Le Chatelier’s principle Collision Theory And Le Chatelier’s principle

How Chemical Reactions Occur Reactants on the left and products on the right separated by an arrow. How do the atoms in the reactants reorganize to form the products? It is believed that molecules react by colliding with each other. Some collisions are successful and allows the reactants to form the products.

Collision model: reactions occur during molecular collisions Model explains: why a reaction proceeds faster if the concentration of the reacting molecules are increased (higher concentrations lead to more collisions); and why reactions go faster at higher temperatures.

Conditions That Affect Reaction Rates Higher concentrations (more molecules per unit volume) leads to more collusions and so more products formed. Higher temperatures lead to increased speed of molecules and thus the average collusion is more energetic and more likely to break bonds and produce products. Temperature affects speed and thus collision energy but not orientation of molecules.

Activation Energy When molecules collide, a certain minimum energy called the activation energy is needed for the reaction to occur. Activation energy is also defined as the energy needed to reach the activated complex. If the energy contained in the collision is greater than Ea, the reaction can go “over the jump” to form products.

Activation Energy If the collision energy is less than Ea , the colliding molecules bounce apart unchanged. Is it possible to speed up a reaction without changing the temperature or reactant concentrations?

It is possible to speed up a reaction using a catalyst, a substance that speeds up a reaction without being consumed. A catalyst works because it provides a new pathway for the reaction – a pathway that has a lower activation energy than the original pathway. Lower activation energy = more collisions are successful = faster reaction

In an exothermic reaction, the potential energy of the reactants is greater than the potential energy of the products. Remember in an exothermic reaction, energy is given off to the surroundings. In this case the energy of the products is less than the energy of the reactants, both of which are less than the activation energy.

In an endothermic reaction, the potential energy of the products is greater than the potential energy of the reactants. Remember in an endothermic reaction, energy is absorbed from the surroundings. In this case the energy of the reactants is less than the energy of the products, both of which are less than the activation energy.

Le Chatelier’s Principle Various changes in conditions affect the equilibrium position of a reaction system. The effects of changes in concentration, pressure, and temperature on a system at equilibrium can be predicted by using Le Chatelier’s Principle. Henry Louis Le Chatelier

Le Chatelier’s Principle When a change is imposed on a system at equilibrium, the position of the equilibrium shifts in a direction that tends to reduce the effect of that change.

Effect of a Change in Concentration When a reactant or product is added to a system at equilibrium, the system shifts away from the added component. On the other hand, if a reactant or product is removed, the system shifts toward the removed component.

Effect of a Change in Concentration For example, consider the following reaction at equilibrium (picture “a”): N2(g) + 3H2(g) ⇌ 2NH3(g) Suppose we add N2 to this equilibrium mixture (picture “b”). Le Chatelier’s principle predicts that the system will shift in a direction that consumes nitrogen.

Effect of a Change in Concentration This tends to offset the original change – the addition of N2. Therefore, Le Chatelier’s principle correctly predicts that adding nitrogen will cause the equilibrium to shift to the right as some of the added nitrogen is consumed (picture “c”). If NH3 had been added instead of N2, the system would have shifted to the left, consuming NH3 .

Example: Changes in Concentration Arsenic can be extracted from its ores by first reacting the ore with oxygen (called roasting) to form solid As4O6, which is then reduced using carbon: As4O6(s) + 6C(s) ⇌ As4(g) + 6CO(g) Predict the direction of the shift of the equilibrium position in response to each of the following changes: Addition of carbon monoxide Addition or removal of carbon or As4O6 Removal of gaseous arsenic (As5)

As4O6(s) + 6C(s) ⇌ As4(g) + 6CO(g) Addition of carbon monoxide The shift will be away from the substance whose concentration is increased. The equilibrium position will shift to the left. Addition or removal of carbon or As4O6 Since a pure solid has no effect on the equilibrium position, there is no shift. Removal of gaseous arsenic (As4) The shift will be toward the substance whose concentration is decreased. The equilibrium position will shift to the right.

The Effect of a Change in Pressure There are three ways to change the pressure of a reaction system involving gaseous components: Add or remove a gaseous reactant or product. Add an inert gas (one not involved in the reaction). Change the volume of the container. When an inert gas is added, there is no effect on the equilibrium position. When the volume of the container holding a gaseous system is reduced, the system responds by reducing its own volume. This is done by decreasing the total number of gaseous molecules in the system.

Example: Using Le Chatelier’s Principle Predict the shift in equilibrium position that will occur for each of the following processes when the volume is reduced: P4(s) + 6Cl2(g) ⇌ 4PCl3(l) Only consider Cl2 (only gas). The volume is decreased, so the position will shift to the right to decrease the number of gaseous molecules. PCl3(g) + Cl2 (g) ⇌ PCl5(g) Equilibrium position will shift to the right since the product side contains only one gaseous molecule while the reactant side has two. PCl3(g) + 3NH3(g) ⇌ P(NH2)3(g) + 3HCl(g) Both sides contain four gaseous molecules so no shift is seen in this case.

The Effect of a Change in Temperature To use Le Chatelier’s principle to describe the effect of temperature change, treat energy as a reactant (in an endothermic process) or as a product (in an exothermic process), and predict the direction of the shift in the same way as when an actual reactant or product is added or removed.

Example: Using Le Chatelier’s Principle For each of the following reactions, predict the shift in equilibrium position as the temperature is increased. N2(g) + O2(g) ⇌ 2NO(g) ΔHo = 181 kJ The reaction is endothermic so write energy as a reactant. N2(g) + O2(g) + energy ⇌ 2NO(g) Increase in temperature will cause the equilibrium to shift to the right. b. 2SO2(g) + O2(g) ⇌ 2SO3(g) ΔHo = -198 kJ The reaction is exothermic so write energy as a product. 2SO2(g) + O2(g) ⇌ 2SO3(g) + energy Increase in temperature the equilibrium will shift to the left.

Summary of Le Chatelier’s Principle