Extra Credit 38 (double

Q17.5.6

Question: Why do batteries go dead, but fuel cells do not?

Solution/Tutorial:

Batteries store all the chemicals inside, and undergo oxidation-reduction reactions. When the reaction goes toward completion, the battery uses up all of the reagents. However, Fuel cells won't die because there is a constant supply of reactants going into the cell. Most fuels cells use hydrogen and oxygen as the chemicals.

Q12.3.1

Question: How do the rate of a reaction and its rate constant differ?

Solution/Tutorial:

The rate of reaction varies with the concentrations of the reactants. The rate constant stays the same at a given temperature, no matter the concentrations of the reactants.

Q12.5.10

Question: The rate constant for the decomposition of acetaldehyde, CH₃CHO, to methane, CH₄, and carbon monoxide, CO, in the gas phase is 1.1 × 10−2 L/mol/s at 703 K and 4.95 L/mol/s at 865 K. Determine the activation energy for this decomposition.

Tutorial:

The rate constant stays the same at a given temperature, however at different temperatures it varies. Therefore the question gave two different rate constants for the two different temperatures. The activation energy will be in joules per mole and R is a constant (8.314 J/mol*K). The Arrhenius equation ln(k₂/k1)=(Ea/R)(1/T₁-1/T₂) will be used to solve for activation energy.

Solution:

ln(k₂/k1)=(Ea/R)(1/T₁-1/T₂) the Arrhenius equation

ln(1.1x10-2/4.95)=(Ea/8.314J/mol*K)(1/865K-1/703K) the Arrhenius equation with the rate constants, temperature, and R substituted

Ea=1.91x10⁵J/mol calculated activation energy in Joules per mole.

Q21.4.5

Question: Why is electron capture accompanied by the emission of an X-ray?

Solution/Tutorial:

The electron pulled into the nucleus was most likely found in the 1s orbital. As an electron falls from a higher energy level to replace it, the difference in the energy of the replacement electron in its two energy levels is given off as an X-ray.

Q20.2.9

Question:

Balance each redox reaction under the conditions indicated.

Tutorial:

oxidation definition: loss of electrons, therefore increasing the oxidation number

reduction definition: gain of electrons, therefore decreasing the oxidation number

-the sum of the oxidation numbers in a compound is zero

-the sum of the oxidation numbers in a polyatomic ion is equal to the charge

-there should be equal numbers of each element on both sides of the equation

-balance the oxygens by adding H₂O and add the corresponding number of H⁺ to the other side of the equation

-for acidic solutions, the H⁺ can remain as is

-for basic solutions, add OH^- to cancel out any H⁺ and instead form water

-cancel out anything that repeats on both sides of the equation

1. CuS(s) + NO₃⁻(aq) → Cu²⁺(aq) + SO₄²⁻(aq) + NO(g); acidic solution
2. Ag(s) + HS⁻(aq) + CrO₄²⁻(aq) → Ag₂S(s) + Cr(OH)₃(s); basic solution
3. Zn(s) + H₂O(l) → Zn²⁺(aq) + H₂(g); acidic solution
4. O₂(g) + Sb(s) → H₂O₂(aq) + SbO₂⁻(aq); basic solution
5. UO₂²⁺(aq) + Te(s) → U⁴⁺(aq) + TeO₄²⁻(aq); acidic solution

Answers:

a. 8H⁺+3CuS+8NO₃^-→3Cu²⁺+3SO₄^2-+8NO+4H₂O

b. 5H₂O+6Ag+3HS^-+2CrO₄^2-→3Ag₂S+2Cr(OH)₃+7OH^-

c. 2H⁺+Zn+H₂O→Zn²⁺+H₂+H₂O

d. 2OH^-+O₂+Sb→H₂O₂+SbO₂^-

e.4H⁺+3UO₂²⁺+Te→3U⁴⁺+TeO₄^2-+2H₂O

Q20.5.4

Question:

For any spontaneous redox reaction, E is positive. Use thermodynamic arguments to explain why this is true.

Tutorial/Solution:

ΔG=-nFE; In order for a reaction to be spontaneous ΔG must be negative. F is Faraday's constant and is always positive and n is the moles of electrons transferred in the reaction which is also positive. As a result, E must be positive to ensure that ΔG is a negative number and make the Redox reaction spontaneous.

Q24.5.1

Question:

1. How many unpaired electrons are found in oxygen atoms ?
2. How many unpaired electrons are found in bromine atoms?
3. Indicate whether boron atoms are paramagnetic or diamagnetic.
4. Indicate whether F- ions are paramagnetic or diamagnetic.
5. Indicate whether Fe2+ ions are paramagnetic or diamagnetic.

Tutorial:

Electrons are distributed into molecular orbitals, the s, p, d, and f blocks. An orbital will have a number in front of it and a letter that corresponds to the block. The s block holds two electrons, the p block holds six, the d block holds ten, and the f block holds fourteen. So, based on the number of electrons an atom has, the molecular orbitals are filled up in a certain way. An electron is first placed in each of the orbitals, and then any additional electrons are added to make the electrons paired in their orbital. Paramagnetic means there are electrons that are unpaired diamagnetic means all of the electrons are paired. Using the electron configuration of the outermost orbital can be used to determine whether there are electrons that are unpaired.

Solution:

1. The O atom has 2s²2p⁴ as the electron configuration. Therefore, O has 2 unpaired electrons.
2. The Br atom has 4s²3d¹⁰4p⁵ as the electron configuration. Therefore, Br has 1 unpaired electron.
3. The B atom has 2s²2p¹ as the electron configuration, so it has one unpaired electron, it is paramagnetic.
4. The F^- ion has 2s²2p⁶ has the electron configuration, so it has no unpaired electrons which means p sublevel is fully occupied by electrons, it is diamagnetic.
5. The Fe²⁺ ion has 3d⁶ has the electron configuration, so it has 4 unpaired electrons, it is paramagnetic.

Answers:

1. paramagnetic 2. paramagnetic 3. paramagnetic 4. diamagnetic 5. paramagnetic

Q14.7.8

Question:

The text identifies several factors that limit the industrial applications of enzymes. Still, there is keen interest in understanding how enzymes work for designing catalysts for industrial applications. Why?

Tutorial/Solution:

Enzymes have really high efficiency on promoting the rate of the reaction. Catalysts can increase reaction rates by enormous factors (up to 10¹⁷ times the uncatalyzed rate) and tend to be very specific, typically producing only a single product in quantitative yield. Without the use of catalyst, the industry can't support high needs of huge population. Its use in meat tenderizers and laundry detergent encourage scientists to explore more applications in our industrial area.