question 13

Which of the following would NOT be considered a standard state for an element at 298.15 K and 1 bar?

Accepted Answer

Br2(g)

Question 14

A chemist wants to calculate the enthalpy change for the reaction: 2A + B → 3C. The standard enthalpies of formation are: ΔHf°(A) = -100 kJ/mol, ΔHf°(B) = 0 kJ/mol, ΔHf°(C) = -200 kJ/mol. What is ΔH°rxn?

Accepted Answer

ΔH°rxn = -200 kJ

Question 15

Which of the following best describes the process for calculating the enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

Subtract the sum of the standard enthalpies of formation of the reactants (each multiplied by their stoichiometric coefficients) from the sum for the products.

Enthalpy, Heats of Reaction, and Hess's Law in General Chemistry ....

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