Week 9 Chemical Equilibria - Part 1 Workbook PDF

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Week 9: Chemical Equilibria - Part 1 Workbook

Site: Monash University Unit: CHM1011 - Chemistry I - S1 2018 Book: Week 9: Chemical Equilibria - Part 1 Workbook Printed by: Kellie Vanderkruk Date: Monday, 30 April 2018, 12:29 PM

Table of contents Week 9: Pre-lectorial Work 1 Introduction Video 1 Introduction to Chemical Equilibria 1 Introduction to Chemical Equilibria 2 Meaning of Kc The Reaction Quotient - Q Video 2 Question: Kc Calc. Video 3 Le Châtelier’s Principle - Temperature Summary of Ways to Change Position of Equilibrium Question: Le Châtelier Video 4 Question: Ksp Week 9: Pre-lectorial Work 2 Acids / Bases Brønsted-Lowry Acid-Base Definition The Concept of pH, pOH & Kw Video 1: Kw Question: Strong Acids Strong/Weak Acids and Bases The Acid Dissociation Constant Ka Conjugate Pairs Example Weak Acid pH - Methanoic Acid Questions: Weak acids Video 2: Titration Curves Acid-Base Titration Curves Polyprotic Acids Week 9: Pre-lectorial Test Pre-lectorial test Week 9: Lectorial 1 (17) Introduction Activity 1: Equilibria Poll 1 Activity 2: Equilibria Activity 3A: Equilibria Equilibria & Gibbs Energy Activity 3B: Equilibria Activity 4: Chemical Equilibria Changing the Position of the Equilibrium - Summary Week 9: Lectorial 2 (18) Introduction Activity 1: Cobalt Equilibrium World's stongest acid World's strongest base Poll 1 Activity 2: Acids Summary

Week 9: Pre-lectorial Work 1

This section covers your pre-lectorial work for this week. Please go to the next page for an overview of the required pre-work for this week’s lectorials.

Introduction Page 1 This section of the workbook will take you through the required pre-lectorial readings and activities. It is important that you take the time to carefully review the material provided and complete any activities. We recommend that you allocate at least 50 minutes to work through the topics and activities in this section of the workbook, with additional time to complete the prompted readings as necessary (this will vary depending on your reading speed and any prior knowledge you may have). Note that this section will cover pre-work for both of your lectorials for this week. What are we learning about? In this section we will look at the topics of Chemical Equilibrium, Gibbs Free Energy and Le Châtelier’s Principle. By the end of this section you should be able to … To be able to write the equilibrium expressions for Kc, K sp, K p To understand chemical equilibrium as a thermodynamic property the equilibrium constant, K the relationship between K, the reaction quotient, Q and Gibb's Free Energy (ΔG) Equilibria and Gibbs Free Energy Le Chatelier’s Principle: the effect on the equilibrium of changing concentration, pressure and volume, the effect of temperature on the equilibrium constant, the effect of an inert gas or a catalyst on the equilibrium The relationship of K, T and enthalpy (ΔH) Understand and be able to discuss and apply knowledge of: The poor solubility of some ionic salts The solubility product constant, Ksp Using Qsp to determine whether a substance will dissolve or precipitate The Common Ion effect Apply Le Chatalier's Principle to an equilibrium involving Co(II) ions and perform calculations involving Kc Understand and be able to discuss and apply knowledge of: the classical definitions of acids and bases calculating pH the acid-dissociation constant, K a the relationship of K a and pK a the nature of polybasic acids the relative strengths of acids and bases the conjugate acid-base pair the net direction of acid-base reactions

What do I need to do?

The following activities must be completed prior to attending your first lectorial for this week … The assessed Pre-lectorial Test for this week

Read through the Pre-lectorial Workbook pages Read Chapters 9.1 and 9.2 Blackman 3rd edition Watch Videos 1-4 Answer a question on Ksp

Video 1 Page 2

In the video below, take note of the following information about Chemical Reactions and Constants.

Week 9: Chemical Equilibria

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Note: Recommend Chrome or Firefox for playback on a MAC

Introduction to Chemical Equilibria 1 Page 3

Please read Section 9.1 and 9.2 of Chemistry, Blackman et al, 3rd Edition

Thermodynamic Equilibria Do all chemical reactions go to completion? Compare these two reactions: (i) Mg(s) + 2 HCl(aq) → MgCl2 (aq) + H2 (g) This reaction goes to completion, which is indicated by the single headed arrow only going in one direction (ii) CH COOH(aq) + CH CH OH(aq) ⇌ CH COOCH CH (aq) + H O(l) 3

3

2

3

2

3

2

This reaction does not go to completion. The double headed arrow indicates the reaction occurs in both directions, i.e. it is an equilibrium Chemical equilibria: Are dynamic at a molecular level Can be understood by considering: concentrations of reactants and products relative concentrations of reactants and products relative partial pressures in gas phase reactions thermodynamics

Introduction to Chemical Equilibria 2 Page 4

Please read Sections 9.1 and 9.2 of Chemistry, Blackman et al, 3rd Edition

Consider the following general reaction: aA+bB⇌cC+dD The double headed arrow indicates that this reaction can proceed at the same time in both the forward and reverse direction, i.e.: A + B react to give C + D (i.e. the forward reaction) C + D react to give A + B (i.e. the reverse reaction) Once equilibrium is established: reaction still proceeds in both forward and reverse directions, i.e. the system is in a state of dynamic equilibrium and the reaction has not stopped rates of forward and reverse reaction are equal (if the temperature remains constant) concentration of chemical species is constant partial pressures of gaseous species is constant no net change in the overall composition of the reaction mixture Note: Not all reactions achieve equilibrium Example: Production of ammonia (NH ) 3

N (g) + 3H (g) ⇌ 2 NH (g) 2

2

3

Starting with just N and H in the reaction vessel, the following occurs during the reaction: 2

2

[N2 ] and [H2 ] both decrease as the forward reaction proceeds, but the rate of the forward reaction decreases Concurrently, NH3 molecules react to form N2 and H2 (i.e. the reverse reaction), thus the rate of the reverse reaction increases as [NH 3] increases. Eventually the rate of both forward and reverse reactions are equal when equilibrium is established (see diagram below)

Once equilibrium has been established, the concentrations (and also partial pressures) of H 2, N 2 and NH 3 remain constant (see figure below)

Meaning of Kc Page 6 Kc indicates the extent of a reaction (i.e. how far the forward reaction proceeded before equilibrium was established) -3

If Kc is small (< 10 ), then the reactants are favoured 3

If Kc is large (> 10 ), then the products are favoured -3

3

If 10 > pKa (1), the pH of an aqueous solution of H2 S can be determined from pKa (1) alone Oxalic Acid: a weak diprotic organic acid. Belongs to a class of polyprotic acids used as strong metal chelating agents.

These types of ligands and their metal salts are important in biology, medicine and geochemistry (next semester!).

Oxalic acid can be found in black pepper, parsley, poppy seed, rhubarb and spinach, Lesser amounts in chocolate, nuts, berries, and beans. The oxalate dianion binds to metals very strongly: eg calcium oxalate - kidney stones (Ouch!).

The complexes can be very insoluble : can inhibit uptake of nutritionally important metals OR can be used to sequester unwanted metals.

Cancer treatment: colorectal tumours Oxaliplatin (marketed as Elaxotin)

Please read Section 11.7 of Chemistry, Blackman et al, 3rd Edition

Deprotonation of polyprotic acids is highly pH-dependant.

Week 9: Pre-lectorial Test

Go to the next page of this Workbook to get the link for your assessed pre-lectorial test for this week.

Pre-lectorial test Now that you have completed your Pre-lectorial work, you now need to complete your assessed pre-lectorial test. Ensure that you check the due date and time for the test as it may be due several days before your first lectorial for the week. What do I need to do? Go to the following Pre-lectorial Test and attempt the quiz activity. Take note of the instructions provided for the test. Pre-lectorial Test for Week 9.

Week 9: Lectorial 1 (17)

Go to the next page of this Workbook to get an overview of what you will cover in this first lectorial for this week.

Introduction This section of the workbook is a guide to support you through the topics and activities covered during the first lectorial of the week. Ensure that you have completed your pre-lectorial test and any prior pre-learning activities outlined in the previous section of this workbook. What are we learning about? In this lectorial we will look at the topics of Chemical Equilibrium, Gibbs Free Energy and Le Châtelier’s Principle. By the end of this lectorial you should be able to: To be able to write the equilibrium expressions for Kc, Ksp, K p To understand chemical equilibrium as a thermodynamic property the equilibrium constant, K the relationship between K, the reaction quotient, Q and Gibb's Free Energy (ΔGr) Equilibria and Gibbs Free Energy Le Châtelier’s Principle: the effect on the equilibrium of changing c, p and V, the effect of temperature on the equilibrium constant, the effect of an inert gas or a catalyst on the equilibrium The relationship of K, T and enthalpy (ΔH)

The following activities will be worked on during the lectorial time. Ensure that you follow up any remaining work after the end of the lectorial.

Discussion about Equilibria Activity on Equilibria Group activity

Activity 1: Equilibria

Discussion about Equilibria

The following activity is to be completed in groups during class. Please follow the prompts and instructions given by your educator. 1. In groups of four discuss write down what is meant by "Chemical Equilibrium" 2. Using the green circles for reactant particles and the yellow circles for product particles, show the relationship between the equilibrium constant (K c) and the position of the equilibrium for the following scenarios: (a) Kc1. 3. Match the following pictures where the reactions are: (a) Kc1 c

Poll 1

Go to: mars.eng.monash.edu to log into MARS using your Monash login. Then complete the Poll question below.

Which of the following expressions gives the equilibrium constant (K c) for the reaction:

CH4(g) + 2 H2S(g) Question and Answer

CS2 (g) + 4 H2 (g)

Activity 2: Equilibria In-class activity The following activity is to be completed individually during class. Please follow the prompts and instructions given by your educator. 2+

-

2-

Consider the following equilibrium: [Co(H2O)6] (aq) + 4SCN (aq) ⇌ [Co(SCN)4 ] (aq) + 6H2 O(l) -

2+

After the reactants are mixed, SCN , [Co(H2 O)6 ]

2-

and [Co(SCN)4 ] are present in the solution.

For the following two scenarios: -

+

(i) Adding SCN and (ii) Adding Ag (Hint: AgSCN precipitates from solution): Discuss and explain whether: (a) the equilibrium will shift to the reactants or products; and (b) Q is larger or smaller then Kc :

Activity 3A: Equilibria

In-class activity

The following activity is to be completed individually during class. Please follow the prompts and instructions given by your educator. For the following equilibrium: H2(g) + I2(g) ⇌ 2 HI(g) (a) Write down the expression for Kc (b) Calculate Kc for the following equation if a 2.00 L vessel contains an equilibrium mixture of 0.0860, 0.124 and 0.726 mol of H2 , I2 and HI, respectively at 460˚C (c) The reaction was commenced again and at a particular time in the reaction the following concentrations were found at 460˚C: 0.0500, 0.0700 and 0.600 M of H , I and HI, respectively 2

2

Determine the value of Q. Is the reaction at equilibrium, if not explain why and how the reaction will proceed in order to achieve equilibrium. Note: The values calculated for K and Q will be used in Activity 3B c

Equilibria & Gibbs Energy

The following information references Section 9.3 of Chemistry, Blackman et al, 3rd Edition

As can be seen in this table, a -ve ΔH value, does NOT always mean a +ve value for Kc.

As seen on the previous slide, Kc is not necessarily related to ΔH. The connection between thermodynamics and equilibria is actually between Kc and ΔG.

Recall that when ΔGr = 0, the reaction is NOT spontaneous, nor is the reverse reaction spontaneous. It is at equilibrium.

The Gibbs free energy under standard conditions (1M for solutions, 1 atm for gases) o← has the label, ΔGr o

The free energy change under any conditions, ΔG r, is related to ΔG r and Q by:

The Gibbs free energy under standard conditions (1M for solutions, 1 atm for gases) o has the label, ΔGr ← o

The free energy change under any conditions, ΔG r, is related to ΔG r and Q by:

↑ A variable

↑ A constant

↑ A variable

BUT, at equilibrium, ΔGr = 0, and Q = K!

↑ constant

↑ constant

Using this equation, we can make a simple observation, about whether the reactants or products are favoured in a reaction. o

If ΔGr is > 0, then Kc < 1, so reactants are favoured! o

If ΔGr is < 0, then Kc > 1, so products are favoured!

Activity 3B: Equilibria

In-class activity

The following activity is to be completed individually during class. Please follow the prompts and instructions given by your educator. For the following equilibrium: H2(g) + I2(g) ⇌ 2 HI(g) 0

(a) Using the value obtained for Kc in Activity 3A part (b), calculate the value of Δr G at 460˚C and determine whether the reaction will be spontaneous or not and why. 0

(b) Using the value of Q obtained in Activity 3A part (c) and also the value of Δr G from part (c) above, determine the value of Δr G at 460˚C. Explain whether or not the value of Δr G indicates the reaction is spontaneous and which way the reaction will proceed to achieve equilibrium. Discuss whether or not this correlates with the answer in Activity 3A, part (c).

Activity 4: Chemical Equilibria

In-class activity

The following activity is to be completed in groups during class. Please follow the prompts and instructions given by your educator. For the following chemical equilibrium: 0

-1

N2O 4(g) ⇌ 2 NO2(g) ; ΔrH = +56.9 kJ mol

(a) Write down the expression for the equilibrium constant (Kc) (b) For this part of the activity coloured circles will be used. The green circles represent the N 2O 4 molecules and the yellow circles represent the NO 2 molecules. For the forward reaction: one green circle forms two yellow circles For the reverse reaction: two yellow circles combine to form a green one Game details The Monash School of Chemistry has decided to hold an inaugural disco for the resident N O and NO molecules as they want to demonstrate how certain 2 4 2 equilibria behave under different conditions. The DJ is playing the best tunes of the year and the music is pumping away. To start the disco off an initial equilibrium position is established such that that there are 6 yellow circles and four green circles on the dance floor. Show what will happen to the position of the above equilibrium using the circles in each one of the following scenarios, starting with the initial position each time: 1. 4 more green circles decide to join the disco 2. 2 green circles leave the disco as they are too tired to continue 3. After a while with all the dancing, the temperature in the room has increased 4. The dry ice machine is used to generate smoke on the dance floor which lowers the temperature 5. This disco room has a special feature in that the room itself is able to decrease in size. At one point the DJ decides to decrease the volume of the room so that all the circles get closer to each other. 6. Some gate crashers in the form of 10 Argon molecules (represented by white circles) decide they want in on the party and enter the disco room. 7. The resident disco cat in the form of a black circle (who also happens to act as the catalyst for this particular reaction) walks slowly into the disco room to see what's happening and starts meowing along with the music.

Changing the Position of the Equilibrium - Summary Le Châtelier’s Principle 'If an equilibrium system is subjected to change, the system will adjust itself to partially oppose the effect of the change.’ Used to predict what will happen to the equilibrium position if the conditions are changed (i.e. pressure increased, reactants added, etc) Refer to the following table for a summary: Summary of Ways to Change Position of Equilibrium

Week 9: Lectorial 2 (18)

Go to the next page of this Workbook to get an overview of what you will cover in this second lectorial for this week.

Introduction This section of the workbook is a guide to support you through the topics and activities covered during the second lectorial of the week. Ensure that you have completed your pre-lectorial test and any prior pre-learning activities outlined in the previous section of this workbook. What are we learning about? In this lectorial we will look at the topics of Cobalt equilibria as well as acids and bases. By the end of this lectorial you should be able to: Apply Le Chatalier's Principle to an equilibrium involving Co(II) ions and perform calculations involving Kc Understand and be able to discuss and apply knowledge of: the classical definitions of acids and bases calculating pH the acid-dissociation constant, K a the relationship of K a and pK a the nature of polybasic acids the relative strengths of acids and bases the conjugate acid-base pair the net direction of acid-base reactions

What do we need to do?

The following activities will be worked on during the lectorial time. Ensure that you follow up any remaining work after the end of the lectorial. Co(II) equilibrium Discussion of stong/weak acid Calculating pH of a diprotic acid

Please read Section 11.0 of Chemistry, Blackman et al, 3rd Edition

Our discussion however will open with a definition of acids and bases. (base = alkali) It’s important to choose the right acid.

In a scene from Breaking Bad, Jesse pours hydroflouric acid onto a corpse in the bathtub to dissolve it.

Activity 1: Cobalt Equilibrium lass Activity 1: Cobalt equilibrium The following activity is to be completed in groups during class. Please follow the prompts and instructions given by your educator.

In the activity today, the following equilibrium will be explored: 2+

-

2-

[Co(H2 O)6 ] (aq) + 4 Cl (aq) ⇌ [CoCl 4] (aq) + 6 H2 O(l) (pink)

(blue)

The equilibrium reaction above is sensitive to various changes, some which will be seen in today's activity. The lecturer will demonstrate some of these changes as part of the activity. Give a written answer (and chemical equation if appropriate) to explain your observations. 1. Addition of concentrated hydrochloric acid (HCl): 2. Addition of silver nitrate solution (AgNO 3): 3. Based on how this reaction responds to a change in temperature, determine whether this is an exothermic or endothermic chemical reaction. Re-write the chemical reaction, includ...