Q: Which of the following is the correct conversion between calories and joules?

$$1 \text{ cal} = 4.184 \text{ J}$$

Question 1

Which of the following statements best describes the First Law of Thermodynamics?

Accepted Answer

The total energy of the universe is a constant; energy can be transferred or transformed but not created or destroyed.

Question 2

A chemical reaction occurs in an insulated calorimeter. The temperature of the water increases. Which statement is true about the reaction?

Accepted Answer

The reaction is exothermic because heat is released to the surroundings.

Question 3

Which equation correctly relates the change in internal energy $$(\Delta E) to $$heat $$(q)$$ and work $$(w)$$?

Accepted Answer

$$\Delta E = q + w$$

Question 4

A 100.0 g sample of water at $$25.0^\circ C$$ absorbs $$418.4 J of $$heat. What is the final temperature of the water? (Specific heat of water $$= 4.184 J/g$\cdot ^\circC)$$

Accepted Answer

$$26.0^\circ C$$

Question 5

Which of the following is the correct formula for calculating the heat absorbed by a substance using its specific heat?

Accepted Answer

$$q = s \cdot m \cdot \Delta T$$

Question 6

A student dissolves $$0.10$$ mol of $$NH_4Cl in$$ $$100.0 g of $$water in a calorimeter. The temperature drops from $$25.0^\circ C to$$ $$18.0^\circ C. $$The specific heat of the mixture is $$4.184 J/g$\cdot ^\circC $$and the calorimeter has a heat capacity of $$9.72 J$$/$$^\circC. $$What is the total heat absorbed by the system?

Accepted Answer

$$-3.0 	imes 10^3$ J$

Question 7

Which of the following best describes an endothermic reaction?

Accepted Answer

A reaction that absorbs heat from the surroundings, resulting in a positive $$\Delta H.$$

Question 8

If $$2.00 g$$ ($$0.0125$$ mol) of $$Br_2$$ reacts with excess $$NaOH$$ and the measured $$\Delta H is$$ $$-516 J$$, how much heat would be released if $$1.00$$ mol of $$Br_2$$ reacted under the same conditions?

Accepted Answer

$$-41.3 kJ$$

Question 9

Which of the following statements about thermochemical equations is correct?

Accepted Answer

Multiplying all coefficients in a thermochemical equation by a factor multiplies $$\Delta H by $$the same factor.

Question 10

Which of the following is a correct application of Hess's Law?

Accepted Answer

The enthalpy change for a reaction is the sum of the enthalpy changes for each step leading from reactants to products.

Question 11

Given the following reactions:
$$S(s) + O_2$(g) \rightarrow SO_2$(g) \quad \Delta H = -296.8 kJ$$
$$SO_2(g$$) + \frac{1}{2}$$O_2(g$$) \rightarrow $$SO_3(g$$) \quad \Delta H = -98.9$$ kJ
What is $$\Delta H$$ for S(s$$) + \frac{3}{2}$$O_2(g$$) \rightarrow $$SO_3(g$$)$$?

Accepted Answer

$$-395.7 kJ$$

Question 12

Which of the following is the correct conversion between calories and joules?

Accepted Answer

$$1 	ext{ cal} = 4.184 	ext{ J}$$

Question 13

A sample of copper ($$s = 0.385 J/g$\cdot ^\circC) $$weighing $$50.0 g is $$heated from $$20.0^\circ C to$$ $$40.0^\circ C. $$How much heat is absorbed?

Accepted Answer

$$385 J$$

Question 14

Which of the following best explains why temperature is used in calorimetry experiments to determine $$q$$?

Accepted Answer

Temperature change is directly proportional to the amount of heat absorbed or released by the system.

Question 15

Which of the following is the correct formula for calculating the heat absorbed by $$n$$ moles of a substance using its molar heat capacity?

Accepted Answer

$$q = C_m \cdot n \cdot \Delta T$$

Question 16

Which of the following statements is true about the sign of $$\Delta H in a $$chemical reaction?

Accepted Answer

$$\Delta H is $$negative for exothermic reactions and positive for endothermic reactions.

Question 17

A calorimeter contains $$200.0 g of $$water at $$22.0^\circ C. A $$reaction causes the temperature to rise to $$25.0^\circ C. $$What is the heat change for the water? (Specific heat $$= 4.184 J/g$\cdot ^\circC)$$

Accepted Answer

$$2,510 J$$

Question 18

Which of the following is NOT a correct statement about calorimetry?

Accepted Answer

Calorimetry uses temperature changes to indirectly calculate heat.

Calorimetry, Enthalpy, and Hess’s Law: Energy in Chemical Reactions

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