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Chemistry 15: 1-2

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Chemical Equilibrium


Principles of Chemical Equilibrium

In theory, given the appropriate amount of energy, all chemical reactions are reversible. If bonds can be created, they can be broken. We should write every reaction:

mA + nB ↔ xC + yD+ ΔH

If ΔH is positive (energy is released as a product), then the reaction is exothermic and product-favored and will run forward under normal conditions. If ΔH is negative (energy is consumed), then the reaction is endothermic and reactant-favored and will run backward.

In a real solution reaction situation, there are many reactant and product molecules. At any given moment, some are engaged in the forward reaction, some in the reverse reaction. A product-favored reaction simply means that more forward reactions are taking place than backward reactions: the concentration of the products is increasing, while the concentration of the reactants is dropping.

Over time, the reaction rate will drop to zero: no further changes to the concentration of products or reactants will be observed. In fact, some forward reactions are still occuring, but the same number of backward reactions is also occuring, with the result that the total number of reactant and product molecules remains constant.

Equilibrium Constant and Reaction Quotients

At any moment during a reaction, we can measure the concentrations of the reactants and products. A useful ratio is the mathematical product of the product concentrations divided by the mathematical product of the reactanct concentrations.

Q = [C]x[D]y / [A]m[B]n

If Q is increasing over time, the reaction is product-favored; when it is decreasing, the reaction is reactant-favored.

There are a few conventions in writing the reaction quotient.

When the reaction is at equalibrium, Q becomes the equilibrium constant, which is represented by K. If K >> 1, the product of the concentrations of the products is greater than the product of the concentration reactants (which just means there's more of the products than the reactants), which means the reaction was product-favored.

Note that equilibrium does not mean that the products and reactants exist in equal concentrations or amounts, only that the conditions are such that the net concentrations of each is no longer changing.

Practice with the Concepts

Making and Measuring Solutions

Discussion Questions

Optional Readings

Need more drill work on significant figures? Scientific notation? Concepts of mass, density, and volume? Percents? ...You name it, you can probably work on it at the Chemistry drill site, courtesy Scott Van Bramer at Widener University.