Chem. 2, Exam 2
Which of the followings statements is (are) true for a galvanic (voltaic) cell when E° = +1.00 V? 1. The reaction is spontaneous 2. At equilibrium, K = 1. 3. ∆G° is negative
1. The reaction is spontaneous 3. ∆G° is negative
Using the given reaction profile, what must compound K be? I A -- B I --------- I I I K-A+B I ------- A-K-B I -------- I I -------- I KA+B I I I I --------- I K + A + B --------- I AB+ K -------------------------------------------------------->
A catalyst is not used up in the reaction, and therefore should occur as both a reactant and product.
If a plot of 1/[A] versus time produces a straight line with a positive slope for the reaction A → B + C, what is the order of the reaction?
A graph of 1/[A] versus time yields a straight line with a positive slope for second order reactions.
Galvanized is a term associated most closely with which metal?
A metal such as iron is galvanized by coating it with a more easily oxidized metal that protects the iron from corrosion, acting as an anode and forcing the iron to act as a cathode. Zn is the most easily oxidized metal in the list, and is the one commonly used to galvanize other metals.
A battery that cannot be recharged is classified as a _____
A primary battery is one that designed to be used once and discarded. The electrochemical reaction in the cell is not reversible. A secondary battery can be recharged.
Why does aluminum (Al) metal not undergo corrosion like iron does?
Aluminum is easily oxidized to Al₂O₃, but the surface layer of Al₂O₃ formed serves as a protective coating to the Al.
A galvanic cell Zn | Zn²⁺ || Ni²⁺ | Ni runs spontaneously. If a current is imposed to turn this into an electrolytic cell, which of the following will occur?
An electrolytic cell runs in the opposite direction of a galvanic cell. Applying a current to the galvanic cell Zn | Zn²⁺ || Ni²⁺ | Ni would convert the Zn anode to a cathode and cause Zn²⁺ to be reduced.
An electrolytic cell converts:
An electrolytic cell uses an electric current to produce a chemical reaction, so it converts electrical energy into chemical energy.
Using the information in the table, the rate law for the reaction 2 A(g) + B(g) → C(g) + D(g) is [A]0 (M) [B]0 (M) Rate (M/s) 0.100 2.50 0.460 0.200 2.50 0.920 0.300 1.25 1.38
As the concentration of the of A is doubled, the rate is doubled. As the concentration of B is reduced by half, the rate stays the same. The formula representing rate would be: Rate = k[A].
For the electrochemical cell represented by Cr²⁺ | Cr³⁺ || Li⁺ | Li, E°(cell) = -2.63 V. This cell:
Does not run spontaneously and is electrolytic
Consider the multistep reaction. What is the best rate law for the overall reaction? k1 2A + B → C + D (slow) k2 C → 2E (fast) <--
Fast reactions following the slow step in a mechanism do not enter into the rate law, so C and k₂ will not be part of the rate law. The slow step involves only A and B and it is a termolecular reaction. Therefore the best rate law of the overall reaction would be Rate = k₁[A]²[B].
What is the smallest whole-number coefficient for H₂S when the equation H₂S + MnO₄⁻ → Mn²⁺ + SO₄²⁻ is balanced in acidic solution?
In the equation S increases in oxidation number from -2 to +6, losing 8 electrons, and Mn decreases in oxidation number from +7 to +2, gaining 5 electrons. The only way to balance the electrons is to give H₂S a coefficient of 5 and MnO₄⁻ a coefficient of 8, resulting in a transfer of 40 electrons in the balanced equation.
Which of the following is the cell diagram for the reaction 3 Pb²⁺+(aq) + 2 Al(s) → 3 Pb(s) + 2 Al³⁺(aq)?
In the reaction 3 Pb²⁺+(aq) + 2 Al(s) → 3 Pb(s) + 2 Al³⁺(aq) Al is being oxidized at the anode and Pb²⁺ is being reduced at the cathode, so the cell diagram is Al(s) | Al³⁺(aq) || Pb²⁺(aq) | Pb(s)
The Haber Process: N₂ + 3H₂ → 2NH₃ is an important industrial route for the production of ammonia. However, the reaction does not occur fast enough at room temperature to be useful. What could be done to increase the rate of this reaction the most?
Increasing the temperature increases the average kinetic energy of molecules resulting in more successful collisions. Catalysts increase the rate of reactions by lowering the activation energy. Both are useful strategies, and both are used in combination to increase the reaction rate the most in the Haber Process
What species are made at the anode and cathode, respectively, during the electrolysis of molten KBr?
Oxidation occurs at the anode, so Br₂(g) is formed from oxidation of Br⁻, at the anode. Reduction occurs at the cathode, so K(l) is formed from reduction of K⁺at the cathode.
In a voltaic (galvanic) cell, oxidation occurs at the _____ and is where _____ in the salt bridge moves toward.
Oxidation occurs at the anode. As the electrons are removed from the solution, electrical neutrality is maintained as anions (negative ions) in the salt bridge move into the solution. (anode; anions)
As E°(cell) increases:
Since ∆G°=-nFE°, ∆G° decreases as E°(cell) increases.
What is the rate law for the proposed mechanism? H₂(g) + 2NO(g) N₂O(g) + H₂O(g) Step 1 (slow) N₂O(g) + H₂(g) N₂(g) + H₂O(g) Step 2 (fast)
Step one is termolecular with a rate law: rate = k₁[H₂][NO]². This is the slow step in the mechanism. Fast steps following the slow step do not enter into the rate law, so the overall rate law is rate = k[H₂][NO]².
Which of the following affects E(cell) under nonstandard conditions?
The E(cell) under nonstandard conditions is affected by all three conditions: -Concentration of reactants -temperature -concentration of products
What is the standard electrode potential for the reaction 2 Cr + 3 Pb²⁺ → 3 Pb + 2 Cr³⁺? Standard Reduction potentials: E° (V): Pb²⁺(aq) + 2e⁻→ Pb(s) -0.13 Cr³⁺(aq)+ 3e⁻→ Cr(s) -0.74
The E° for a cell is given by the equation E°(cell)=E°(cathode)−E°(anode) In the reaction 2 Cr + 3 Pb²⁺ → 3 Pb + 2 Cr³⁺ Cr is oxidized at the anode Cr → Cr³⁺ + 3e⁻ Pb²⁺ is reduced at the cathode. Pb²⁺ +2e⁻ → Pb So, E°(cell)=E°(cathode)−E°(anode) E°(cell)=−0.13 V−(−0.74 V)= +0.61 V
Which of the following species is the best reducing agent?
The best reducing agent is the one most easily oxidized, which is the one with the lowest reduction potential. From the choices on the list, Mg, with a reduction potential of -2,36 V will be the best reducing agent
The reaction C₃H₈(g) + 5O₂(g) ⟶ 3 CO₂(g) + 4 H₂O(l) can be continuously conducted in what type of device?
The energy from a reaction such as C₃H₈(g) + 5O₂(g) ⟶ 3 CO₂(g) + 4 H₂O(l) can be harnessed to produce electricity directly in an electrochemical cell called a fuel cell, so called because the source of energy is a replaceable fuel such as C₃H₈(g).
A reaction has a rate law of Rate = (1.25 M⁻¹s⁻¹)[A][B]. What is the rate of the reaction if [A] = 0.301 M and [B] = 0.280 M?
The following solution may contain one or more values that are different from the problem provided to you, however, the steps to solve the problem are the same. .Plug the units into the given rate for this second order reaction. rate=1.25 1 M --- (0.301M)(0.280M)=0.105 --- M⋅s s
The reaction rate of a reaction at 60 °C will be greater than at 30°C because _____
The molecules are moving faster at higher temperatures. This increases the likelihood that they have enough kinetic energy to react.
For the reaction 2 A + 3 B → 4 D + 5 E, how is the rate of reaction expressed in terms of rate of disappearance of B?
The rate of a reaction is equal to the rate of change of a reactant or product, divided by its coefficient in the balanced equation, with negative values for reactants because they are disappearing and positive values for products. For the given reaction 2A+3B⟶4D+5E2A+3B⟶4D+5E the rate in terms of the change in [B] is given by Rate=−1Δ[B] -------- 3Δt
For the reaction 3 A + 4 B → 2 C + 4 D, what is the magnitude of the rate of change for [B] when [C] is increasing at 2.0 M/s?
The rate of a reaction is equal to the rate of change of a reactant or product, divided by its coefficient in the balanced equation. Since reactants are disappearing they have negative values and products have positive values. For the reaction 3A+4B⟶2C+4D the rate in terms of the change in B and C is given by Rate= -1 Δ[B] 1 Δ[C] -- ------ = - ---- 4 Δt 2 Δt Substituting 2.0 M for Δ[C] and solving for --- ----- s Δt the absolute magnitude ofΔ[B] ----- Δt 1 Δ[B] 1 M - ---- = -- (2.0Ms)=1.0 --- 4 Δt 2 s Δ[B] M M ----- =4×1.0 ---- =4.0 --- Δt s s
Which of the following will decrease the rate of a reaction?
The rate of reaction is affected by the concentration of the reactants, the presence of a catalyst, the surface area of the reactants, and the temperature of the reaction. Removing or decreasing each of these would result in a decreased reaction rate.
Identify the molecularity of the elementary reaction below: Br₂(g) → 2Br･(g)
The reaction is unimolecular. Unimolecular reactions involve the rearrangement of one reactant molecule to produce two or more. One Br₂ molecule splits into two ions.
The half-life of element X is 500 years. If there are initially 8 g of X, how much will remain after 1500 years?
There are 3 half-lives. Find the amount after each half life. 1.) 8 - = 4 2 2.) 4 - = 2 2 3.) 2 - = 1 2
Considering the redox reaction Cr + Ni²⁺ → Cr³⁺ + Ni, how many electrons are transferred in the balanced reaction?
To balance the charges in the equation Cr must have a coefficient of 2 and Ni a coefficient of 3, 2 Cr + 3 Ni²⁺ → 2 Cr³⁺ + 3 Ni2 Cr + 3 Ni²⁺ → 2 Cr³⁺ + 3 Ni so a total of 6 electrons are transferred in the balanced reaction.
What is ∆H (in kJ/mol) for a reaction where the activation energy is 250 kJ/mol while the activation energy for the reverse reaction is 150.0 kJ/mol?
To calculate ∆H, subtract the positive reverse Ea from the positive forward Ea. kJ 250.0−150.0= 100.0 ----- mol
Which equation is used to determine the amount of time required for the initial concentration to decrease by 45% if the rate constant has units of s⁻¹?
To determine the order, you should look at the units of the rate. It has units of s⁻¹, which indicates that it is a first order reaction. The equation for a first order rate law is: [A] ln ------ = −kt [A]o
What does a 'dead' battery mean chemically?
When a battery is "dead" it means that the chemical components are at equilibrium. When that occurs, ∆G = 0, and Q = K.
A reaction is determined to have the rate law rate = k[NO]²[H₂]. What is the rate-determining step in the mechanism? Step 1 2NO → N₂O₂ <-- Step 2 N₂O₂ + H₂ → N₂O + H₂O Step 3 N₂O + H₂ → N₂ + H₂O
You can determine which step is rate-determining. To determine the rate law for a mechanism containing an equilibrium step, you must redefine the intermediate in terms of the equilibrium step. You cannot have any intermediates in the rate law. The rate determining step is Step 2.
What is the activation energy for the isomerization of methyl isocyanide? Rate Constant Temperature 4.30 x 10⁻³ min⁻¹ 472 K 5.54 x 10⁻² min⁻¹ 503 K
You can solve for the EAEA by rearranging this equation and plugging in given values. k1 EA 1 1 ln(-----)= --- (--- - ---) R T2 T1 (R)(T1)(T2) k1 EA= ---------- ln(-----) (T1−T2) k2 (8.314)(472)(503) 5.54×10⁻² EA= ------------------- ln(---------------) (31) 4.30×10⁻³ EA = 162,747 J/mol Convert this to kJ by dividing by 1000. 162,747 J ---------- =163 kJ 1000
How long will it take for the concentration of A to decrease from 1.25 M to 0.205 for the second order reaction A → Products? (k = 1.52 M⁻¹min⁻¹)
You can solve this problem using the integrated form of the second order rate law. 1 1 -- = ---- +kt [A] [A]0 1 1 1 ----- = -----+(1.52 -------- ×t) 0.205 1.25M M⋅min t= 2.68min
What is the order of the reaction if A decomposes to B and C with a rate constant of 8.43 × 10⁻⁴ s⁻¹ at a certain temperature?
You can tell the order of a reaction from the units of the rate constant. The rate constant of this reaction has the units s⁻¹, so it is a first order reaction.