Examples of Zero Order Reactions:

A first-order reaction can be defined as a chemical reaction in which the reaction rate is linearly dependent on the concentration of only one reactant. In other words, a first-order reaction is a chemical reaction in which the rate varies based on the changes in the concentration of only one of the reactants.

If a plot of reactant concentration versus time is not linear but a plot of the natural logarithm of reactant concentration versus time is linear, then the reaction is first order.

• Photochemical reaction between hydrogen and chlorine:
• Decomposition of N2O on hot platinum surface:
• Decomposition of NH3 in presence of molybdenum or tungsten is a zero-order reaction.
• Iodations of acetone in presence of H+ ions.

The fundamental difference between zero and first-order kinetics is their elimination rate compared to total plasma concentration. First-order kinetics proportionally increases elimination as the plasma concentration increases, following an exponential elimination phase as the system never achieves saturation.

For a zero-order reaction, increasing the concentration of the reacting species will not speed up the rate of the reaction. A reaction is zero-order if concentration data is plotted versus time and the result is a straight line.

Zero-Order Reactions

The time required for half of the original population of radioactive atoms to decay is called the half-life. The relationship between the half-life, T1/2, and the decay constant is given by T1/2 = 0.693/λ.

Zero-order kinetics is described when a constant amount of drug is eliminated per unit time but the rate is independent of the concentration of the drug.

Graphical Relations and Half Lives For a zero order reaction (Half life decreases with decreasing concentration.) For a 1st order reaction (Half life is constant.)

Named after Leonor Michaelis and Maud Menten, this model of enzyme kinetics describes the relationship between the concentration and the rate of enzyme-mediated reaction. In short, at low concentrations, the more substrate you give the faster the reaction rate. Beyond this concentration, clearance will be zero-order.

A few substances are eliminated by zero-order elimination kinetics, because their elimination process is saturated. Examples are Ethanol, Phenytoin, Salicylates, Cisplatin, Fluoxetin, Omeprazol.

FIRST-ORDER KINETICS For most drugs, we need only consider first-order and zero-order. Most drugs disappear from plasma by processes that are concentration-dependent, which results in first-order kinetics. With first-order elimination, a constant percentage of the drug is lost per unit time.

Alcohol elimination was originally believed to be a zero-order process, meaning that alcohol was removed from the body at a constant rate, independent of the concentration of alcohol. Because of this concentration dependence, it is not possible to estimate one single rate of alcohol metabolism.

This does not work if the elimination of the drug occurs at a constant rate which is independent of concentration. Thus, in zero-order kinetics of elimination the concept of half-life becomes meaningless because the drug concentration does not decrease by half with every predictable time interval.

Even further, 94 to 97% of a drug will have been eliminated after 4 to 5 half-lives. Thus, it follows that after 4 to 5 half-lives, the plasma concentrations of a given drug will be below a clinically relevant concentration and thus will be considered eliminated.

3 This means that if you begin taking a medication with a half-life of 24 hours, after four days, or on the fifth day, the rate of intake of the drug will approximately equal the rate of elimination. If the half-life is 12 hours, you’ll reach a steady state at the beginning of the third day (after 48 hours).

What is a drug’s half-life? The half-life of a drug is the time it takes for the amount of a drug’s active substance in your body to reduce by half. This depends on how the body processes and gets rid of the drug. It can vary from a few hours to a few days, or sometimes weeks.

Half-life is used to describe exponential decay. In one half-life period, on average, half of the C14 atoms would decay. So one would expect that if you start with four C14 atoms, you would after one half life have two, and after another half life only one would remain.

Swallowing a drug is a relatively slow method of taking a drug. After the drug is swallowed, it is dissolved in the stomach and then absorbed into the bloodstream from the linings of the stomach and later, the small intestine.