Q: Which equation correctly expresses the standard enthalpy change of a reaction using standard enthalpies of formation?

$$\Delta H_{rxn}^\circ = \sum n \Delta H_f^\circ \text{(products)} - \sum n \Delta H_f^\circ \text{(reactants)}$$

Q: A photon with a wavelength of $$200\ \mathrm{nm}$$ shines on a metal with a binding energy of $$4.0 \times 10$$^{-19}$$\ \mathrm{J}. $$What is the maximum kinetic energy of the ejected electron? ($$h = 6.626 \times 10$$^{-34}$$\ \mathrm{J\cdot s}$$, $$c = 3.00 \times 10^8$\ \mathrm{m/s})$$

$$6.23 \times 10$$^{-19}$$\ \mathrm{J}$$

Question 1

A runaway polymerization reaction of styrene in a tank car can lead to an explosion. Which of the following best explains why water is sprayed on the tank car in such a scenario?

Accepted Answer

To cool the tank and absorb the exothermic heat, preventing the temperature from reaching the boiling point of styrene.

Question 2

Given the reaction: $$\mathrm{Ca(OH)_2(s) + CO_2$(g) \rightarrow $$CaCO_3$$(s) + H_2O(g)}$$, and $$1.00\ \mathrm{kg} of$$ $$\mathrm{Ca(OH)_2}$$, calculate the total heat generated if $$\Delta H_{rxn}^\circ = -69.1\ \mathrm{kJ/mol}. ($$Molar mass of $$\mathrm{Ca(OH)_2} is$$ $$74.09\ \mathrm{g/mol})$$

Accepted Answer

$$-932\ \mathrm{kJ}$$

Question 3

Which equation correctly expresses the standard enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

$$\Delta H_{rxn}^\circ = \sum n \Delta H_f^\circ 	ext{(products)} - \sum n \Delta H_f^\circ 	ext{(reactants)}$$

Question 4

The combustion of styrene ($$\mathrm{C_8H_8(l)}) is $$given by: $$\mathrm{C_8H_8(l) + 10 O_2$(g) \rightarrow 8 CO_2$(g) + 4 H_2O(l)}$$, $$\Delta H_{rxn}^\circ = -4395.0\ \mathrm{kJ}. $$Given $$\Delta H_f^\circ$$ for $$\mathrm{CO_2$(g)} = -393\ \mathrm{kJ/mol}$$ and $$\mathrm{H_2O(l)} = -286\ \mathrm{kJ/mol}$$, what is $$\Delta H_f^\circ$$ for styrene?

Accepted Answer

$$97\ \mathrm{kJ/mol}$$

Question 5

Which of the following statements about bond enthalpy is correct?

Accepted Answer

Bond enthalpy is always positive because energy is required to break a bond.

Question 6

Using average bond enthalpies, estimate $$\Delta H_{rxn}$$ for the reaction: $$\mathrm{CH_4$ + Cl_2$ \rightarrow CH_3Cl + HCl}. $$Use the following bond enthalpies (in $$\mathrm{kJ/mol}$$): $$\mathrm{C-H}$$: 413, $$\mathrm{Cl-Cl}$$: 242, $$\mathrm{C-Cl}$$: 328, $$\mathrm{H-Cl}$$: 431.

Accepted Answer

$$-104\ \mathrm{kJ}$$

Question 7

Which of the following best describes the energy content of foods and fuels?

Accepted Answer

It is approximately equal to the enthalpy change of combustion at constant pressure.

Question 8

Which fuel has the highest energy content per gram according to the provided data?

Accepted Answer

Hydrogen

Question 9

Which equation correctly relates the speed of light ($$c$$), wavelength ($$\lambda$$), and frequency ($$
u) of $$electromagnetic radiation?

Accepted Answer

$$
u = \frac{c}{\lambda}$$

Question 10

A photon with a wavelength of $$200\ \mathrm{nm}$$ shines on a metal with a binding energy of $$4.0 	imes 10$$^{-19}$$\ \mathrm{J}. $$What is the maximum kinetic energy of the ejected electron? ($$h = 6.626 	imes 10$$^{-34}$$\ \mathrm{J\cdot s}$$, $$c = 3.00 	imes 10^8$\ \mathrm{m/s})$$

Accepted Answer

$$6.23 	imes 10$$^{-19}$$\ \mathrm{J}$$

Question 11

Which phenomenon demonstrates the wave nature of light but not of particles?

Accepted Answer

Diffraction

Question 12

According to Einstein’s explanation of the photoelectric effect, what determines whether electrons are ejected from a metal surface?

Accepted Answer

The frequency of the incident light must be above a threshold value specific to the metal.

Question 13

Which equation gives the energy of a photon?

Accepted Answer

$$E = h 
u$$

Question 14

The Rydberg equation for the hydrogen emission spectrum is $$\frac{1}{\lambda} = R_H \left( \frac{1}{m^2$} - \frac{1}{n^2$} \right). $$What does $$R_H$$ represent?

Accepted Answer

The Rydberg constant, a proportionality constant for hydrogen spectral lines.

Question 15

According to Bohr’s model, what happens when an electron in a hydrogen atom jumps from $$n=3 to$$ $$n=2$$?

Accepted Answer

A photon is emitted with energy equal to the difference between the two energy levels.

Question 16

Which of the following is a limitation of the Bohr model of the atom?

Accepted Answer

It only accurately describes hydrogen and hydrogen-like ions.

Question 17

The de Broglie wavelength of a particle is given by $$\lambda = \frac{h}{p}. $$What does $$p$$ represent?

Accepted Answer

The momentum of the particle.

Question 18

Which of the following best explains why the color of an object is primarily determined by the colors of white light that it reflects?

Accepted Answer

Because most objects absorb some wavelengths and reflect others, and the reflected wavelengths determine the observed color.

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