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Heterogeneous Equilibrium - Homgeneous Equilibrium - Concept

Teacher/Instructor Jacqueline Spivey
Jacqueline Spivey

Ph.D.,U.C.Santa Cruz
Teaching at a top-ranked high school in SF

She teaches general and chemistry at a top-ranked high school in San Francisco. Prior to that, she lead and published a number of research studies and lectured at SF State University.

Homogeneous and heterogeneous equilibrium is a system of chemical equilibrium which depend upon the states of matter of the substances involved. Homogeneous equilibrium involves substances in the same state. Heterogeneous equilibrium involves substances in different states. The position of heterogeneous equilibrium does not depend on the amount of pure solid and liquid present.

Hi guys. So in this segment I'd like to discuss with you the difference between homogenous and heterogeneous equilibria. This far the systems that you've probably encountered all have the same, have the reactants and the products that are in the same phase. Most likely they were in the gas phase and that is what we call a homogenous equilibria where all the substances are in the same state be they all gas or liquid or solid. But most specifically for equilibrium problems usually in the gas phase. So many equilibria obviously involves substances that are in different states and we call those heterogeneous equilibria to imply that we have substances, reactants and or products that are not all in the same phase. So the position of the heterogeneous equilibrium does not depend on the amounts of pure solid or pure liquid presence.

And why is that? That's because their concentrations do not change during the course of the reaction. So basically you can say those are a constant, so that would mean that then the solids and the liquids are not included in the equilibrium expression. So let's go ahead and kind of look at that really quickly. So for the first example let's say we have calcium carbonate solid, in equilibrium with calcium oxide solid and carbon dioxide gas. So if we were to write the equilibrium expression the way that we're familiar with we would write the product of our products over our reactants right? So we'd have the concentration of calcium oxide times the concentration of carbon dioxide divided by the concentration of calcium carbonate.

However, I just said that since these 2 guys the calcium oxide and the calcium carbonate are solids they will effectively fall out of the equilibrium expression making it only dependent on the gaseous products in the reaction which are carbon dioxide. So for this particular reaction your keq would only be based on the concentration of carbon dioxide. For another example, let's say we had pure water liquid, decomposing to form hydrogen gas and oxygen gas. Again we'd write the equilibrium expression keq is equal to H2 remember it's squared because of the coefficient out in front times the concentration of oxygen gas divided by the concentration of water which should be squared because of the coefficient in front but remember this is pure water, so it's a liquid therefore it effectively falls out of the equilibrium expression making it only dependent on the concentration of the hydrogen gas and the oxygen gas.

Just to switch it up a little bit if this changed and then your reactants were water in the form of a gas or water vapor, decomposing to form hydrogen gas and oxygen gas then your equilibrium expression would include the concentration of water because, water vapor in fact can have changes in concentration and that's heterogeneous and homogeneous equilibrium.