Name Date Class
Chapter 18Reaction Rates and EquilibriumSMALL-SCALEEXPERIMENT
LE CHÂTELIER’S PRINCIPLE ANDCHEMICAL EQUILIBRIUM
Small-Scale Experiment for text Section 18.2
OBJECTIVES
- Observeandrecordhow a chemical system at equilibrium responds to changes inconcentration of reactants or products.
- Describeshifts in equilibrium in terms of Le Châtelier’s principle.
INTRODUCTION
If you’ve ever watched a game of football, you know that it has become very specialized.Coaches design special offenses and defenses for various down and distance situations,as well as for positions of the ball on the field, the score, and the time remaining in thegame. For example, offensive and defensive strategies are quite different for a thirddown and one situation than for a first down and ten. As a result, several offensive anddefensive players often run on and off the field after each play. One thing remainsconstant: only 11 players per team are on the field for any given play. This is an exampleof a dynamic equilibrium. A dynamic equilibrium is one in which forward reactions(players running on the field) take place at the same rate as reverse reactions (playersrunning off the field). There is no net change in the number of players on the field.
You learned in Section 18.2 that chemical systems often reach a state of dynamicequilibrium. In a system at chemical equilibrium, the rate of the forward reactionequals the rate of the reverse reaction. As a result, the amounts of the reactants andproducts of equilibrium remain constant as long as no stress is placed on the system.
Le Châtelier’s principle says that if a system at equilibrium is stressed, theequilibrium balance will shift in a direction that will relieve the stress. For example, ifyou add a reactant, the equilibrium will shift toward products (to the right) so thatthere is a different balance of reactants and products. Similarly, if you add a product,the equilibrium will shift toward reactants (to the left).
PURPOSE
In this experiment, you will investigate chemical systems at equilibrium. You will disturbthem by adding or subtracting reactants or products and observe how the equilibriumsystem responds. You will explain those changes in terms of Le Châtelier’s principle.
SAFETY
- Wear safety goggles, an apron, and gloves when working with corrosive chemicals.
- Use full small-scale pipets only for the controlled delivery of liquids.
- Don’t chew gum, drink, or eat in the laboratory. Never taste a chemical in the laboratory.
- Avoid inhaling substances that can irritate your respiratory system.
Experiment 29 Le Châtelier’s Principle and Chemical Equilibrium203
Name Date Class
MATERIALSSmall-scale pipets of the following solutions:
bromthymol blue (BTB) / potassium iodide (KI)
hydrochloric acid (HCl) / nitric acid (HNO3)
sodium hydroxide (NaOH) / silver nitrate (AgNO3)
ammonia (NH3) / sodium carbonate (Na2CO3)
copper(II) sulfate (CuSO4) / sodium thiosulfate (Na2S2O3)
lead(II) nitrate (Pb(NO3)2) / sodium phosphate (Na3PO4)
EQUIPMENT
small-scale reaction surface
empty pipet for stirring
plastic cup
204Small-Scale Chemistry Laboratory Manual
Name Date Class
EXPERIMENTAL PAGE
Mix the following. Stir each mixture thoroughly by blowing air through a pipet. Thechemical equations describe the changes you observe.
1.Mix one drop BTB and one drop HCl (H+). Record your observations in Table29.1. Now add just enough NaOH to induce another change. Alternately addmore HCl and NaOH.
HBTBH+ + BTB–
yellowblue
2.Mix one drop BTB and one drop NH3. Record your observations in Table 29.1.Now add just enough HCl (H+) to effect a change.
NH3+ HOHNH4++ OH–NH3 + H+NH4+
3.Mix one drop NH3and two drops CuSO4(Cu2+). Record your observations inTable 29.1. Now add, with stirring, just enough NH3to effect a change. Add HCl,with stirring, until the light blue precipitate returns. Add more HCl until theprecipitate disappears. Repeat this procedure until you are sure of what you see.
Cu2+ + 2OH–Cu(OH)2(s) (light blue precipitate)
Cu2+ + 4NH3Cu(NH3)42+(royal blue solution)
H+ + OH–HOH
4.Mix one drop Pb(NO3)2(Pb2+) and one drop KI (I–). Record your observations inTable 29.1. Now add, with stirring, enough NaOH to effect a change. Now addHNO3.
Pb2+ + 2I–PbI2(s) (bright yellow precipitate)
Pb2+ + 2OH–Pb(OH)2(s) (milky white precipitate)
5.Mix one drop Pb(NO3)2and one drop NaOH with stirring. Record yourobservations in Table 29.1. Now add more NaOH, drop by drop with stirring, untila change occurs. Finally, add nitric acid, HNO3(H+), drop by drop, slowly, withstirring, until a change occurs.
Pb2++ 2OH–Pb(OH)2(s)Pb2+ + 3OH–Pb(OH)3–
H+ + OH–HOH
6.Add two drops AgNO3to a plastic cup. Now add one drop of each of the chemicalsin the left column, one at a time. Stir and observe after each addition. When achange occurs, record your observations in Table 29.1. Go on to the nextchemical. Near the last addition, you may need to add a few more drops ofAgNO3.
Add these chemicals in this order / Equations that account for changesa.1 drop Na2CO3 / a.2Ag+ + CO32–Ag2CO3(s)
b.HNO3 / b.CO32– + 2H+HOH + CO2(g)
c.NaOH / c.2Ag+ + 2OH–Ag2O(s) + H2O
d.HCl / d.H+ + OH–HOH
Ag+ + Cl–AgCl
e.NH3 / e.Ag+ + 2NH3Ag(NH3)2+
f.KI / f.Ag+ + I–AgI(s)
g.Na2S2O3 / g.Ag+ + S2O32–AgS2O3–
h.Na3PO4 / h.3Ag+ + PO43–Ag3PO4(s)
Experiment 29 Le Châtelier’s Principle and Chemical Equilibrium205
Name Date Class
206Small-Scale Chemistry Laboratory Manual
Name Date Class
EXPERIMENTAL DATA
Record your results in Table 29.1 or in a copy of the table in your notebook.
Table 29.1 Dynamic Equilibria
1.BTB + HCl / 5.Pb(NO3)2 + NaOH+ NaOH: / More NaOH
+ HNO3
2.BTB + NH3 / 6.AgNO3 + Na2CO3
+ HCl / + HNO3
3.NH3 + CuSO4 / + NaOH
More NH3 / + HCl
+ HCl / + NH3
More HCl / + KI
4.Pb(NO3)2 + KI / + Na2S2O3
+ NaOH / + Na3PO4
+ HNO3
CLEANING UP
Avoid contamination by cleaning up in a way that protects you and your environment.Carefully clean the small-scale reaction surface by absorbing the contents onto apaper towel. Rinse the small-scale reaction surface with a damp paper towel and dryit. Dispose of the paper towels in the waste bin. Wash your hands thoroughly withsoap and water.
Experiment 29 Le Châtelier’s Principle and Chemical Equilibrium207
Name Date Class
QUESTIONS FOR ANALYSES
Use what you learned in this experiment to answer the following questions.
1. Bromthymol blue (BTB) is an acid, so it has a hydrogen ion (H+). For the purposesof this question, you will write it HBTB. HBTB ionizes in water to producehydrogen ion (H+) and bromthymol blue ion (BTB–).
HBTBH+ + BTB–
yellowblue
a. What color appeared when you added HCl to BTB? What color is HBTB?Explain the shift in equilibrium in terms of Le Châtelier’s principle.
b. What color appeared when you added NaOH to the mixture? What color isBTB–? Explain the shift in equilibrium in terms of Le Châtelier’s principle.
2.Ammonia reacts with water to produce hydroxide ion. Ammonia is neutralized byhydrogen ion:
NH3 + HOHNH4+ + OH–
NH3 + H+NH4+
a. What color did BTB change in the presence of ammonia (NH3)? Explain thecolor change in terms of Le Châtelier’s principle. Include an equation in yourexplanation.
b. What happened when HCl was added? Explain in terms of Le Châtelier’sprinciple.
208Small-Scale Chemistry Laboratory Manual
Name Date Class
3.Copper ions react differently in the presence of varying amounts of ammonia(NH3). The hydroxide produced from a small amount of ammonia produces aprecipitate. Excess ammonia produces a highly colored solution:
Cu2+ + 2OH–Cu(OH)2(s) (precipitation equilibrium)
Cu2+ + 4NH3Cu(NH3)42+(complex ion in solution)
a.What color is the precipitatewhen only a little ammonia is added? What coloris the solutionin the presence of excess ammonia?
b.Explain in terms of Le Châtelier’s principle the disappearance of theprecipitate when excess ammonia is added.
c.What is the effect of the HCl? Explain in terms of Le Châtelier’s principle.
4.Lead(II) ion reacts with iodide ion to produce a bright yellow precipitate, andwith hydroxide ion to form a milky white precipitate.
Pb2+ + 2I– PbI2 (s)bright yellow
Pb2+ + 2OH–Pb(OH2)(s) milky white
a.Why does the yellow precipitate disappear when sodium hydroxide is added?Explain in terms of Le Châtelier’s principle.
b.Why does the yellow precipitate reappear when nitric acid is added?
Experiment 29 Le Châtelier’s Principle and Chemical Equilibrium209
Name Date Class
5.Lead(II) ion reacts with varying amounts of hydroxide ions in different ways. Asmall amount of hydroxide produces a precipitate; an excess of hydroxide ionproduces a complex ion:
Pb2+ + 2OH–Pb(OH)2(s)
Pb2+ + 3OH–Pb(OH)3– (complex ion in solution)
a.Why does the precipitate disappear when excess NaOH is added? Explain interms of Le Châtelier’s principle.
b.Why does the precipitate reappear when a little nitric acid is added?
c.Why does the precipitate disappear when excess nitric acid is added?
6.Explain the observations in Step 6 by writing net ionic equations to describe eachreaction of silver ion (Ag+) you observed.
7.Explain why BTB is green in neutral water solution. In terms of Le Châtelier’sprinciple, why does it turn yellow when acid is added, and blue when base isadded?
210Small-Scale Chemistry Laboratory Manual
Name Date Class
NOW IT’S YOUR TURN!
1.Repeat Step 1 with other indicators such as phenolphthalein (HPhen);bromphenyl blue (HBPB); methyl red (HMR); alizarine yellow R (HAYR); andthymol blue (HTB). Explain in terms of Le Châletier’s principle what happenswith each when HCl and NaOHare added. Write net ionic equations to describetheir reactions in water.
2.Repeat the precipitation reaction of lead nitrate with potassium iodide, as in Step4. Can you alternately add NaOH and HNO3to change the precipitates? Explain.
3.Repeat the precipitation reaction of lead nitrate with sodium hydroxide, as inStep 5. Name two ways you can make the precipitate disappear. Prove youranswer by carrying out the experiments.
4.Perform the following experiment and explain your results in terms of Le Châtelier’s principle.
Experiment 29 Le Châtelier’s Principle and Chemical Equilibrium211
Name Date Class
a.Mix one drop FeCl3with one drop KSCN. What happened? Write an equationto explain.
b.Add 10 drops water, separate into four parts with a soda straw, and add theindicated solutions. Tell what happens in each case. Explain in terms of therespective equilibria.
c.
d.
e.
f.
212Small-Scale Chemistry Laboratory Manual