Describing a Reaction Equilibria, Rates, and Energy Changes

Q Describing a Reaction Equilibria, Rates, and Energy Changes 

Every chemical reaction can go in either forward or reverse direction. Reactants can go forward to products, and products can revert to reactants. As you may remember from your general chemistry course, the position of the resulting chemical equilibrium is expressed by an equation in which Kgq, the equilibrium constant, is equal to the product concentrations multiplied together, divided by the reactant concentrations multiplied together, with each concentration raised to the power of its coefficient in the balanced equation. For the generalized reaction 

The value of the equilibrium constant tells which side of the reaction arrow is energetically favored. If Kgq is much larger than 1, then the product concentration term [C] [D] is much larger than the reactant concentration term [A] [B] and the reaction proceeds as written from left to right. If Afeq is near 1, appreciable amounts of both reactant and product are present at equilibrium. And if Kgq is much smaller than 1, the reaction does not take place as written but instead goes in the reverse direction, from right to left. 

In the reaction of ethylene with H2O, for example, we can write the following equilibrium expression and determine experimentally that the equilibrium constant at room temperature is approximately 25. 

Because Xgq is a bit larger than 1, the reaction proceeds as written but a substantial amount of unreacted ethylene remains at equilibrium. For practical purposes, an equilibrium constant greater than about 10 is needed for the amount of reactant left over to be barely detectable (less than 0.1%). 

In the reaction of ethylene with HBr, for example, we can write the follow- 

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