Extra Credit 41

1. Which member of each pair of metals is more likely to corrode (oxidize)?

1. Mg or Ca
2. Au or Hg
3. Fe or Zn
4. Ag or Pt

ANS: Corrosion is when metals act as an anode and then get oxidized. Metals with smaller standard reduction potential are more likely to corrode. These are determined experimentally.

https://chem.libretexts.org/Core/Ana...rrosion_Basics

a. Ca has less standard reduction potential than Mg

b. Hg has less standard reduction potential than Au

c. Zn has less standard reduction potential than Fe

d. Ag has less standard reduction potential than Pt

2. How much and in what direction will each of the following affect the rate of the reaction:

CO(g)+NO₂(g)⟶CO₂(g)+NO(g)CO(g)+NO₂(g)⟶CO₂(g)+NO(g)

if the rate law for the reaction is rate=k[NO₂]²?

a. Decreasing the pressure of NO₂ from 0.50 atm to 0.250 atm. ANS: In this problem we are seeing what the difference would be when we replace the NO₂ in the rate equation given from .5 to .25 thus cutting it in half. If we write it out, it gives us k[.5]² which equals .25k, k being a constant which wont affect the answer. k[.25]² gives us .0625k. So by decreasing the pressure by half it brought our answer down by a factor of four from .25k to .0625k.

b. Increasing the concentration of CO from 0.01 M to 0.03 M. ANS: This would have no effect since CO is not in the rate law/equation given.

3. Hydrogen iodide, HI, decomposes in the gas phase to produce hydrogen, H2, and iodine, I2. The value of the rate constant, k, for the reaction was measured at several different temperatures and the data are shown here:

What is the value of the activation energy (in kJ/mol) for this reaction?

ANS: Activation energy is the minimum energy required by the reactants to undergo a collision and cross the energy barrier to move in the forward direction and form products.

Using the equation:

Ln(K1/K2)=(E_a/R) x ((1/T2)-(1/T1))

Where R is a constant (8.314), K1 is the rate of the reaction at T1 temperature, and K2 is the rate of the reaction at T2 temperature.

From the data we use 6.23x10^-7 as K1 and its corresponding temperature 555K as T1. We use 2.01x10^-3 as K2 and its corresponding temperature 700K as T2.

We can plug in the data given into this equation and find E_a which is the activation energy.

Ln(6.23x10^-7 L Mol-1 s-1/2.01x10^-3 L Mol-1 s-1)=(E_a/8.314)*((1/700K)-(1/555K))

-8.07=E_a*-4.48x10^-5 J Mol-1

E_a= 177 kJ mol-1

https://chem.libretexts.org/Core/Phy...on_Energy_-_Ea

4. The following nuclei do not lie in the band of stability. How would they be expected to decay? Explain your answer.

a. ¹⁵₂₈P

b. ⁹²₂₃₅U

c. ²⁰₃₇Ca

d. ³₉Li

e. ⁹⁶₂₄₅Cm

ANS: Radioactive decay occurs to change the neutron to proton ratio to a stable state. If the ratio is too high, then the particle goes through Alpha decay. Positron emission is used when the ratio is too low. Beta also decreases the ratio.

a. Beta decay to decrease the ratio

b. Alpha decay to decrease the ratio

c. Positron emission to increase the ratio

d. Beta emission to decrease the ratio

e. Alpha emission to decrease the ratio

https://chem.libretexts.org/Textbook...nd_Gamma_Decay

5. Using the activity series, predict what happens in each situation. If a reaction occurs, write the net ionic equation; then write the complete ionic equation for the reaction.

1. A few drops of NiBr₂ are dropped onto a piece of iron
2. A strip of zinc is placed into a solution of HCl.
3. Copper is dipped into a solution of ZnCl₂.
4. A solution of silver nitrate is dropped onto an aluminum plate.

Ans:

a. Iron will be oxidized because it is higher on the activity series. This also means it will replace Nickel in NiBr₂ in a single replacement reaction. Nickel will be reduced because it is lower on the activity series than Iron. Nickel will go from a 2+ charge to a neutral charge. Iron goes from a neutral charge to a +2 charge.

Ni²⁺ + 2e^- ⇒ Ni

Fe ⇒ 2e^- + Fe²⁺

NiBr₂ + Fe ⇒ FeBr₂ + Ni

b. This is a single replacement reaction where Zinc displaces Hydrogen. This forms Hydrogen gas and a salt Zinc Chloride. The Zinc begins with a charge of zero and loses an electron and becomes positively charged aka becoming oxidized. Hydrogen gets reduced and becomes a neutral molecule.

Zn ⇒ 2e^- + Zn²⁺

H⁺ + e^- ⇒ H₂

Zn + HCl ⇒ H₂ + 2ZnCl₂

c. In this equation Cl is a spectator ion. Zinc will be oxidized because it is higher on the activity series and Copper ion will be reduced to Copper metal because it is lower on the activity series.

Zn ⇒ 2e^- + Zn²⁺

Cu²⁺ + 2e^- ⇒ Cu

Zn (s) + Cu²⁺ ⇒ Zn²⁺ + Cu (s)

d. Silver Nitrate is a single replacement reaction where Aluminum takes the place of Silver. Aluminum is oxidized from a +1 charge to a 2+ charge because it is higher on the activity series. Silver is lower on the activity series so it will be reduced from a 2+ charge to a +1 charge.

Al⁺ ⇒ Al²⁺ e^-

Ag²⁺ + e^- ⇒ Ag⁺

Ag(NO₃) + Al⁺ ⇒ Al(NO₃) + Ag⁺

https://chem.libretexts.org/LibreTex...ctivity_Series

6. Occasionally, you will find high-quality electronic equipment that has its electronic components plated in gold. What is the advantage of this?

ANS: The advantage of this is that gold has a highly positive reduction potential. This means that it is very resistant to corrosion because it does not form oxides with hydrated iron oxide (rust). It is the least reactive metal.

7. Will the value of Δo increase or decrease if I− ligands are replaced by NO2− ligands? Why?

ANS: NO2 is a strong field and I- is a weak field. This means NO2 has a high spin complex and I- has a low spin complex. Lower spin complexes have greater crystal field splitting energy where as high spin complexes have lower crystal field splitting energy. So, if you replace I- with NO2 it will have a lower crystal field splitting energy.

8. The following data were obtained for the reaction of methane with oxygen:

CH4(g)+2O2(g)→CO2(g)+2H2O(l)CH4g2O2gCO2g2H2Ol

1. How many moles of CO2 are produced for each mole of CH4CH4 that is used up?
2. What concentration of CH4 is used up after 10 minutes?
3. What is the concentration of carbon dioxide produced after 20 minutes?
4. Write an equation for reaction rate in terms of Δ[CO2]ΔCO2 over a time interval.
5. What is the reaction rate for the formation of carbon dioxide between 10 and 20 minutes?
6. What is the average reaction rate between 0 and 30 minutes?
7. Write an expression for reaction rate relating Δ[O2] to Δ[CO2].
8. At what rate is O2 used up between 10 and 20 minutes?

Ans: The stoichiometric equation for this reaction is

CH₄ (g) + 2O_{2 (g)} ⇒ CO₂ (g) +2H₂O (l)

1. The number of moles of CO₂ produced is equal to the number of CH₄ that is used up. 1:1 ratio.

2. Concentration of CH₄ after 10 minutes equals .05-.03 = .02M. The difference between the starting concentration of CH₄ and the concentration after 10 minutes.

3. The concentration of CO₂ produced after 20 minutes is .03M.

4. Rate = d[CO₂]/dt = [.03-.02]/(20-10) = .001 min^-1

5. This answer will be equal to the solution in 4.

6. Average Rate = [.035-0.00]/(30-0) = .001167 minutes

7. 2k[O₂]ⁿ = k[CO₂]ⁿ Based off the stoichiometric equation we can look at the coefficients of O₂ and CO₂ we can see the reaction rate of O₂ must be twice that of CO₂.

8. O₂ is used up twice as fast as CH₄ as a reactant so it will have half the concentration from 10 minutes to 20 minutes as CH₄.

https://chem.libretexts.org/Textbook...Reaction_Rates