Equilibrium and Stress

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Equilibrium and Stress

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Resources Available

• Exp. L Equilibrium and Stress

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Observations

• Fe(NO3)3
• NH4SCN
• KSCN
• NaNO3
• H2SO4

• CuSO4
• Co(NO3)2

What color was observed for each compound?
Each compound breaks into one type of positive
ion (cation) and one type of negative ion. What
ions are formed?
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Observations
NO3- SCN- SO42-
Fe3 colorless
NH4 colorless
K colorless
Na colorless
H colorless
Cu2 blue
Co2 pink
Which ions are responsible for color?
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Concentration Effects

• Thiocyanatoiron(III) Ion System

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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.
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Molecular Equation
Fe(NO3)3
NH4SCN
NH4NO3
FeSCN(NO3)2
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Ionic Equation
Break each compound into one cation and one anion.
Fe3(aq)
3 NO3-(aq)
SCN-(aq)
NH4(aq)
FeSCN2
NO3-(aq)
NH4(aq)
3
thiocyanatoiron(III) ion
Complex ions are sometimes written in
brackets without phase annotation.
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Net Ionic Equation
Fe3(aq)
SCN-(aq)
FeSCN2
Cancel identical ions on both sides of the
equation.
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What species is responsible for the red color?
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Thiocyanatoiron(III) Ion Equilibrium Mass-Action
Expression
FeSCN2
products
reactants
Fe3
SCN-
Write the mass-action expression for
the formation of the thiocyanatoiron(III) ion.
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What does a darker red color indicate?
What ions are added when Fe(NO3)3 is added to the
solution?
Why does the solution turn a darker red color?
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What ions are added when KSCN is added to the
solution?
Why does the solution turn a darker red color?
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Why does the solution turn a darker red color?
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Why does the solution turn a lighter red color?
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Why does the solution turn a lighter red color?
Does the equilibrium shift?
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Pressure Effects

• Nitrogen Dioxide-Dinitrogen Tetroxide System

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Nitrogen Dioxide - Dinitrogen Tetroxide
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Nitrogen Dioxide - Dinitrogen Tetroxide
0 sec
1 sec
10 sec
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Nitrogen Dioxide - Dinitrogen TetroxideMass-Actio
n Expression
products
reactants
Why does the gas initially get darker and then
lighten?
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Dilution Effects

• Tetrachlorocuprate(II) Ion

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Tetrachlorocuprate(II) Ion
Which ion is responsible for the color in each
solution?
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Tetrachlorocuprate(II) Ion and Water
top view
Deionized water causes the pale blue color, salt
water gives a pale green solution.
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Tetrachlorocuprate(II) Ion and Water
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Tetrachlorocuprate(II) Ion Equilibrium
Mass-Action Expression
CuCl42-
products
reactants
Cu2
Cl-4
Why does the addition of water change the
solution color, but the addition of salt water
have no effect?
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Temperature Effects

• Nitrogen Dioxide-Dinitrogen Tetroxide
• Tetrachlorocobaltate(II) Ion

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Nitrogen Dioxide - Dinitrogen Tetroxide
Is the equilibrium reaction endothermic or exother
mic?
ice water
room temp
hot water
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Nitrogen Dioxide - Dinitrogen TetroxideMass-Actio
n Expression
Why does the color change?
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Tetrachlorocobaltate(II) Ion
Which ion is responsible for the blue ion?
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Tetrachlorocobaltate(II) Ion Equilibrium
Mass-Action Expression
pink
cobalt blue
When the solution is heated, is the equilibrium
constant increasing or decreasing?
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