ELECTROCHEMISTRY APCHEM STUDY GUIDE

If the oxygen isotope 20 - O has a half - life of 15 seconds, what fraction of a sample of pure 20 - O remains after 1.0 minute?

1/16

What is the enthalpy for the reaction: CO2 → C (graphite) + O2? C3H8 (g) + 5 O2 (g) → 3 CO2 + 4H2O (l) ΔH = -220 kJ H2 (g) + ½ O2 (g) → H2O (l) ΔH = -258 kJ C3H8 (g) → 3 C (graphite) + 4 H2 (g) ΔH = 104 kJ

394.7kJ

Assume that a 1.5 g sample of lauric acid is combusted and all of the heat energy released is transferred to a 325 g sample of water initially at 25oC. calculate the final temperature of the water if ΔH combustion of lauric acid is -37 kJ/g and the specific heat of water is 4.18 J/gK.

66 degrees C

Use table 20.1 on page 841 to rank the following ions in order of increasing strength as oxidizing agents: NO3 - (aq), Ag + (aq), Cr2O7 2- (aq).

Ag +< NO3 - < Cr2O7 2- (most positive standard reduction potential)

Calculate the emf at 298 K generated by a voltaic cell in which the reaction is Cr2O7 2- (aq) + 14 H+ (aq) + 6 I - (aq) → 2 Cr 3+ (aq) + 3 I2 (s) + 7 H2O (l) when [Cr2O7 2-] = 2.0M, [H+] = 1.0 M, [I-] = 1.0M, and [Cr3+] = 1.0 x 10 -5 M

E = 0.89 V

Determine the value of Ka for propanoic acid at 25oC.

1.3 x 10 5

Al 3+ (aq) + 3 e- → Al (s) Eo = -166 V Ag + (aq) + e - → Ag (s) Eo = + 0.80 V According to the standard reduction potentials given above, what is the standard cell potential for the reaction represented below? 3 Ag + (aq) + Al (s) → 3 Ag(s) + Al 3+ (aq)

+2.46V

A student prepares a solution by dissolving 60.00 g of glucose (molar mass 180.2 g/mol) in enough distilled water to make 250.0 mL of solution. The molarity of the solution should be reported as

1.332 M

2 MnO4 - (aq) + 5 C2O4 2- (aq) + 16 H+ (aq) → 2 Mn 2+ (aq) + 10 CO2 (g) + 8 H2O (l) Permanganate and oxalate ions react in an acidified solution according to the balanced equation above. How many moles of CO2 (g) are produced when 20. mL of acidified 0.20 M KMnO4 solution is added to 50. mL of 0.10 M Na2C2O4 solution?

.01

If 50. mL of 1.0 M NaOH is diluted with distilled water to a volume of 2.0 L, the concentration of the resulting solution is

0.025 M

A student is given the task of determining the concentration of a propanoic acid solution of unknown concentration. A 0.173 M NaOH solution is available to use as the titrant. The student uses a 25.00 mL volumetric pipet to deliver the propanoic acid solution to a clean, dry flask. After adding an appropriate indicator to the flask, the student titrates the solution with the 0.173 M NaOH, reaching the end point after 20.52 mL of the base solution has been added. Calculate the molarity of the propanoic acid solution.

0.142 M

The equilibrium constant for the gas phase reaction below is 95 at 25oC. N2O5 (g) ⬄ 2 NO2 (g) + ½ O2 (g) What is the value of the equilibrium constant for the following reaction at 25oC?

1 / (95) 2

An unknown acid is dissolved in 25 mL of water and titrated with 0.100 M NaOH. The results are shown in the titration curve above. Which of the following could be the unknown acid?

3.8

Which of the following diagrams best represents the Ag NO3 solution before the reaction occurs?

A

Propanoic acid, CH3CH2COOH, is a carboxylic acid that reacts with water according to the equation above. At 25 oC the pH of a 50.0 mL sample of 0.20 M CH3CH2COOH is 2.79. Identify a Bronsted-Lowry conjugate acid-base pair in the reaction. Clearly label which is the acid and which is the base.

CH3CH2COOH acid, CH3CH2COO - base, or H3O +acid, H2O base

A nonstandard cell is made by replacing the 1.0 M solutions of Cu(NO3)2 and Sn (NO3) 2 in the standard cell with 0.50 M solutions of Cu(NO3)2 and Sn (NO3) 2. the volumes of solutions in the nonstandard cell are identical to those in the standard cell. Is the cell potential of the nonstandard cell greater than, less than, or equal to the cell potential of the standard cell? Justify your answer.

It is the same. In the cell reaction Q = [Sn2+] / [ Cu 2+] and the concentrations of each ion are equal to each other in both cases.

Use the standard reduction potentials in table 20.1 to calculate the standard free-energy change ΔGo, and the equilibrium constant K, at 298 K for the reaction: a) 4 Ag (s) + O2 (g) + 4 H+ (aq) → 4 Ag + (aq) + 2 H2O (l) b) Suppose the reaction in part a) is written 2 Ag (s) + ½ O2 (g) + 2 H+ (aq) → 2 Ag + (aq) + H2O (l) What are the values of Eo, ΔGo, and K when the reaction is written this way?

K = 4 x 10 14

Sn (s) is used to reduce other compounds in chemical reactions. If a chemist needs a substance that is more effective in its reducing ability, which of the following species would be the best choice?

K+

M+ is an unknown metal cation with a +1 charge. A student dissolves the chloride of the unknown metal, MCl, in enough water to make 100.0 mL of solution. The student then mixes the solution with excess AgNO3 solution, causing AgCl to precipitate. The student collects the precipitate by filtration, dries it, and records the data shown below. (The molar mass of AgCl is 143g/mol). Mass of unknown chloride, MCl 0.74g Mass of filter paper 0.80 g Mass of filter paper plus AgCl precipitate 2.23 g

KCl

Which of the following identifies the element(s) being oxidized and reduced in the reaction?

Oxygen is both oxidized and reduced

In the expanded view of the center portion of the salt bridge shown in the diagram below, draw and label a particle view of what occurs in the salt bridge as the cell begins to operate. Omit solvent molecules and use arrows to show the movement of particles.

Show at least one K+ ion moving toward the Cu compartment on the left and at least one NO3 - ion moving in the opposite direction.

A student is given a standard galvanic cell, represented above, that has a Cu electrode and a Sn electrode. As current flows through the cell, the student determines that the Cu electrode increases in mass and the Sn electrode decreases in mass. Identify the electrode at which oxidation is occuring. Explain your reasoning based on the student's observations.

Since the Sn electrode is losing mass, Sn atoms must be forming Sn 2+ (aq). This is oxidation. OR Because the cell operates, Eo must be positive, and based on the Eo values from the table, it must be Sn that is oxidized.

A 100 g sample of a metal was heated to 100 oC and then quickly transferred to an insulated container holding 100 g of water at 22oC. The temperature of the water rose to reach a final temperature of 35oC. Which of the following can be concluded?

The metal temperature changed more than the water temperature did, but the metal lost the same amount of thermal energy as the water gained

Both the standard and nonstandard cells can be used to power an electronic device. Would the nonstandard cell power the device for the same time, a longer time, or a shorter time as compared with the standard cell? Justify your answer.

The nonstandard cell would power the device for a shorter time because the supply of Cu 2+ would be exhausted more quickly. OR the nonstandard cell would power the device for a shorter time because the reaction will reach E = 0 more quickly.

Which of the following statements is a correct interpretation of the data regarding how the order of the reaction can be determined?

The reaction is first order if the plot of ln [H2O2] versus time is a straight line

During the course of the experiment, which of the following happens to the NO3 - ions?

They remain dissolved in the filtrate solution

A 0.10 M solution of a weak monoprotic acid has a pH equal to 4.0. The ionization constant, Ka, of the acid is

1 x 10 to the -7th

The O2 (g) produced from the decomposition of the 1.0 L sample of H2O2 (aq) is collected in a previously evacuated 10.0 L flask at 300. K. What is the approximate pressure in the flask after 400. s? (For estimation purposes, assume that 1.0 mole of gas in 1.0 L exerts a pressure of 24 atm at 300.K)

1.2 atm

When a 3.22 g sample of an unknown hydrate of sodium sulfate, Na2SO4 . X H2O (s), is heated, H2O (molar mass 18 g) is driven off. The mass of the anhydrous Na2SO4 (s) (molar mass 142 g) that remains is 1.42g. the value of X in the hydrate is

10

In an insulated cup of negligible heat capacity, 50.g of water at 40. oC is mixed with 30. g of water at 20. oC. the final temperature of the mixture is closest to

33 degrees C

5 H2O2 (aq) + 2 MnO4 - (aq) + 6 H+ (aq) → 2 Mn 2+ (aq) + 8 H2O (l) + 5 O2 (g) In a titration experiment, H2O2 (aq) reacts with aqueous MnO4 - (aq) as represented by the equation above. The dark purple KMnO4 solution is added from a buret to a colorless, acidified solution of H2O2 (aq) in an Erlenmeyer flask. (Note: Ath the end point of the titration, the solution is a pale pink color.) At a certain time during the titration, the rate of appearance of O2 (g) was 1.0 x 10 -3 mol/Ls. What was the rate of disappearance of MnO4 - at the same time?

4.0 x 10 -4 mol/L

Which of the following best describes what happens to the pH of the H2O2 solution as the titration proceeds?

As H+ ions are consumed, the solution becomes less acidic and the pH increases.

Using information from the table above, write a net-ionic equation for the reaction between the Sn electrode and the Cu(NO3)2 solution that would be thermodynamically favorable. Justify that the reaction is thermodynamically favorable.

Cu 2+ + Sn (s) → Cu (s) + Sn 2+ Eo is positive (0.34 V + 0.14 V = 0.48 V), therefore the reaction is thermodynamically favorable. OR The cell observations from earlier parts of the question are evidence that the Sn is oxidized and Cu is reduced, therefore Eo must be positive.

Which of the following particulate diagrams best shows the formation of water vapor from hydrogen gas and oxygen gas in a rigid container at 125 oC, but should also include molecules of excess oxygen?

D (make sure there is excess!)

The student is asked to redesign the experiment to determine the concentration of a butanoic acid solution instead of a propanoic acid solution. For butanoic acid the value of pKa is 4.83. The stuent claims that a different indicator will be required to determine the equivalence point of the titration accurately. Based on your response in #55, do you agree with the student's claim? Justify your answer.

Disagree with the student's claim. From above, pKa for propanoic acid is log (1.3 x 10 -5) = 4.89. Because 4.83 is so close to 4.89, the pH at the equivalence point in the titration of butanoic acid should be close enough to the pH in the titration of propanoic acid to make the original indicator appropriate for the titration of butanoic acid.

Determine if it is true or false and support your answer. The pH of a solution prepared by mixing the 50.0 mL sample of 0.20 M CH3CH2COOH with a 50.0 mL sample of 0.20 M NaOH is 7.00.

False. The conjugate base of a weak acid undergoes hydrolysis at equivalence to form a solution with a pH >7. hydrolysis: CH3CH2COO- + H3O+ ⬄ CH3CH2COOH + OH-

In both experiments it was observed that the measured final temperature of the water was less than the final temperature calculated in problem 46. Identify one source of experimental error that might account for this discrepancy and explain why the error would make the measured final temperature of the water lower than predicted.

Heat is lost to the environment. The T of the water is lower because less heat is transferred to the water. OR Heat is transferred to the can or thermometer. The T of the water is lower because less heat is transferred ot the water. OR Incomplete combustion of the lauric acid. The temperature of the water is lower because less heat was released from the combustion reaction.

C2H4 (g) + H2 (g) -(in the presence of Pt)→ C2H6 (g) C2H4 (g) is reduced by H2 (g) in the presence of a solid platinum catalyst, as represented by the equation above. Factors that could affect the rate of the reaction include which of the following? I. Changes in the partial pressure of H2 (g) II. Changes in the particle size of the platinum catalyst III. Changes in the temperature of the reaction system

I, II, and III

Which compound, lauric acid or sucrose, is more soluble in water? Justify your answer in terms of the intermolecular forces present between water and each of the compounds

Sucrose is more soluble than lauric acid. Stronger interactions occur between sucrose and water molecules due to a greater capacity for hydrogen bonding as a result of a larger number of -OH groups in sucrose. Although lauric acid molecules have one site for hydrogen bonding, the long hydrocarbon chain causes London dispersion forces to be the predominant, yet weaker, interaction with water molecules.

The experiment described above is repeated using a 1.5 g sample of sucrose. The combustion reaction for sucrose in air is represented below. C12H22O11 (s) + 12 O2 (g) → 12 CO2 (g) + 11 H2O (g) Even though ΔGo for the combustion of sucrose in air has a value of -5837 kJ/mol rxn, the combustion reaction does not take place unless it is ignited. Explain.

The reaction has a high activation energy. The ignition source provides energy to the molecules; some of them can then overcome the activation energy barrier. The reaction is exothermic, so the heat released allows the reaction to continue.

Determine if it is true or false and support your answer. If the pH of a hydrochloric acid solution is the same as the pH of a propanoic acid solution, then the molar concentration of the hydrochloric acid solution must be less than the molar concentration of the propanoic acid solution.

True. HCl is a strong acid that ionizes completely. Fewer moles of HCl are needed to produce the same [ H3O+] as the propanoic acid solution, which only partially ionizes.

Call the top line (X), middle line (Y) and bottom line (Z). Which of the following statements would correctly describe the total change in enthalpy for the system?

X-Z

Will an aqueous solution of iron (II) chloride oxidize magnesium metal? If so, write the balanced molecular and net ionic equations for the reaction.

Yes. Mg is above Fe. Mg (s) + Fe 2+ (aq) → Mg 2+ + Fe (s)

If the potential of a Zn - H+ cell (like that in figure 20.9 page 840) is 0.45 V at 25 oC when [Zn2+] = 1.0 M and PH2 = 1.0 atm, what is the H+ concentration?

[H+] = 6 x 10 -6 M

The reaction is thermodynamically favorable. The signs of ΔGo and ΔSo for the reaction are which of the following?

delta g neg and delta s postive

Using the diagram above, what kind of reaction is this, and what law is being represented?

exothermic, Hess's Law

As the mass of the Sn electrode decreases, where does the mass go?

into the solution as Sn 2+ ions.

A molecular solid coexists with its liquid phase at its melting point. The solid-liquid mixture is heated, but the temperature does not change while the solid is melting. The best explanation for this phenomenon is that the heat absorbed by the mixture

is used in overcoming the intermolecular attractions in the solid

The role of a catalyst in a chemical reaction is to

lower the activation energy for the reaction

Which element is being oxidized during the titration, and what is the element's change in oxidation number?

oxygen, -1 to 0

Which electron would the metal ions in the salt solution plate out to form a solid on the electrode?

right electrode (cathode)

Which of the following substances is a strong electrolyte when dissolved in water?

sodium nitrate

In an attempt to determine ΔH combustion of lauric acid experimentally, a student places a 1.5 g sample of lauric acid in a ceramic dish underneath a can made of Al containing 325 g of water at 25oC. The student ignites the sample of lauric acid with a match and records the highest temperature reached by the water in the can. The experiment is repeated using a can of the same mass, but this time the can is made of Cu. The specific heat of Cu is 0.39 J/gK, and the specific heat of Al is 0.90 J/gK. Will the final temperature of the water in the can made of Cu be greater than, less than, or equal to the final temperature of the water in the can made of Al? Justify your answer.

the temperature will be greater in the can made of Cu. since Cu has a lower specific heat than al, more heat will be absorbed by the water in the Cu can than in the water of the Al can.

A student is trying to determine the heat of reaction for the acid-base neutralization reaction represented below: NaOH (aq) + HCl (aq) → H2O (l) + NaCl (aq) the student uses 0.50 M NaOH and 0.50 M HCl solutions. which of the following situations, by itself, would most likely result in the LEAST error in the calculated value of the heat of reaction?

the thermometer was incorrectly calibrated and read 0.5 Celsius degree too high during the procedure

Find the oxidation numbers for every element in the following a) sodium phosphate (s) b) calcium acetate (s) c) carbonate ion d) elemental magnesium

you know this

Calculate the value of oG for the reaction. Include units with your answer.

ΔG = -nFEo ΔG = (-2 mol e- / mol rxn) (96485 C/ mol e-) (0.48 J/C) = -93,000 J/mol = -93 kJ/mol

The precipitation reaction between AgNO3 (aq) + NaCl (aq) proceeds as follows: AgNO3 (aq) + NaCl (aq) → NaNO3 (aq) + AgCl (s) Use the following information to calculate ΔHo for the net ionic equation.

ΔHo = -41.7 kJ

Predict the sign of ΔSo for the reaction in problem 48 and justify your prediction

ΔSo for the reaction is positive. There are 23 moles of gaseous product for every 12 moles of gaseous reactants.