Kinetics
Kinetics refers to how fast the reaction happens (rate of reaction) and factors affecting it.
Collision Theory states that in order for the reaction to happen, reacting particles must have proper orientation when colliding and have enough energy. The collisions that result in the formation of the product are called effective collisions.
Factors affecting the rate of reaction are :
Catalyst = speeds up reaction by lowering activation energy. Catalyst provides a new reaction pathway.
Concentration = increasing the concentration of reactants increases the rate of the reaction.
Surface area = increasing the surface area of the reactants increases the rate of the reaction.
Temperature = increasing the temperature of the reaction increases the rate of the reaction.
Heat
Reactions can be divided into two categories: exothermic and endothermic.
Exothermic reactions release energy and have a negative ΔH (heat of the reaction).
Endothermic reactions absorb energy and have a positive ΔH.
Table I on the Periodic table shows ΔHs for various reactions. ΔH is H products - H reactants.
Potential Energy Diagrams
Facts to remember:
Spontaneous reactions (occur on its own without external input) and systems in nature tend to undergo changes towards less energy (lower enthalpy) and more disorder (higher entropy).
Equilibrium
At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction and the concentration of reactants and products stay constant.
A physical equilibrium refers to the reaction with a phase change such as going from solid to liquid and back.
Chemical equilibrium refers to a reaction with a chemical change (identity of the substance is changed).
Le Chatelier's Principle
Le Chatelier's principle says that when there is a stress on a system that takes it away from equilibrium, the system will react in such a way as to get back to the favorable equilibrium position.
There are 3 stressors on the system. Lets list them and see what happens with each one.
Lets say our equation is
2A (aq) + B(s) <---> C(g)
1. Concentration.
When concentration is increased, equilibrium shifts to the opposite side to make up for it. For example, adding more A shifts the equilibrium to the right side, thus producing more C.
When concentration is decreased, equilibrium shifts to the same side.
Changes in solids and liquids DO NOT shift the equilibrium.
2. Pressure and volume.
When volume is decreased, pressure is increased and equilibrium shifts to the side with less GAS moles. In the case of our equation, there are 0 moles of gas on the left and 1 on the right. Equilibrium will shift to the left.
When volume is increased, pressure is decreased and equilibrium shifts to the side with more gas moles.
3. Temperature.
In order to see the effect of changing temperature, we must first figure out on what side temperature is on. If the reaction is endothermic, heat is on the left side. If the reaction is exothermic, heat is on the right side.
Once this has been determined, we can treat temperature as one of the reactants or products (depending on which side it is on) and we can refer to point 1.
PS: Catalyst
Catalyst speeds up the reaction in both forward and reverse directions. Catalyst does not effect the equilibrium and does not shift it.
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