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Reaction Direction

>> Parts of this equation/concept include:

A -	Reaction Quotient
B -	Le Chatelier's Principle >> Concentration Changes >> Pressure Changes >> Temperature Changes

The reaction quotient (Q) has the same form as the equilibrium constant, except that the concentrations are not necessarily equilibrium concentrations. Because a reaction will continue (either forward or backward) until it reaches the equilibrium concentrations, Q can be used to determine the direction of the reaction.

Because product concentration is proportional to K (or Q), it is easiest to determine reaction direction by considering the required change in product concentration. If Q is greater than K, the product concentration must decrease for the values to match, thus the reaction direction is backward (to decrease products). If Q < K, the product concentration must increase for the value of Q to reach K, so the reaction must go forward (create products).

>> Example 1

If the concentration of all products and reactants is 0.1 M, what will be the direction of the gaseous reaction

C₂H₄ + H₂ C₂H₆       K_C = 0.99

Solution:

Q = [C₂H₆]/[C₂H₄][H₂] = [0.1]/[0.1][0.1] = 10

Q > K

Since Q is "too high," it can be reduced by using up products and creating reactants. Therefore the reaction goes in reverse.

>> Example 2

the partial pressures are P_CO2 = 1.1 atm, P_Cl2 = 2.0 atm, P_CCl4 = 0.0010 atm and P_O2 = 0.0030 atm, will the gaseous reaction, CO₂ + 2 Cl₂ CCl₄ + O₂, go in the forward or reverse direction? (K_P = 6.4 x 10^–18)

Solution:

Q

=

PCCl4PO2 PCO2PCl22

=

(0.0010)(0.0030) (1.1)(2.0)2

=

3 x 10–6 4.4

=

6.8 x 10–7

Since Q > K, the product concentration must decrease. The reaction will go in the reverse direction.

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Le Chatelier's principle says that a system adjusts to minimize stress. Consequently, the equilibrium shifts such that whatever is added is removed.

>> Concentration Changes

The concentration can be changed by adding or removing a substance. If added, the equilibrium shifts to remove the substance. If removed, the equilibrium shifts to replace it. Adding or removing (provided some remains) solid, pure liquid, or solvent does not affect the equilibrium, since the concentration of these substances is constant.

>> Example 3

For the reaction, CH₃OH(g) + O₂(g) HCOOH(g) + H₂O(g)

1. What direction does the equilibrium shift if more oxygen is added?
2. What direction does the equilibrium shift is water is removed?
3. How does the concentration of methanol (CH₃OH) change if more oxygen is added?
4. How does the concentration of methanol change if more water is added?
5. How does the concentration of methanol change when more methanol is added?

Solution:

1. Because reactant is added the equilibrium will shift toward the products (right).
2. Because product is removed, the equilibrium will shift toward the products to replace it (right).
3. If more oxygen (reactant) is added, the equilibrium shifts toward products. That uses up the other reactant (methanol); therefore the concentration of methanol decreases.
4. If water is added, the equilibrium shifts toward the reactants. The creation of more reactants increases the concentration of methanol.
5. When more methanol is added, the concentration of methanol increases. Then the equilibrium shifts to remove some of the extra methanol. Therefore, the concentration of methanol is still higher than it was in the original equilibrium. However, it is less than the total of the methanol from the first equilibrium and the amount added.

>> Example 4

For the reaction, C(s) + 2 F₂(g) CF₄(g)

1. How does the equilibrium shift if carbon tetrafluoride is added?
2. How does the concentration of fluorine change if carbon tetrafluoride is added?
3. How does the concentration of fluorine change if carbon is added?

Solution:

1. As product is added, the equilibrium shifts toward reactants (left).
2. Since the equilibrium shifts toward reactants, the concentration of the reactant fluorine will increase.
3. Carbon is a solid. Adding more will not change its concentration. Therefore the concentration of fluorine remains the same.

>> Pressure Changes

Changing pressure only affects gases. Higher pressures favor fewer moles of gas and lower pressures favor more moles of gas. Only consider moles of gas (not other physical states) in determining equilibrium shifts.

>> Example 5

How does an increase in pressure affect the concentration of the first reactant in the following reactions?

1. C₂H₄(g) + H₂(g) C₂H₆(g)
2. Xe(g) + 3 F₂(g) XeF₆(g)
3. C(s) + 2 F₂(g) CF₄(g)
4. H₂S(g) + Hg(l) HgS(s) + H₂(g)

Solution:

Since pressure is increasing, each reaction will shift to the side with fewer moles of gas.

1. There are 2 moles of gas on the reactant side and 1 mole of gas on the product side. The equilibrium will shift toward products (right). This will use up reactants, so the concentration (and partial pressure) of C₂H₄ will decrease.
2. There are 4 moles of gas on the reactant side and 1 mole of gas on the product side, so the equilibrium will shift toward the product (right) side. Therefore the concentration of xenon will decrease.
3. There are 2 moles of gas on the reactant side and 1 mole of gas on the product side. The equilibrium will shift toward products (right), but since carbon is a solid, its concentration will not change.
4. There is one mole of gas on the reactant side and there is 1 mole on the product side. A change is pressure will favor neither side, so the concentration of all products and reactants will not change.

>> Temperature Changes

The easiest way to predict equilibrium shifts is to consider energy (or heat) as a product or a reactant. It is a product in exothermic reaction (–H) and a reactant in endothermic reactions (+H). Increasing temperature increases the "concentration of energy."

>> Example 6

How does the concentration of the last product change if the temperature increases?

1. Fe₂S₃ 2 Fe³⁺ + 3 S^2–       +H
2. Si + 2 F₂ SiF₄       –H

Solution:

1. Since this reaction is endothermic, energy is a "reactant."

   energy + Fe₂S₃ 2 Fe³⁺ + 3 S^2–

   So the reaction shifts toward the products, and the concentration of sulfide will increase.

2. Since this reaction is exothermic, energy is a "product."

   Si + 2 F₂ SiF₄ + energy

   So the reactant shifts toward the reactants, and the concentration of silicon tetrafluoride will decrease.

>> Example 7

How does the concentration of the first reactant change if the temperature decreases?

1. C₂H₄ + H₂ C₂H₆       (exothermic)
2. 2 NOCl 2 NO + Cl₂       (endothermic)

Solution:

1. Since this is an exothermic reaction, energy is a "product."

   C₂H₄ + H₂ C₂H₆ + heat

   At lower temperature, heat is removed. So to replace it, the reaction must shift toward the products. Therefore the concentration of C₂H₄ decreases.

2. Since this is an endothermic reaction, energy acts as a reactant

   heat + 2 NOCl 2 NO + Cl₂

   So the decrease in temperature will shift the equilibrium toward reactants. Therefore the concentration of chlorine will decrease.

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