# Exercises

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## Q8-1

A) You are driving down the street at a constant speed of 60mph.
Draw a graph of distance versus time over the course of 4 hours.

B) How would you use the graph to determine the rate (speed in mph) at which the car is travelling (assuming you did not already know the answer)? Hint: What is the slope for the graph in part A? What does the term slope mean?

C) Now draw a new graph (still distance vs. time). Assume that at t=1 hour you removed your foot from the accelerator, and that the car coasts to a stop after 1 hour (we know that is not realistic, just think about the shape of the curve.)

E) Does the car move at a constant rate (speed) as it is slowing down? Why or why not?

F) Why do the two graphs look different? (hint: they should look different)

G) In the time between t=1 and t=2, how would you estimate your speed (using the second graph you drew? (Why do we need to specify the time?)

## Q8-1

Consider the reaction A2 + B2 ⇌ 2AB. If the initial concentration of both A and B is 4 M, and after 10 minutes the reaction appears to stop. The concentration of [A] is now 2.0M.

A) Draw a graph of [A2] v time, over the span of 20 minutes.

B) On the same axes draw a graph of [B2] v time, over a span of 20 minutes

C) On the same axes draw a graph of [AB} v time over a span of 20 minutes

## Q8-2

We can discuss the rate in terms of [A2], [B2], or [AB]. How are they related to each other? Δ[A2]/Δt = ?

## Q8-3

What is the difference between Δ[A2]/Δt and d[A2]/dt

## Q8-4

At t = 15 minutes has the reaction stopped?

## Section 8-4  ## Q8-1

Here are some concentration time data for a reaction. Determine the rate law for the reaction by plotting three graphs: You should use excel to plot the graphs. (just copy the data out of word and paste it into excel)

A) [A] v time,

B) ln[A] v time

C) 1/[A] v time.

Submit all graphs with your work.

 Time (minutes) [A] (M) 0.0 0.55 0.25 0.42 0.50 0.31 0.75 0.23 1.00 0.17 1.25 0.12 1.50 0.085

A) What is the order of the reaction with respect to [A]?

B) How would you calculate the rate constant?

## Q8-1

What set of concentrations would you use in order to determine the rate law for a given reaction?

Here is one set of conditions – fill out the rest of the table with enough sets of conditions to determine what the rate law is for the reaction

A + B à C + D

 [A] mol/L [B] mol/L Initial rate mol/L.min 1.0 1.0 1.0

A) If the reaction is first order in A and 2nd order in B, fill out the table with the rate you would expect.

 [A] mol/L [B] mol/L Initial rate mol/L.min 1.0 1.0 1.0

B) If the reaction is first order in A and zero in B, fill out the table with the rate you would expect

 [A] mol/L [B] mol/L Initial rate mol/L.min 1.0 1.0 1.0

C) If the reaction is first order in A and first order in B. fill out the table with the rate you would expect

 [A] mol/L [B] mol/L Initial rate mol/L.min 1.0 1.0 1.0

How would you determine the initial rate of the reaction? What measurements would you record and how would you treat the data?

## Q8-2

1. Determine the order of the reaction and rate constant (k) based on the following data. The data are real data generated by watching the dissociation of a protein complex (PA20·PA63) to its separate subunits (PA20 and PA63).

PA20·PA63PA20 + PA63

 Time (min) [PA20·PA63] (μM) 4.67 0.98 7.33 0.81 10.33 0.73 13.00 0.65 16.00 0.60 18.33 0.56 21.00 0.49 24.00 0.55 27.00 0.42 29.33 0.37 32.00 0.38 35.00 0.32 37.67 0.39 40.33 0.33 43.33 0.33 46.00 0.28 48.67 0.28 51.67 0.31 54.33 0.20 57.00 0.19 59.67 0.20 62.67 0.24 65.00 0.24 68.00 0.19 70.67 0.21 73.67 0.19 76.33 0.16 79.00 0.13 81.67 0.15
 84.67 0.14 87.67 0.19 90.67 0.17 93.33 0.19 96 0.15

## Q8-1

If we say something (anything) is at equilibrium, what does that mean to you?

## Q8-2

If a reaction is at equilibrium, what does that mean? (is it different than the definition you gave above?)

## Q8-3

Consider the reaction A ⇌ 2B. The initial concentration of A is 4 M, and after 10 minutes the concentration of A is 2.0 M and does not change any further.

A) Draw a graph of [A] v time, over the span of 20 minutes, and on the same axes draw a graph of [B] v time, over a span of 20 minutes. Show where the reaction reaches equilibrium.

## Q8-4

When the reaction reaches equilibrium, what is the relationship between the rate of the forward reaction and the rate of the reverse reaction?

## Q8-1

What does it mean when we say a reaction has reached equilibrium?

## Q8-2

If ΔG for a system is = 0, what does that mean?

## Q8-3

For the reaction A D B, as the reaction approaches equilibrium what happens to G (does it increase, decrease or stay the same?)

## Q8-4

For the reaction A D B, if K = 1, what are the relative concentrations of A and B?

## Q8-5

For the reaction A D B, for which K = 1. Draw a graph of how the Gibbs Free energy changes as the relative amounts of [A] and [B] change.

G

100% A 50% A 0%A

0% B 50% B 100%B

## Q8-1

A) Write out the equilibrium reaction for the ionization of Propanoic acid (CH3CH2COOH) in water.

B) What is the expression for Ka?

C) If the pH of a 0.15 M solution of propanoic acid is 2.8, what is the Ka? What is the pKa?

## Q8-2

For the acid base reactions given, draw out the full reaction using Lewis structures and curved arrows to indicate how the proton transfer takes place.

Indicate whether the equilibrium position would lie to the right or the left.

pKa H2O = 15.7, NH4+ = 9.8, NH3 = 33

A) NH3 + H2O D NH4+ + OH

B) NH2 + H2O D NH3 + –OH

## Q8-3

Draw a reaction energy profile (H vs reaction progress) for an exothermic reaction.

• Label the axes, the transition state, reactants, products, DH, and the activation energy.

## Q8-1

A) What is the pH of a 0.25 M solution of NH4Cl? (Ka = 5.6 x 10–10)

B) If you added enough NH3 to make the solution 0.25 M solution in NH3, what would happen to the pH (assume the only reaction going on is the one you used in part 1a) – why?

C) Calculate the pH of a solution that is 0.25 M in both NH4Cl and NH3

## Q8-2

For the reaction:

2NH3(g) + CO2(g) D (NH2)2CO(s) + H2O(l) ΔH = – 330 kJ

Predict the effect of the following on the position of equilibrium (use arrows to show the direction of change

C) Increasing pressure

D) Increasing volume