Equilibrium and collision theory Chemistry Chapter 17 Equilibrium and collision theory Chemistry Zuhumdal Last revision Fall 2014

Collision Theory Rates of chemical reactions are related to the properties of atoms, ions, and molecules through a model called collision theory. According to collision theory, atoms, ions, and molecules can react to form products when they collide, provided they have enough kinetic energy.

Think clay If you throw 2 clay balls together gently, they do not stick together, kinda like the lack of reaction between colliding particles with low energy. If you throw the same balls of clay together with great force, they stick together.

Activation Energy Activation energy is a barrier that reactants must cross to be converted to products.

Activated Complex An activated complex is the arrangement of atoms at the peak of the activation-energy barrier Also called transition state.

Factors Affecting Reaction Rates Temperature Concentration Particle Size Catalysts Pressure

Temperature Concentration Usually, raising the temperature speeds up the reactions, while lowering the temperature slows down the reactions. This will increase the number of particles that have enough kinetic energy when they collide. Concentration Cramming more particles into a fixed volume increases the concentration of reactants.

Particle Size Catalyst The smaller the particle size, the larger the surface area for a given mass of particles. Which burns faster, a piece of wood or a kindle? Why? Catalyst A catalyst is a substance that increases the rate of a reaction without being used itself. Catalyst permit reactions to proceed at a lower energy than required.

Reversible Reaction Do reactions only go one way or do reactions go 2 ways? Let’s look at this equation: 2SO2(g) + O2 ---- 2SO3 What happens when sulfur dioxide and oxygen gas are mixed in a sealed container? -----------

Reversible Reactions As the SO3 concentration increases, a small amount collides and reverts to SO2 and O2 by the reverse direction! As the concentration of SO3 becomes higher and higher, the reverse reaction speeds up! Eventually SO3 decomposes to SO2 and O2 as fast as it forms SO3.

Dynamic Chemical Equilibrium Equilibrium occurs when opposing reactions occur at equal rates and the concentrations of the products and reactants stop changing. Reactants and products are not necessarily equal in amount. And the forward and reverse reactions have not stopped.

Le Chatelier’s Principle If a stress is applied to a system in dynamic equilibrium, the system changes to relieve stress. Stresses include changes in the concentration of reactant or products, changes in temperature, and changes in pressure

Le Chatelier's Principle >> Concentration Changes The concentration can be changed by adding or removing a substance. If added, the equilibrium shifts to remove the substance. If removed, the equilibrium shifts to replace it. Adding or removing (provided some remains) solid, pure liquid, or solvent does not affect the equilibrium, since the concentration of these substances is constant.

Le Chatelier's Principle >> Concentration Changes Example 15 For the reaction, CH3OH(g) + O2(g)  HCOOH(g) + H2O(g) What direction does the equilibrium shift if more oxygen is added? What direction does the equilibrium shift if water is removed? How does the concentration of methanol (CH3OH) change if more oxygen is added? How does the concentration of methanol change if more water is added? How does the concentration of methanol change when more methanol is added?

Le Chatelier's Principle >> Pressure Changes Changing pressure only affects gases. Higher pressures favor fewer moles of gas Lower pressures favor more moles of gas. Only consider moles of gas (not other physical states) in determining equilibrium shifts.

Le Chatelier's Principle >> Pressure Changes Example 16 How does an increase in pressure affect the concentration of the reactants in the following reactions? C2H4(g) + H2(g)  C2H6(g) Xe(g) + 3F2(g)  XeF6(g) C(s) + 2F2(g)  CF4(g) H2S(g) + Hg(l)  HgS(s) + H2(g)

Le Chatelier's Principle >> Temperature Changes The easiest way to predict equilibrium shifts is to consider energy (or heat) as a product or a reactant. Energy is a product in exothermic reaction (– H) Energy is a reactant in endothermic reactions (+ H). Increasing temperature increases the "concentration of energy."

Le Chatelier's Principle >> Temperature Changes Example 17 How does the concentration of the product change if the temperature increases? Fe2S3  2Fe 3+ + 3S 2–       + H Si + 2F2  SiF4         – H