Chemical Equilibria ! An Introduction - Santa Fe College

CHM 1041 

J. Bieber 

INTENT 

Chemical Equilibria ! An Introduction 

To study the qualitative aspects of equilibria and be introduced to the quantitative methods of 

investigating chemical equilibria. 

DISCUSSION 

Le Châtelier’s Principle!Qualitative Equilibria 

Recall the physical equilibrium already observed during the Solutions experiment: 

H 2O 

+ KNO3 (s) 

K + 

1+ (aq) NO 3 1- heat 

(aq) 

The equilibrium tells us that the rate of precipitation of KNO 3(s) equals that rate of dissolution of 

KNO 3(s), that is, a saturated solution of this ionic compound is in contact with its solid salt. No net 

change occurs, but ionic movement is continuous. 

Changes in any equilibrium, when placed under stress (changes in temperature, concentration, 

pressure, etc.), will react to counteract the stress and, in so doing, restore the equilibrium at some 

new position. 

Using this principle we would predict correctly for the heterogeneous equilibrium shown above that 

when 

Stress Shift 

the system is heat . . . more KNO 3 will dissolve 

NaNO 3(s) is added . . . KNO 3 will precipitate 

KCl(s) is added . . . KNO 3 will precipitate 

since NaNO 3 and KCl are more soluble than KNO 3. 

Using the same principle that you have seen here to predict changes in an equilibrium involving 

physical properties (solubility), you can also successfully predict the behavior of chemical equilibria. 

Bieber/PC/Chm2 Labs/Int_Eq.wpd 10/02

Acid/Base Equilibria 

In your first course in chemistry you were introduced to the concepts of acids and bases. You have 

seen that different acids have different strengths; and as a result some acids are classified as strong 

acids, others as weak. If an acid reacts to a large extent with water to form its conjugate base and 

hydronium ion, then the acid a strong acid. However, how large is large? 

Now that we will begin to look at acids and bases quantitatively, we need a more precise definition. 

A strong acid is defined as any acid that reacts with water more than 50% to yield its conjugate base 

and hydronium ion. When this happens the principal species in the solution, and the ones which 

should be used in writing any principal equations involving those solutions, are the products of that 

reaction with water. 

For example, an acid (HA) that reacts 75% with pure water is a strong acid. 

HA H3O + 

1+ (aq) A 1- + H2O (aq) 

But is the concentration of HA in the solution significant? Can the equilibrium be shifted to the left 

by the addition of H 3O 1+ ? the answer is yes to both! 

This Discussion will consider the reaction which results when an aqueous solution of iron(III) ion 

is mixed with an aqueous solution of thiocyanic acid, HSCN, to produce a complex ion containing 

one iron(III) ion and one SCN 1! ion. Aqueous iron(III) ion is best represented as [Fe(H 2O) 6] 3+ , with 

6 water molecules around the iron(III); but unless we want to emphasize that a water molecule is 

being displaced, we will just write Fe 3+ (aq). However, since thiocyanic acid is a strong acid, it 

would not be correct to write equations involving aqueous solutions of it as HSCN(aq). Instead, the 

principal equation for the reaction between these two species may be written as either: 

Fe[(H 2 O) 6 ] 3+ (aq) + SCN 1- (aq) –> Fe[(H 2 O) 5 (SCN)] 2+ (aq) + H 2 O(1) 

OR 

Fe 3+ (aq) + SCN 1- (aq) –> FeSCN 2+ (aq) 

The first equation has been written to show the waters explicitly, to emphasize that a water to 

iron(III) interaction is being replaced by a SCN 1- to iron(III) interaction. 

That is not to say that the species HSCN does not exist in an aqueous solution, just that it is not the 

predominant species. HSCN is a strong acid, but not as strong an acid as, say, HCl. Therefore, 

while the equilibrium: 

HSCN (aq) H 3O + 

1+ (aq) SC N 1- + H 2O (l) 

(aq) 

needs to be considered, it lies to the right under the conditions of this experiment. Therefore, the 

principal form of HSCN present will be SCN 1- . 

Bieber/PC/Chm2 Labs/Int_Eq.wpd 10/02 

2

Indicators. Chemicals which change color when the acidity of a solution changes can be very 

useful. If such a substance is a weak acid/weak base conjugate pair, where the weak acid has a 

different color than the weak base, it is called an indicator. 

Weak Acid 

+ H2O (l) 

Conjugate W eak Base 

aA + bB cC + dD 

K c = 

Fe 3+ (aq) SCN 1- + (aq) 

[C] c [D] d 

[A] a [B] b 

[FeSCN] 2+ (aq) 

+ H 3O 1+ (aq) 

Therefore, changing the hydronium ion concentration can shift this equilibrium. Also, changing the 

concentration of any other acid or base in the system can have an effect on the position of this 

equilibrium. In this experiment you will use two different indicators, Methyl Orange which changes 

color at different basicities of the solution. 

Quantitative Equilibria 

Associated with any equilibrium state there is a number called the equilibrium constant, K c, which 

expresses the necessary condition on the concentrations of reactants and products for each reaction. 

For the reaction: 

(Eq. 1) 

(Eq. 2) 

where A, B, ,C, and D are solutes in solution or are gases, and the bracketed expressions are their 

concentrations in units of moles per liter (M) at equilibrium. The equilibrium constant, K c, will have 

a fixed value for the reaction at any given temperature. If A, B, C, and D are mixed in arbitrary 

amounts in a container they will react until their concentrations satisfy Equation (2). Depending on 

the magnitude of K c and the amounts of species used initially. Reaction (1) will proceed to the right 

or to the left until equilibrium is attained. 

Complex Ion Equilibria 

In this experiment you will look at the qualitative equilibrium of the reaction between iron(III) ion 

and thiocyanate ion: 


3 

(Eq. 3) 

When solutions containing Fe 3+ (aq) and thiocyanate ion are mixed. Reaction (3) occurs to some 

extent, forming the FeSCN 2+ complex ion, which has a dark orange-red color. As a result of the 

reaction, the equilibrium amounts of Fe 3+ (aq) and SCN 1! will be less than they would have been if 

no reaction occurred; for every mole of FeSCN 2+ that is formed, one mole of Fe 3+ (aq) and one mole

of SCN 1! will have reacted. According to the Law of Mass Action, K c for Reaction (3) has the 

form: 

K c = 

[FeSCN 2+ ] 1 

[Fe 3+ ] 1 [SCN1- ] 1 


4 

(Eq. 4) 

The concentration of the solvent, here, water, is essentially constant. It, is incorporated into the 

equilibrium constant.

CHM 1041 Chemical Equilibria ! An Introduction J. Bieber 

Report Form 

Name: _____________________________ Partner: _____________________________ 

Section: _______________ Date: __________ 

EXPERIMENT 

Observe the indicator color changes for methyl orange and phenolphthalein indicators that you will 

use in some of these experiments to detect changes in acidity. 

Shifting a Chemical Equilibrium 

Weak Acid and Weak Base Equilibria. The equilibrium involving an acid and its conjugate base 

when both of them are weak can be shifted by the addition of either an acid or a base. 

To 3 mL of 0.1 M HC 2H 3O 2 (acetic acid, or HOAc, or CH 3COOH) add a drop of methyl orange 

indicator, then add M NaC 2H 3O 2 (sodium acetate or, NaOAc, or CH 3COONa), a few drops at a time, 

with mixing. Explain your observations in terms of the equilibrium equation below. At some point 

methyl orange changes from red to orange to yellow as a solution becomes less acidic (as H 3O 1+ is 

consumed). 

HC2H 3O 2 (aq) H2O (l) H 3O 1+ 1- 

+ (aq) + C 2H 3O 2 (aq) 

To each of two 3 mL sample of 0.1 M NH 3 add a drop of phenolphthalein. To one sample add 1 M 

NH 4Cl, a few drops at a time, with mixing. To the other add 1 M HCl, a drop at a time, with 

mixing. In each case, note any changes in color and in odor of the solutions. At some point in time 

phenolphthalein changes from pink to colorless as a solution becomes less basic. Write the equation 

for the equilibrium in dilute NH 3. 

Equilibrium: ___________________________________________________________________ 


5

1. Observations upon the addition of 1 M NH 4Cl: 

Observation (a) color change: __________________ (b) odor change: __________________ 

Inventory of 1 M NH 4Cl: ________________________________________________________ 

Equation: _____________________________________________________________________ 

Explain your observations in terms of the extent of shift in this equilibrium (if any) occurred : 

______________________________________________________________________________ 

______________________________________________________________________________ 

2. Observations upon addition of 1 M HCl: 

(a) color change: _________________________ (b) Odor change: _______________________ 

What substances predominate in a 1M HCl solution? 

Write the net ionic equation for the principal reaction of 1 M HCl and 0.1 M NH 3. 

Equation: _____________________________________________________________________ 


______________________________________________________________________________ 

______________________________________________________________________________ 


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The Thiocyanato-Iron(III) Complex Ion. The aqueous iron(III) ion, Fe 3+ (aq), and thiocyanate ion, 

SCN 1- , react to form a dark orange-red complex ion in aqueous solution: 

Fe 3+ (aq) + SCN 1- (aq) 

FeSCN 2+ (aq) 

(orange-red) 

Add 1.0 mL of 0.20 M Fe(NO 3) 3 to 1.0 mL of 0.20 M KSCN, and dilute this solution, measuring the 

added volume of water accurately, until only a moderately orange-red color is attained; about 80 mL 

of tap water will do it. Record the volume of water that you added: _________. Place 5.0 mL of 

the diluted solution into each of four medium test tubes. 

Perform the indicated operations (below) and compare all four solutions (using one of these four as 

a color standard) by looking down the tube from above, with a white paper background. Correlate 

each observation with Le Châtelier’s Principle and the above equilibrium. 

1. Add a few drops of 0.20 M Fe(NO 3) 3 to the first test tube. 

Observations: _________________________________________________________________ 


______________________________________________________________________________ 

________________________________________________________________________ 

2. To the second 5.0 mL portion add a few drops of 0.20 M KSCN. 

Observations: _________________________________________________________________ 


______________________________________________________________________________ 

______________________________________________________________________________ 


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3. Suspend a third 5.0 mL portion in a beaker of tap water warmed by your Tirril Burner for one 

minute. 

Observations: _________________________________________________________________ 


______________________________________________________________________________ 

______________________________________________________________________________ 

Rewrite the principal equilibrium equation in this solution, this time including “+ heat ” on one side: 

_______________________________________________________________ 


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Chemical Equilibria ! An Introduction - Santa Fe College

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