Q: A student reacts 0.2459 g of Zn ($$\mathrm{Zn(s) + 2\ HCl(aq) \rightarrow $$ZnCl_2$$(aq) + H_2$(g)}) in a $$coffee-cup calorimeter containing 120.0 mL of solution ($$d = 1.00\ \mathrm{g/mL}$$, $$C_s = 4.18\ \mathrm{J/g^\circ C}$$, $$\Delta T = 6.2^\circ \mathrm{C}). $$What is the enthalpy change per mole of Zn?

$$-8.29 \times 10^3$\ \mathrm{J/mol}$$

Q: Which of the following is the correct formula for calculating the heat change in a coffee-cup calorimeter?

$$q = \text{mass} \times C_s \times \Delta T$$

Question 1

Which of the following statements correctly describes an exothermic reaction?

Accepted Answer

It has a negative $$\Delta H$$ and releases heat to the surroundings.

Question 2

Given the reaction $$\mathrm{CH_4$(g) + 2\ O_2$(g) \rightarrow CO_2$(g) + 2\ H_2O(g)}$$ with $$\Delta H_{rxn} = -802\ \mathrm{kJ}$$, how much heat is released when 32.08 g of methane ($$\mathrm{CH_4$}$$, molar mass = 16.04 g/mol) is completely combusted?

Accepted Answer

$$-1604\ \mathrm{kJ}$$

Question 3

Which equation correctly relates the enthalpy change ($$\Delta H) to $$the internal energy change ($$\Delta E) $$and pressure-volume work at constant pressure?

Accepted Answer

$$\Delta H = \Delta E + P\Delta V$$

Question 4

A student performs a reaction in a coffee-cup calorimeter using 100.0 g of solution with a specific heat capacity $$C_s = 4.18\ \mathrm{J/g^\circ C}$$ and observes a temperature increase of $$5.0^\circ \mathrm{C}. $$What is the heat absorbed by the solution?

Accepted Answer

$$2090\ \mathrm{J}$$

Question 5

Which of the following is true about the enthalpy of reaction ($$\Delta H_{rxn}$$)?

Accepted Answer

It is an extensive property and depends on the amount of reactants used.

Question 6

If the reaction $$\mathrm{C(s) + O_2$(g) \rightarrow CO_2$(g)}$$ has $$\Delta H = -393.5\ \mathrm{kJ}$$, what is $$\Delta H$$ for the reverse reaction $$\mathrm{CO_2$(g) \rightarrow C(s) + O_2$(g)}$$?

Accepted Answer

$$+393.5\ \mathrm{kJ}$$

Question 7

A reaction is carried out in a coffee-cup calorimeter. If the mass of the solution is 120.0 g, $$C_s = 4.18\ \mathrm{J/g^\circ C}$$, and $$\Delta T = 6.2^\circ \mathrm{C}$$, what is the heat change for the solution?

Accepted Answer

$$3109.92\ \mathrm{J}$$

Question 8

Which statement best describes the use of Hess's Law?

Accepted Answer

The enthalpy change for a reaction is the sum of the enthalpy changes for each step in a series of reactions.

Question 9

If a chemical equation is multiplied by a factor of 2, what happens to the enthalpy change ($$\Delta H) $$for the reaction?

Accepted Answer

$$\Delta H is $$doubled.

Question 10

A student reacts 0.2459 g of Zn ($$\mathrm{Zn(s) + 2\ HCl(aq) \rightarrow $$ZnCl_2$$(aq) + H_2$(g)}) in a $$coffee-cup calorimeter containing 120.0 mL of solution ($$d = 1.00\ \mathrm{g/mL}$$, $$C_s = 4.18\ \mathrm{J/g^\circ C}$$, $$\Delta T = 6.2^\circ \mathrm{C}). $$What is the enthalpy change per mole of Zn?

Accepted Answer

$$-8.29 	imes 10^3$\ \mathrm{J/mol}$$

Question 11

Which of the following best explains why an endothermic reaction feels cold to the touch?

Accepted Answer

The reaction absorbs heat from the surroundings, lowering the temperature.

Question 12

In a calorimetry experiment, why is the sign of $$q_{rxn}$$ opposite to the sign of $$q_{soln}$$?

Accepted Answer

Because the heat lost by the solution is gained by the reaction.

Question 13

Which of the following is the correct formula for calculating the heat change in a coffee-cup calorimeter?

Accepted Answer

$$q = 	ext{mass} 	imes C_s 	imes \Delta T$$

Question 14

A reaction has $$\Delta H = -500\ \mathrm{kJ}$$ when 2 moles of reactant are used. What is $$\Delta H if $$only 1 mole of reactant is used?

Accepted Answer

$$-250\ \mathrm{kJ}$$

Question 15

Which of the following is NOT a correct application of Hess's Law?

Accepted Answer

The $$\Delta H of a $$reaction is always independent of the pathway taken.

Question 16

If the enthalpy change for $$\mathrm{Al(g) + 1.5\ Cl_2$(g) \rightarrow $$AlCl_3$$(s)} is$$ $$-704\ \mathrm{kJ}$$, what is the enthalpy change for $$2\mathrm{Al(g) + 3\ Cl_2$(g) \rightarrow 2\ $$AlCl_3$$(s)}$$?

Accepted Answer

$$-1408\ \mathrm{kJ}$$

Question 17

Which of the following best describes the role of a coffee-cup calorimeter in measuring enthalpy changes?

Accepted Answer

It measures the heat change at constant pressure for reactions in solution.

Question 18

A chemical reaction is performed and the temperature of the solution decreases. What can be concluded about the reaction?

Accepted Answer

The reaction is endothermic and absorbs heat from the surroundings.

Question 19

Which of the following is a correct statement about standard enthalpy changes?

Accepted Answer

Standard enthalpy changes are measured under standard conditions, typically $$1\ \mathrm{atm}$$ pressure and $$25^\circ \mathrm{C}.$$

Enthalpy, Calorimetry, and Hess's Law (General Chemistry Study Notes)

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