Q: Given the reaction: $$\mathrm{CH_4$(g) + 2O_2(g) \rightarrow CO_2$(g) + 2H_2O(g)}$$, use the following average bond enthalpies to estimate $$\Delta H_{rxn}$$: $$D(C-H) = 413\ \mathrm{kJ/mol}$$, $$D(O=O) = 495\ \mathrm{kJ/mol}$$, $$D(C=O) = 799\ \mathrm{kJ/mol}$$, $$D(O-H) = 463\ \mathrm{kJ/mol}. $$Which is the correct calculation for $$\Delta H_{rxn}$$?

$$\Delta H_{rxn} = [4 \times 413 + 2 \times 495] - [2 \times 799 + 4 \times 463]$$

Q: If a laser pointer emits light at a wavelength of $$650\ \mathrm{nm}$$, what is the frequency of the emitted light? (Speed of light $$c = 3.00 \times 10^8$\ \mathrm{m/s})$$

$$4.62 \times 10$$^{14}$$\ \mathrm{Hz}$$

Q: What is the energy of a single photon emitted by a red laser pointer at $$650\ \mathrm{nm}$$? ($$h = 6.626 \times 10$$^{-34}$$\ \mathrm{J\cdot s}$$, $$c = 3.00 \times 10^8$\ \mathrm{m/s})$$

$$3.06 \times 10$$^{-19}$$\ \mathrm{J}$$

Q: Which formula is used to estimate the enthalpy change ($$\Delta H_{rxn}) of a $$reaction using bond enthalpies?

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

Question 1

Which statement best explains why the bond enthalpy is always a positive value?

Accepted Answer

Because energy is required to break a chemical bond.

Question 2

Given the reaction: $$\mathrm{CH_4$(g) + 2O_2(g) \rightarrow CO_2$(g) + 2H_2O(g)}$$, use the following average bond enthalpies to estimate $$\Delta H_{rxn}$$: $$D(C-H) = 413\ \mathrm{kJ/mol}$$, $$D(O=O) = 495\ \mathrm{kJ/mol}$$, $$D(C=O) = 799\ \mathrm{kJ/mol}$$, $$D(O-H) = 463\ \mathrm{kJ/mol}. $$Which is the correct calculation for $$\Delta H_{rxn}$$?

Accepted Answer

$$\Delta H_{rxn} = [4 	imes 413 + 2 	imes 495] - [2 	imes 799 + 4 	imes 463]$$

Question 3

Which of the following bonds has the highest average bond enthalpy according to the provided table?

Accepted Answer

N≡N

Question 4

A reaction is endothermic if:

Accepted Answer

The total energy required to break bonds is greater than the energy released in forming new bonds.

Question 5

Which equation correctly relates the energy of a photon to its frequency?

Accepted Answer

$$E = hv$$

Question 6

If a laser pointer emits light at a wavelength of $$650\ \mathrm{nm}$$, what is the frequency of the emitted light? (Speed of light $$c = 3.00 	imes 10^8$\ \mathrm{m/s})$$

Accepted Answer

$$4.62 	imes 10$$^{14}$$\ \mathrm{Hz}$$

Question 7

Which of the following phenomena cannot be explained by the classical wave model of light?

Accepted Answer

Photoelectric effect

Question 8

What is the energy of a single photon emitted by a red laser pointer at $$650\ \mathrm{nm}$$? ($$h = 6.626 	imes 10$$^{-34}$$\ \mathrm{J\cdot s}$$, $$c = 3.00 	imes 10^8$\ \mathrm{m/s})$$

Accepted Answer

$$3.06 	imes 10$$^{-19}$$\ \mathrm{J}$$

Question 9

Which statement about electromagnetic radiation is correct?

Accepted Answer

Electromagnetic radiation consists of oscillating electric and magnetic fields.

Question 10

Which of the following correctly describes the relationship between wavelength ($$\lambda) $$and frequency ($$v) $$for electromagnetic waves?

Accepted Answer

$$\lambda$$ and $$v$$ are inversely proportional.

Question 11

Which of the following best explains why ultraviolet light is more dangerous to biological tissues than visible light?

Accepted Answer

Ultraviolet light has a shorter wavelength and higher energy than visible light.

Question 12

A chemist wants to predict whether a reaction will be exothermic or endothermic. Which information is most useful?

Accepted Answer

The average bond enthalpies of all bonds broken and formed.

Question 13

Which formula is used to estimate the enthalpy change ($$\Delta H_{rxn}) of a $$reaction using bond enthalpies?

Accepted Answer

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

Question 14

Which of the following is a consequence of the quantization of energy in atoms?

Accepted Answer

Atoms emit light only at specific wavelengths, producing line spectra.

Question 15

Which real-world application relies on the photoelectric effect?

Accepted Answer

Solar panels converting sunlight into electricity.

Question 16

A molecule has a bond that requires $$799\ \mathrm{kJ/mol} to $$break. What does this imply about the bond?

Accepted Answer

It is a strong bond and requires a lot of energy to break.

Question 17

Which of the following best describes the difference between amplitude and wavelength in a wave?

Accepted Answer

Amplitude is the height of the wave; wavelength is the distance between two corresponding points on adjacent waves.

Question 18

Which of the following is true about the speed of light ($$c) in a $$vacuum?

Accepted Answer

It is a constant value for all electromagnetic radiation.

Question 19

A student calculates the enthalpy change for a reaction and gets a positive value. What does this indicate about the reaction?

Accepted Answer

The reaction is endothermic and absorbs heat.

Bond Enthalpy, Enthalpy of Reaction, and the Wave Nature of Light

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