## An Introduction to Chemical Kinetics

Kinetics is from the Greek, kinein, meaning "set in motion or move." Chemical kinetics studies two things:

1) the rate at which a chemical reaction takes place.
2) the factors which affect the speed of the reaction.

At the introductory level, most study is devoted to how fast a reactant is used up as opposed to how fast a product is formed.

Reminders!!

1) Chemical reactions are usually written in this form:

Reactants ---> Products

2) If there were two reactants and one product, it would be written like this:

A + B ---> C

3) Right now, we will keep all the coefficients at one. Later, we will change them.

Kinetics studies the rate of a chemical reaction. That means it is a time-based study. In every case, we will ask how fast did something take place. Some time unit (seconds, minutes, etc) will always be involved and it will always be in the denominator. In kinetics, something will happen "per second" or "per minute."

That means we will see this often: (something in the numerator divided by a time unit in the denominator).

The most typical thing in the numerator is molarity. Remember that the symbol for molarity is M and the units are moles/liter. So there are three possible ways to discuss the above reaction:

1) a change in the molarity of A
2) a change in the molarity of B
3) a change in the molarity of C

In this introductory part of kinetics, we will only study reactions that go from left to right. Later on, it will get more complicated, but not right now.

Since A and B are reactants, their amounts will go down in amount. They are reactants being used up in order to make the product C.

Therefore:

1) as the reaction proceeds, the total molarity of A remaining unreacted will decrease.
2) as the reaction proceeds, the total molarity of B remaining unreacted will decrease.

Since C is a product, it is being made. Its amount will go up.

Therefore

3) as the reaction proceeds, the total molarity of C present will increase.

However, the above points 1, 2, 3 DO NOT have a time unit in them. In order to create a rate, we need time in the denomiator:

A - the rate of reaction for substance A will be the amount used up divided by the elapsed time.
B - the rate of reaction for substance B will be the amount used up divided by the elapsed time.
C - the rate of reaction for substance C will be the amount produced divided by the elapsed time.

The last point to make is that, since A and B are decreasing in amount, we will indicate that with a negative sign in front of the number. You'll see how this is done in the next several tutorials.