Reversible Reactions 6-1

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Reversible Reactions6-1

• Reactions are spontaneous if DG is negative.
• If DG is positive the reaction happens in the
  opposite direction.
• 2H2(g) O2(g) 2H2O(g) energy
• 2H2O(g) energy 2H2(g) O2(g)
• 2H2(g) O2(g) 2H2O(g) energy

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Equilibrium
Forward reaction
H2O(g) CO (g\ H2 (g) CO2 (g)
Reverse reaction
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Dynamic Equilibrium If kept in a closed system,
then both reactions will occur. If this reaction
is left then it will reach a point of equilibrium
where the rate of the forward reaction equals the
rate of the reverse reaction. So the
concentrations of the reactants doesnt change.
The equilibrium is dynamic, because even though
the concentrations stay the same and nothing
appears to be happening, both the forward and
reverse reactions are continuing to occur.
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of A decreases while of B increases till
equilibrium is reached.
Equilibrium is reached when rate of forward
reaction is the same as the reverse reaction.
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Changes in reaction rates of the forward and
reverse reactions for H2O CO H2 CO2
Rate of forward reaction decreases while reverse
increases till the concentrations reach a level
at which the rate of the forward and reverse
reactions is the same. The system has reached
equilibrium.
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Equilibrium

• When you first put reactants together the for
  ward reaction starts.
• Since there are no products there is no reverse
  reaction.
• As the forward reaction proceeds the reactants
  are used up so the forward reaction slows.
• The products build up, and the reverse reaction
  speeds up.

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• Eventually you reach a point where the reverse
  reaction is going as fast as the forward
  reaction.
• This is dynamic equilibrium.
• The rate of the forward reaction is equal to the
  rate of the reverse reaction.
• The concentration of products and reactants stays
  the same, but the reactions are still running.

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Equilibrium

• Equilibrium position- how much product and
  reactant there are at equilibrium.
• Shown with the double arrow.
• Reactants are favored
• Products are favored
• Catalysts speed up both the forward and reverse
  reactions so dont affect equilibrium position.

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• A homogenous reaction is one in which all the
  substances are in the same state.
• A heterogeneous reaction is one in which all the
  substances are not in the same state.

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Measuring equilibrium

• At equilibrium the concentrations of products and
  reactants are constant.
• Keq is the equilibrium constant
• Keq Productscoefficients
  Reactantscoefficients
• Square brackets means concentration in
  molarity (moles/liter)

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Writing Equilibrium Expressions

• General equation
• aA bB cC dD
• Keq Cc Dd
• Aa Bb

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• N2 (g) 3H2 (g) 2NH3 (g)

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• H2 (g) F2 (g) 2HF(g)

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CaCO3 (s) CO2 (g) CaO(s)
When writing equilibrium constant expressions for
heterogeneous equilibria, you dont include pure
solids or pure liquids. Their concentrations
dont change
Keq CO2 CaO Keq CO2
CaCO3
Becomes 1 due to solid phase
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Keq expression for this reaction 2H2O (l)
2H2 (g) O2(g)
Keq H22O2
H2O is a liquid and is not included in the
equilibrium constant
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• Write the equilibrium expressions for the
  following reactions.
• 4NH3 (g) 7O2 (g) 4NO2 (g) 6H2O (g)
• 2H2O(g) 2H2(g) O2(g)

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Keq expression for this reaction CuSO4 . 5H2O
(s) CuSO4 (s) 5H2O (g)
Keq H2O5
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Keq expression for this reaction

• 2CaCO3(s) 2Ca(s) 2CO2 (g) O2(g)
• 2 H2 (g) O2 (g) 2 H2 O (l)

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• Try writing the equilibrium constant for
• N2 (g) 3H2 (g) 2NH3 (g)
• C(s) 2H2 (g) CH4 (g)
• CO(g) H2 (g) CO2 (g) H2 (g)
• C(s) H2O(g) CO (g) H2 (g)
• H2O(g) H2O(l)