Equilibrium and Stress

Edward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

Resources Available

Exp. L: Equilibrium and Stress

Observations

• Fe(NO3)3

• NH4SCN

• KSCN• NaNO3

• H2SO4

• CuSO4

• Co(NO3)2

Each compound breaks into one type ofpositive ion (cation) and one type of negative ion.

What ions are formed?

What color was observed for each compound?

Observations

NO3- SCN- SO4

2-

Fe3+ colorless

NH4+ colorless

K+ colorless

Na+ colorless

H+ colorless

Cu2+ blue

Co2+ pink

Which ions are responsible for color?

Concentration Effects

Thiocyanatoiron(III) Ion System

Write a balanced molecular equation for this reaction.

Write a balanced ionic equation for this reaction.

Write a balanced net ionic equation for this reaction.

What species is responsible for the red color?

[reactants]

[products]

Thiocyanatoiron(III) Ion Equilibrium Mass-Action Expression

Q =[Fe3+] [SCN-]

[[FeSCN]2+]

Write the mass-action expression for theformation of the thiocyanatoiron(III) ion.

Fe3+(aq) + SCN-(aq) [FeSCN]2+

[FeSCN]2+ + Fe(NO3)3

What does adarker red color indicate?

Why does the solution turna darker red color?

[reactants]

[products]Q =

[Fe3+] [SCN-]

[[FeSCN]2+]= K?

What ions are added whenFe(NO3)3 is added to the solution?

Fe3+(aq) + SCN-(aq) [FeSCN]2+

[FeSCN]2+ + KSCN

Why does the solution turna darker red color?

[reactants]

[products]Q =

[Fe3+] [SCN-]

[[FeSCN]2+]= K?

What ions are added whenKSCN is added to the solution?

Fe3+(aq) + SCN-(aq) [FeSCN]2+

[FeSCN]2+ + NH4SCN

Why does the solution turna darker red color?

[reactants]

[products]Q =

[Fe3+] [SCN-]

[[FeSCN]2+]= K?

Fe3+(aq) + SCN-(aq) [FeSCN]2+

[FeSCN]2+ + EDTA

Why does the solution turna lighter red color?

[reactants]

[products]Q =

[Fe3+] [SCN-]

[[FeSCN]2+]

Fe3+(aq) + EDTA(aq) [FeEDTA]- + 4 H+(aq)

Fe3+(aq) + SCN-(aq) [FeSCN]2+

[FeSCN]2+ + water

Why does the solution turna lighter red color?

[reactants]

[products]Q =

[Fe3+] [SCN-]

[[FeSCN]2+]

Does the equilibrium shift?

Fe3+(aq) + SCN-(aq) [FeSCN]2+

Pressure Effects

Nitrogen Dioxide-Dinitrogen Tetroxide System

Nitrogen Dioxide - Dinitrogen Tetroxide

NO2(g) + NO2(g) N2O4(g)

NO

ON

O

ON

O

ON

O

O

Nitrogen Dioxide - Dinitrogen Tetroxide

0 sec 1 sec 10 sec

NO2(g) + NO2(g) N2O4(g)

[reactants]

[products]

Nitrogen Dioxide - Dinitrogen Tetroxide

Mass-Action Expression

Q =

Why does the gas initially get darker and then lighten?

PNO2( )2

PN2O4

NO2(g) + NO2(g) N2O4(g)

Dilution Effects

Tetrachlorocuprate(II) Ion

Tetrachlorocuprate(II) Ion

Which ion is responsible for the color in each solution?

Cu2+(aq)+ 4 Cl-(aq) [CuCl4]2-

Tetrachlorocuprate(II) Ion and Water

Deionized water causes the pale blue color,salt water gives a pale green solution.

top view

Cu2+(aq)+ 4 Cl-(aq) [CuCl4]2-

[reactants]

[products]

Tetrachlorocuprate(II) Ion Equilibrium Mass-Action Expression

Q =[Cu2+] [Cl-]4

[[CuCl4]2-]

Why does the addition of water change the solution color,but the addition of salt water have no effect?

Cu2+(aq)+ 4 Cl-(aq) [CuCl4]2-

Temperature Effects

Nitrogen Dioxide-Dinitrogen Tetroxide

Tetrachlorocobaltate(II) Ion

Nitrogen Dioxide - Dinitrogen Tetroxide

Is theequilibrium

reactionendothermic

orexothermic?

icewater

roomtemp

hotwater

NO2(g) + NO2(g) N2O4(g)

Nitrogen Dioxide - Dinitrogen Tetroxide

Mass-Action Expression

Q =

Why does the color change?

PNO2( )2

PN2O4

NO2(g) + NO2(g) N2O4(g)

Tetrachlorocobaltate(II) Ion

Which ion is responsible

for theblue ion?

Co2+(aq)+ 4 Cl-(aq) [CoCl4]2-

Tetrachlorocobaltate(II) Ion Equilibrium Mass-Action Expression

K =[Co2+] [Cl-]4

[[CoCl4]2-]

pink cobaltblue

When the solution is heated,is the equilibrium constant increasing or decreasing?

Co2+(aq)+ 4 Cl-(aq) [CoCl4]2-

04/21/23

04/21/23