Reaction Rates and Equilibrium

Some reactions, such as the combustion of rocket fuel, happens extremely quickly. Other reactions such as rusting take a long time to develop

Reaction rate is the rate at which reactants change into products

The collision model states that atoms, ions, and molecules must collide in order to react

Understanding the collision model will help to explain why changing the conditions of a chemical reaction can have an effect on the reaction rate

Temperature

concentration

volume and pressure

surface area

catalysts and inhibitors

We often store perishable foods in a refrigerator to keep them from going bad quickly

remember that temperature is the measure of average kinetic energy so cooling something down will slow the particles and the reaction rate

Likewise heating a substance will increase the speed and motion of the particles and increase the reaction rate

Increasing the concentration of a solution adds more reactant particles which gives more opportunities for collisions to occur

more collisions = faster reaction rate

Volume and pressure are important because they relate to the concentration of reacting gases

Smaller volume = greater pressure = faster reaction rate

The more surface area that is in contact between the reactants the faster the reaction will occur

Think about what dissolves quicker...sugar crystals or a sugar cube

A catalyst speeds up a chemical reaction

an inhibitor slows down the rate of chemical reactions or prevents them from happening at all

Catalysts do NOT change the mass of the products they only change how fast they are produced

Your body uses a special catalyst called enzymes to break down food

Inhibitors do NOT change the ammount of product produced, only change the rate at which they are produced

A common example of an inhibitor would be food preservatives that we add to keep our food from spoiling as quickly as they would otherwise

So far the reactions we have looked at have been reactions that "go to completion," meaning they only happen in one direction and continue until all the reactants are consumed/transformed into the products

There are reversible reactions that can occur in backward and forward directions

we signify reversible reactions with a double arrow

When a reversible reaction's forward and reverse directions happen at the same rate we reach a state of equilibrium or balance

Equilibrium means our forward and reverse reactions or processes happen at the same rate

we can have equilibrium in physical changes as well as chemical changes

Just like we have factors that affect reaction rates, there are things we can do to change equilibrium

Le Chatelier's principle states that if a stress is applied to a system at equilibrium, the equilibrium shifts in the direction that opposes the stress