As a reactant gets used up in a chemical reaction, the reaction rate typically decreases. This is because the concentration of the reactant decreases, which means there are fewer reactant particles available to react with each other. As a result, the frequency of collisions between reactant particles decreases, and the reaction rate slows down.

The relationship between the reaction rate and the concentration of a reactant can be described by the following equation:

```

rate = k[A]^x[B]^y

```

In this equation, the rate is the rate of the reaction, k is the rate constant, [A] and [B] are the concentrations of the reactants A and B, and x and y are the orders of the reaction with respect to A and B, respectively.

As the concentration of a reactant decreases, the corresponding term in the rate equation decreases. This causes the overall reaction rate to decrease.

The rate of a reaction can also be affected by other factors, such as temperature, pressure, and the presence of a catalyst. However, the decrease in the reaction rate as a reactant gets used up is a general principle that applies to most chemical reactions.