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Look at a closed soda bottle. What do you see? Without shaking it up, do you see any gas in there? Any bubbles? Any mist? You most likely don't see gas, mist, or many bubbles. That is because the pressure in the bottle keeps all the bubbles and gas in the solution. The liquid and gas are in equilibrium.
Now open the bottle. What do you see? You probably see bubbles, foam, or mist. Why? Because you have changed the amount of pressure on the liquid and the gas no longer stays in solution. The change in pressure caused the liquid and gas solution to no longer be in equilibrium. Eventually, as you know, most of the gas will leave the liquid and the soda will go flat. When this happens, the liquid and gas are back in equilibrium, just a different equilibrium than the closed bottle.
Equilibrium is explained in chemistry by Le Chatelier's Principle, which states that any change in a substance on one side of the equation in concentration, temperature, or pressure results in an equilibrium shift to oppose the change until a new equilibrium is reached. Another way of saying this is that when a system that is in equilibrium is disturbed, the system adjusts itself to reduce the change.
For instance, you know that the volume of a gas decreases with increased pressure. So, if you have two volumes of gas in equilibrium, if one volume decreases with increased pressure, the other volume must increase with decreased pressure. Think of it like a teeter-totter. As one side goes up, the other must go down.
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Chemical reactions that are in equilibrium are affected by three different changes: change in concentration of products or reactants, change in temperature, and change in pressure. When one of these changes or 'stressors' is applied to a reaction in equilibrium, the rates of the forward and reverse reactions are no longer equal. The system will change so that either more product or more reactants are made. In time, though, a new chemical equilibrium will be reached, and the forward and reverse reactions will again be equal.
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If a chemical reaction in equilibrium has changes in the concentration of the products or reactants, the reaction changes until it comes back into equilibrium. For instance, if you increase the concentration of the reactant, this added stress on the system causes the reaction to make more product, essentially making its forward rate greater than its reverse rate. Since the forward reaction is increasing, the equilibrium is said to shift right. This will continue until the concentration of the reactant has lessened. At this point, the forward and reverse rates will be equal again, and the reaction will be in equilibrium.
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Temperature is also a stress on the reaction system. If a reaction is exothermic, meaning it gives off heat as it proceeds forward, increasing the temperature of the reaction leads to a shift to the left. Therefore, more product is being broken down and more reactant is being made. As product of a usually exothermic reaction is broken down, energy is absorbed, so by making more of the reactants, some of the energy that is added to the system through the increased temperature is then removed.
The opposite is also true. If energy in the form of heat is added to an endothermic reaction, the equilibrium will shift to the right and more product is made.
The last stressor on a system is pressure. Pressure has little effect on reactions that are in solution, but it can affect gas reactions. The reason for this is that the volume a gas takes up is related to how much pressure the gas is under. This is explained by Boyle's Law, which says that the volume of a gas increases as the pressure on that gas decreases.
An increase in pressure favors the reaction that produces fewer gas molecules. So, if you have a gas reaction, A2 + 3 B2 --> 2 AB3, you can see that on the left side of the reaction, there are four molecules of gas (one molecule of A2 and three molecules of B2), and on the right side, there are two. So, increasing the pressure of this reaction would cause the equilibrium to shift right to make fewer molecules overall. If the reaction were the other way, though - 2 AB3 --> A2 + 3 B2 - then increased pressure would favor the reactant because there are only two molecules of reactant for every four molecules of product.
When a system that is in equilibrium is disturbed, the system adjusts itself to reduce the change. Equilibrium is explained in chemistry by Le Chatelier's Principle, which states that any change in a substance on one side of the equation in concentration, temperature, or pressure results in an equilibrium shift to oppose the change until a new equilibrium is reached.
When a chemical reaction system is stressed by an increase in the concentration of the reactants, the system shifts to the right, toward the products. If the concentration of products increases, the system shifts to the left, toward the reactants.
When an exothermic chemical reaction system is stressed by a rise in temperature, the reaction shifts to the left. If the chemical reaction is endothermic and the temperature rises, the reaction shifts to the right.
When a reaction that involves gases has an increase in pressure, the system will shift in the direction that has the fewest gas molecules, whether that is reactants or products.
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Chemistry 101: General Chemistry14 chapters | 132 lessons | 11 flashcard sets
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