Video: EC17-18-S2-Q19B



Video Transcript

Write the balanced chemical equation for the reaction represented by the equation below.

The K in this equation is an equilibrium constant. The subscript b indicates that the equilibrium constant is a base dissociation constant. Base dissociation constants are measured where water is the solvent and the reactant. Let’s have a quick review of what an equilibrium constant is. The equilibrium constant for equilibrium is equal to the concentration of the products multiplied by their stoichiometric coefficients all multiplied together, divided by the equivalent for the reactants. So we know that the ammonium ion and the hydroxide ion are products in the equation and that ammonia is one of the reactants. Now we can start constructing the chemical equation.

The reactants are ammonia and water. We know that water is there because base dissociation constants express the equilibrium between a base and water. As this is an equilibrium, we use the equilibrium arrow. And the products are ammonium and hydroxide. The question asks for the balanced chemical equation. So we should check that this equation is balanced. There’s one equivalent of nitrogen on both sides of the equation, five equivalents of hydrogen, and one equivalent of oxygen. The equation therefore is balanced.

One more thing we can do to make sure this is the right answer is work backwards and see whether the base dissociation constant makes sense. The top half of the base dissociation constant looks right. But what about the bottom half? The concentration of water does not appear in the original base dissociation constant expression. If we look back at the equation, we can tell why. Ammonia, ammonium, and hydroxide are all dissolved in the water whereas water, the liquid, is a solvent. Therefore, its concentration is assumed to be constant and is eliminated from the dissociation constant expression. So we’re back to our original base dissociation constant expression.

And our balanced chemical equation for the reaction represented by the base dissociation constant expression is NH₃ plus H₂O in equilibrium with NH₄+ plus OH-.

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