Reaction Rate
The rate of a chemical reaction is how fast it is proceeding.
Rate is how fast the reactants are being used up and how fast products are being made.
When more atoms and molecules are colliding and reacting per second, the reaction is faster.
Collision Theory
The reactant atoms / molecules must collide with each other, with sufficient energy, for a reaction to happen.
Collision Probability
The higher the probability (i.e. chances) of a collision between reactants, the faster a reaction rate will be.
Factors Affecting Reaction Rates
1) Temperature. Particles with higher temperature move faster and have more energy. That means they have more chances to collide with other molecules. Faster molecules also collide with more energy, which gives each collision a higher chance of causing the reaction.
2) Reactant Surface Area. The more contact area that exists between the reactants, the faster they can react. More molecule collisions can happen when more molecules are next to other molecules of different types.
Imagine a block of solid carbon sitting on top of a block of solid Iodine. They can only react at the surface between the two materials. The material at the other ends of the blocks is not touching the other material, so they are not reacting. Now imagine grinding each block into a powder and mixing them together in a bowl. Now you have a much higher amount of surface area contact between the two materials, so a reaction would be much faster. Many mole pairs of atoms would be able to collide at the same time.
3) Stirring / Mixing: Stirring a liquid reaction causes more molecule collisions. It also helps to break up areas where one reactant is clumped and helps those molecules “find” the other reactant so they can react. This results in a faster reaction rate.
4) Reactant Concentration: The reactant molecules you have in a given volume, the more likely the reaction collision will be. Therefore, higher reactant concentration causes faster reaction rates.
“Concentration” means amount of a chemical in a certain volume.
Example 1 : “4 grams of oxygen per liter of air” is a concentration.
Example 2: “10 grams of oxygen per liter of air” is a higher concentration since it’s more of a substance in the same amount of volume.
Example 3: 10 grams of oxygen per milliliter of air is a much higher concentration than example 2, since now you have the same mass of oxygen in a volume that is 1000 times smaller. Even though it’s the same mass, the molecules would collide much more often when they are so much closer together.
Higher reactant concentrations can increase a reaction rate, but higher product concentrations do not have that same effect.
Example:
In that reaction (burning of natural gas), the reaction goes faster if the concentration of CH4 or O2 is increased.
If you supply pure oxygen gas to burn natural gas, it burns much faster than regular air, which is only 20% oxygen. This is because the oxygen molecules are much more likely to collide with CH4 when there are more of them, so those collisions happen more often. This increases the reaction rate.
On the other hand, increasing the concentration of the products CO2 and H2O would not speed up this reaction. Those are not the ingredient molecules that make the reaction happen, so they just get in the way. Having other molecules in the way actually makes the collision between CH4 and O2 less likely.
5) Catalysts are chemicals that can speed up a reaction. They help reactant molecules collide without being used up in the reaction, so they are not the same thing as a reactant. Sometimes, molecular collisions don’t result in a reaction because the reactant molecules are pointed in the wrong directions to make the reaction start. Catalysts can cause just the right type of collision to be more likely. Often, a very small amount of catalyst can speed up a reaction.
Catalysts lower the activation energy of a reaction. This means they don’t need as much energy when they collide to have a successful reaction.