# CHM 1046L Chemical Equilibrium Lab Report

CHM 1046LSpring 2023

Pre-Lab for Module 6 (Chemical Equilibrium Part 2)

Equilibrium Worksheet – Finding equilibrium concentrations and finding Kc

❖ Keep in mind that you can only calculate the equilibrium constant from

equilibrium concentrations and vice-versa.

❖ So if the problem only has initial (or non-equilibrium) concentrations, you first

need to calculate the equilibrium concentrations before finding the equilibrium

constant.

❖ Alternatively you can also use the initial (or non-equilibrium) concentrations

and the equilibrium constant to determine equilibrium concentrations.

❖ We can do this because stoichiometry relates the reactants to the products and

we can use this to determine how much products is formed and how much

reactant is lost.

Calculate equilibrium concentrations and the equilibrium constant from given initial

concentrations and the equilibrium concentration of a single reactant or product.

A flask is filled with 2.00 M CO2, which decomposes according to the following reaction

2CO2 (g) → 2CO (g) + O2 (g)

At the very beginning of the reaction how much CO2 will be present in the flask initially?

At the very beginning of the reaction how much CO and O2 will be present in the flask initially?

Is the reaction currently at equilibrium? Why?

Which direction will the reaction shift to reach equilibrium?

Will reactants be consumed or created? Would that be a + or a -?

Will products be consumed or created? Would that be a + or a -?

In a sentence describe what is happening in the reaction in terms of stoichiometry.

If a change of one mole was represented by x what would be the change of each of the reactants

and products (do not forget the signs).

Change in CO2 =

Change in CO =

Change in O2 =

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CHM 1046L

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In the table below fill in the blanks based on what you already know

2CO2 (g) → 2CO (g) + O2 (g)

CO2

CO

O2

Initial concentration (I)

Change in concentration (C)

Equilibrium concentration (E)

(add rows I and C)

Problem

A flask is filled with 2.00 M CO2, which decomposes according to the following reaction

2CO2 (g) → 2CO (g) + O2 (g)

Find the equilibrium constant of the reaction of the flask contains 0.90 M CO2 at equilibrium.

SHOW ALL YOUR WORK!

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CHM 1046L

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Part 1: Phase 1

The chemical reaction for the formation of the red-orange iron(III) thiocyanate complex ion is

Fe3+ (aq) + SCN (aq) [FeSCN2+] (aq)

What is the source of the Fe3+ ions?

What is the source of the SCN- ions?

The equilibrium constant for this reaction is given by

The stock solution in the stock beaker contains 10.00 mL of 0.1000 M iron(III) nitrate

solution, 10.00 mL of 0.1000 M potassium thiocyanate solution, and 10.00 mL of 0.5000 M

nitric acid solution.

What is the total volume of the stock solution? SHOW YOUR WORK!

How many moles of iron (III) nitrate are present in the stock solution? SHOW YOUR WORK!

How many moles of potassium thiocyanate are present in the stock solution? SHOW YOUR

WORK!

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CHM 1046L

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Based on the moles of iron (III) nitrate and moles of potassium thiocyanate, how many moles

of iron (III) thiocyanate do you expect to be formed (Hint: think limiting reagent)? We can do

it this time because we have enough of one reagent to force the reaction to completion.

SHOW YOUR WORK!

Once you determine the moles of iron (III) thiocyanate formed, determine the concentration of

iron (III) thiocyanate in the final solution. SHOW YOUR WORK!

Part 1: Phase 2

1) What is the concentration of iron (III) thiocyanate in solution 2 that contains 10.00 mL

of stock solution, labeled “Solution 1” (you just calculated the concentration of this

solution in phase 1) and 40.00 mL of nitric acid solution? SHOW YOUR WORK!

2) What is the concentration of iron (III) thiocyanate in solution 3 containing 10.00 mL of

solution 2 (you just calculated the concentration of this solution in phase 2 question

1) and 40.00 mL of nitric acid solution? SHOW YOUR WORK!

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CHM 1046L

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3) What is the concentration of iron (III) thiocyanate in solution 4 containing 10.00 mL of

solution 3 (you just calculated the concentration of this solution in phase 2 question

2) and 40.00 mL of nitric acid solution? SHOW YOUR WORK!

Which substance did you use as the background blank to calibrate the spectrophotometer?

Explain your reasoning.

Part 2

No calculations required.

Part 3

We will only be doing calculations for Solution 1 going forward. You will do the

calculations for Solutions 2 and 3 on your own. That should be relatively easier once

we go through the calculation for Solution 1 here.

Solution 1 calculations

1) Solution 1 contains 10.00 mL of 0.002 M iron (III) nitrate solution, 4.00 mL of

0.002 M potassium thiocyanate solution, and 6.00 mL of 0.05 M nitric acid solution.

a) Calculate the concentration of iron (III) nitrate in the final solution in Solution 1.

SHOW ALL WORK!

b) Calculate the concentration of potassium thiocyanate in the final solution in

Solution 1. SHOW ALL WORK!

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CHM 1046L

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Summarize the concentration of iron (III) nitrate and concentration of potassium thiocyanate

in Solution 1 from your calculations in questions 1a and 1b above. You can fill in the same

for Solutions 2-3 later after doing the calculations on your own time.

Solution

#

mL of iron

(III) nitrate

mL of

potassium

thiocyanate

mL of

nitric acid

M of iron

(III)

nitrate in

the final

solution

M of

potassium

thiocyanate

in the final

solution

Absorbance

of the

solution

1

2

3

You cannot do the limiting reagent thing here because you do not know that one reagent is

in so much excess like before that the reaction will go to completion. So you must calculate

the concentration of the [FeSCN 2+ ] from the graph you created before.

Is the absorbance value of Solution 1 within the range of values on the graph you created

using the data from Phase 2? So can you use this absorbance value to determine the

concentration of [FeSCN 2+ ] from the graph? (think about the definition of the slope and what

the x and y axis are in this case and how it relates to the Beer-Lambert law).

Doing the calculation to find the equilibrium constant

Consider Solution 1 of Part 3,

What is the concentration of iron (III) nitrate aka iron (Fe 3+ ) ions in this tube?

What is the concentration of potassium thiocyanate aka thiocyanate ions (SCN ) ions in this

tube?

Refer back to the graph produced for the Pre-lab Assignment to solve for [FeSCN 2+ ]

According to Beer and Lambert’s law

A = l C

A = absorbance

= molar absorptivity

l = optical path length in cm (usually 1 cm is the length of the cuvettes used)

C = concentration

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The equation of a line is y = mx + b

Where

y = value for the Y axis

x = value of the X axis

m = slope

b = y intercept

When the graph passes through the origin, the y intercept (b) is zero.

So your equation becomes y = mx

What is the y axis in this case?

What is the x axis in this case?

y = mx

A = l C

You can get the value of ( x l) from the slope of the graph and so can use this to find

concentration given the absorption and vice versa.

What is the value of the slope from the graph?

What is the concentration of iron (III) thiocyanate in this tube calculated from the graph?

SHOW ALL WORK!

Making an ICE table

A flask is filled with Fe3+ and SCN-, which decomposes according to the following reaction

Fe3+ (aq) + SCN (aq) [FeSCN2+] (aq)

At the very beginning of the reaction how much Fe3+ will be present in the flask initially? You

did this calculation and put down the value previously.

At the very beginning of the reaction how much SCN- will be present in the flask initially? You

did this calculation and put down the value previously.

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CHM 1046L

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At the very beginning of the reaction how much [FeSCN2+] will be present in the flask initially?

Think about how much product will be present at the very beginning of the reaction.

Was the reaction at equilibrium when the reaction started? Why?

Which direction will the reaction shift to reach equilibrium?

Will reactants be consumed or created? Would that be a + or a -?

Will products be consumed or created? Would that be a + or a -?

In a sentence describe what is happening in the reaction in terms of stoichiometry.

If a change of one mole was represented by x what would be the change of each of the reactants

and products (do not forget the signs).

Change in Fe3+ =

Change in SCN- =

Change in [FeSCN2+] =

In the table below fill in the blanks based on what you already know

Fe3+ (aq) + SCN (aq) [FeSCN2+] (aq)

Fe3

SCN –

[FeSCN2+]

Initial concentration (I)

Change in concentration (C)

(in terms of x)

Equilibrium concentration (E)

(add rows I and C)

We know the value of x because this is the concentration of [FeSCN2+] which you calculated

from the graph. So you can use this to calculate the value of x and therefore the equilibrium

concentrations of Fe3+ and SCN-. SHOW ALL YOUR WORK!

[Fe3+]eq =

[SCN-]eq =

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[FeSCN2+]eq =

Now that you have these equilibrium concentrations (values) plug them into the equilibrium

constant expression to calculate the value of Kc. SHOW ALL YOUR WORK!

This is the value of Kc that you found from Solution 1

Do you expect the value of Kc that you will find from Solutions 2-3 to be around this number or

be different from the number you just determined? Why or why not? Justify and explain your

reasoning.

Before you leave the lab, you need to do the same calculation that you did for solution 1

(beginning of Part 3 on page 5. Use this worksheet as a template to do those calculations on

solutions 2 and 3 and calculate an average value of Kc from all the 3 values.

Make sure you show the calculation showing how you determined the average. SHOW ALL

YOUR WORK!

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