Q: Which equation is used to estimate the enthalpy change of a reaction using bond enthalpies?

$$\Delta H_{rxn} = \sum \text{bond enthalpies of bonds broken} - \sum \text{bond enthalpies of bonds formed}$$

Question 1

Which of the following statements about electrostatic potential energy ($$E_{el}) is $$correct for two particles with opposite charges?

Accepted Answer

The electrostatic potential energy is always negative, regardless of the distance between the particles.

Question 2

A chemical system absorbs 150 kJ of heat and has 40 kJ of work done ON it by the surroundings. What are the signs of $$q$$ and $$w$$ for the system?

Accepted Answer

$$q is $$positive, $$w is $$positive

Question 3

Which equation correctly represents the relationship for calculating the change in internal energy ($$\Delta E) of a $$system?

Accepted Answer

$$\Delta E = q + w$$

Question 4

If 5.00 g of propane ($$C_3H_8) is $$combusted and the reaction releases $$-2220 kJ $$per mole of propane, how much heat is released?

Accepted Answer

$$-505 kJ$$

Question 5

Which of the following best describes an endothermic reaction under constant pressure?

Accepted Answer

$$\Delta H \u003e 0$$ and heat is absorbed by the system

Question 6

A student mixes 100.0 mL of 1.0 M HNO$$_3$$ and 100.0 mL of 1.0 M KOH in a coffee-cup calorimeter. The temperature rises from 20.0°C to 26.0°C. Assuming the density is 1.0 g/mL and specific heat is 4.18 J/(g·K), what is the enthalpy change per mole of acid?

Accepted Answer

$$-25.1 kJ/$$mol$

Question 7

Which property is intensive and why: heat capacity or specific heat?

Accepted Answer

Specific heat, because it is independent of the amount of substance.

Question 8

Which of the following is true about the measurement in a bomb calorimeter?

Accepted Answer

It measures the change in internal energy ($$\Delta E) at $$constant volume.

Question 9

What is the standard enthalpy of formation for $$O_2(g$$)$$?

Accepted Answer

$$0 kJ/$$mol

Question 10

Which equation is used to estimate the enthalpy change of a reaction using bond enthalpies?

Accepted Answer

$$\Delta H_{rxn} = \sum 	ext{bond enthalpies of bonds broken} - \sum 	ext{bond enthalpies of bonds formed}$$

Question 11

Given the reaction: $$2 	ext{H-Cl} + 	ext{F-F} ightarrow 2 	ext{H-F} + 	ext{Cl-Cl}$$, and the bond enthalpies: H-Cl = 431 kJ/mol, F-F = 155 kJ/mol, H-F = 567 kJ/mol, Cl-Cl = 253 kJ/mol, what is $$\Delta H_{rxn}$$?

Accepted Answer

$$-359 kJ$$

Question 12

Which of the following reactions represents the standard enthalpy of formation for sodium carbonate ($$Na_2CO_3$$)?

Accepted Answer

$$2Na(s) + C(graphite) + \frac{3}{2}O_2$(g) \rightarrow Na_2CO_3$$

Question 13

Which of the following statements about the sign conventions for $$q$$ and $$w is $$correct when heat leaves the system and work is done ON the system?

Accepted Answer

$$q is $$negative, $$w is $$positive

Question 14

Which of the following best explains why water has a higher heat capacity than iron?

Accepted Answer

Water requires more energy to raise its temperature by 1°C compared to iron.

Question 15

Which formula is used to calculate the heat absorbed or released by a substance in calorimetry at constant pressure?

Accepted Answer

$$q = mC\Delta T$$

Question 16

If a reaction has $$\Delta H \u003c 0$$, which of the following is true?

Accepted Answer

The reaction is exothermic and releases heat.

Question 17

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

Accepted Answer

The enthalpy change of a reaction is the sum of the enthalpy changes of the individual steps.

Question 18

Which of the following is true about the standard enthalpy of formation for elements in their most stable form?

Accepted Answer

It is always zero.

Chapter 5: Thermochemistry and Chemical Energy – Study Notes

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