Reactants are transformed into products in a chemical reaction by definition. Chemical kinetics, the study of how quickly change happens, focuses on the reaction rate, or the change in reactant (or product) concentrations as a function of time.

Varying reactions have different rates: in a quicker reaction (higher rate), the reactant concentration falls quickly, whereas, in a slower reaction (lower rate), the reactant concentration decreases slowly.

Chemical processes occur at a variety of speeds (Figure 16.2). Some processes, such as neutralization, precipitation, or explosion, can be completed in a fraction of a second or less. Processes involving many reactions, such as fruit ripening, can take days to months.

Humans age over decades, and coal is formed from dead plants over hundreds of millions of years.

Knowing the response rate is important because how quickly a drug acts may mean the difference between life and death, and how long it takes a commercial product to develop can be the difference between profit and loss.

Under any given set of conditions, the reaction rate is determined by the nature of the reactants; some reactions are intrinsically fast, whereas others are significantly slower. At room temperature, for example, hydrogen reacts explosively with fluorine but extremely slowly with nitrogen:

• H2(g) + F2(g) ⟶ 2HF(g) [very fast]

• 3H2(g) + N2(g) ⟶ 2NH3(g) [very slow]

Under different situations, each given reaction, quick or slow, has a variable pace.

The concentrations of reactants, their physical condition, the temperature of the reaction, and the employment of a catalyst are all parameters that may be controlled.

• The term Concentration refers to molecules that must collide in order to react. Reactant concentration is a significant factor determining response rate.

Physical state: In order for molecules to collide, they must first mix. Collision frequency, and therefore reaction rate, are also affected by the physical condition of the reactants, which influences how easily they mix.

When the reactants are in the same phase, such as in an aqueous solution, random thermal motion brings them together, but gentle stirring further mixes them.

Contact occurs only at the interface between the phases when the reactants are in different phases, thus strong stirring or even grinding may be required. As a result, the finer a solid or liquid reactant is split, the larger it is surface area, the more contact it has with the other reactant, and the faster the reaction happens.

• The term temperature: refers to enough energy that must be present for molecules to collide.

The temperature often has a significant impact on the rate of a reaction.

This effect is used by two kitchen appliances: a refrigerator slows down chemical processes that degrade food, but an oven speeds up other chemical processes that cook it.

Temperature influences reaction rate by increasing collision frequency and, more crucially, collision energy: The frequency with which collisions occur. Remember that molecules in a gas sample have a range of speeds, with the most likely speed increasing with temperature (as shown in the image attached)

When a result, as the temperature rises, reactant particles travel faster, collisions become more frequent, and more molecules react:

• Rate ∝ collision frequency ∝ temperature

Collision energy. Even more importantly, the temperature has an effect on the kinetic energy of the molecules.

Most collisions between NO and O3 molecules in the reaction vessel contain only enough energy for the molecules to bounce off one other without reacting. However, certain impacts have enough energy to cause the molecules to react (as shown in the image attached ). More collisions occur at higher temperatures, resulting in more molecules reacting:

• Rate ∝ collision energy ∝ temperature

Chemical kinetics studies reaction rates and the factors that influence them.

Each response has its own pace under a particular set of circumstances.

Concentration influences rate by influencing the frequency and, more importantly, the energy of collisions between reactant molecules.

Physical state influences rate by determining how well reactants can mix. Temperature influences rate by influencing the frequency and, more importantly, the energy of collisions between reactant molecules.

The initial rate. The starting rate is the instantaneous rate at the time the reactants are combined (that is, at t = 0). We utilize this rate to prevent a complication: when a reaction moves forward (reactants products), the product rises, forcing the reverse reaction (reactants products) to occur faster.

To determine the total (net) rate, we would need to subtract the forward and reverse rates. However, because t = 0 for the initial rate, product concentrations are insignificant, as is the reverse rate.

The slope of the line tangent to the curve at t = 0 s is used to calculate the starting rate. Because the reactant concentrations are highest at t = 0, the initial rate I.