The rate equation of a reaction maps out the rate of disappearance/appearance of a compound over time.
rate = k[A]
Where [A] is the molar concentration of compound A, and 'k' is the rate constant. Rates can have a certain order to them. They can be zero, first, second, etc. depending on the reaction. First order reactions (like the one above) are directly proportional to the concentration, so its 'k' value has a unit equal to (1/s) so as to make the rate (M/s).
Second order reactions could look like so:
rate = k[A]2
where the constant 'k' units are (1/M•s).
Though reaction orders are often whole numbers, they may also be fractions or negative in value.
Zero Order ReactionEdit
A zero-order reaction is independent of the concentration of the reactants, in which even a higher concentration of reactants will not speed up the rate of the reaction. This kind of reaction if found when a catalyst or other material required for the reaction is saturated by the reactants. Zero-order reactions frequently have to occur first in order to provide reactive substances for the higher order reactions.
The rate equation of this reaction is illustrated as: Rate = K
Furthermore, a plot of concentration versus time should yield a straight line.
First Order ReactionEdit
A first-order reaction depends on the concentration of only one reactant. This order occurs mostly in the reactions where there is only one reactant.
The rate equation of this reaction is illustrated as: Rate = K[A]
Furthermore, a plot of the natural log of concentration versus time should yield a straight line.
Second Order ReactionEdit
A second-order reaction is characterized by the property that their rate is proportional to the product of the concentrations of two reactants.
The rate equation of this reaction is illustrated as: Rate = K[A][B]
This means that if you double the concentration of the reactants, it will result in a four time increase in rate.
Furthermore, a plot of the inverse of concentration versus time should yield a straight line.