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Collisions, rate measurement and rate representation

Effective collisions and reactions

A chemical reaction between two particles only occurs if certain conditions are met:

• Collision: The reactant molecules need to collide in order to form new product molecules.
• Orientation: The right parts of reactant molecules must collide together.

When $\ce{CH3CH2CH2Br}$ reacts with hydroxide ($\ce{OH^-}$) to form $\ce{CH3CH2CH2OH}$, the $\ce{OH^-}$ ions must collide specifically with the carbon atom containing bromine for the substitution to occur.

• Energy: The reactants must bring enough energy into the collision. Think of the energy of the reactants as the speed with which they hit each other.

An effective collision occurs when reactants collide in the right orientation and with sufficient energy.

Factors like temperature, pressure and the presence of catalysts influence the rate of reaction by affecting the likelihood of effective collisions.

Reaction rate

The rate of reaction (or reaction rate) measures how quickly a reaction occurs.

The rate of reaction can be measured by

• the rate at which a reactant disappears or
• the rate at which a product appears

during a reaction.

$$\text{Rate of reaction}=\displaystyle{\frac{\text{Change in concentration of reactant/product}}{\text{Time}}}$$

The rate of reaction can predict the time needed to reach a given decrease in reactant concentration or a given increase in product concentration.

The rate of reaction depends on the frequency of effective collisions.

If effective collisions are more frequent, the product molecules form faster and the reactant molecules disappear faster. The rate of reaction is therefore higher.

Measuring reaction rate with solutions

Absorbance is the percentage of light that is absorbed by molecules in a solution.

When a beam of light is directed on a solution, some of the light will be taken in by the solution instead of passing through the solution.

The reaction rate in a solution is often determined by measuring the absorbance over time. As the concentration of a product or reactant in solution increases, the absorbance of light at a set wavelength increases.

Different chemicals absorb light better at different wavelengths. The wavelength of applied light can therefore be set to a specific value to measure a specific chemical over time.

A sample of a UV-visible spectrum of chlorophyll.

Chlorophyll b absorbs the most light when its wavelength is about 453 nm. The absorbance increases (seen as a higher peak) when the concentration increases.

Studying the effect of a given variable on reaction rates

The rate of reaction must be determined experimentally. It cannot be determined from the chemical equation alone.

The chemical equation does not provide any information about the rate of reaction or how the rate of reaction will change as the reaction continues.

Scientists must carry out the reaction in a laboratory setting to examine the rate of the reaction.

In order to determine the effect of an important variable (concentration, temperature, etc.) on the rate, the variable should be changed while the others are held constant.

In the reaction of hydrochloric acid ($\ce{HCl}$) with ammonia ($\ce{NH3}$), the concentration of $\ce{HCl}$ is increased while that of $\ce{NH3}$ is held constant.

This allows for the determination of the effect of $\ce{HCl}$ concentration on the reaction rate.

The same procedure is then repeated with $\ce{HCl}$ held constant to determine the effect of $\ce{NH3}$ concentration on the reaction rate.

Graphical representation of the reaction rate

A concentration ($\text{mol/}\cdm$) versus time (s) graph charting the changes in concentrations of products and reactants during a reaction.

The concentration of products increases as time passes, and this is shown as a curve with a positive slope. A curve representing the reactants will have a negative slope since its concentration decreases.

When the reaction ends, no more products are formed and no more reactants are consumed. The concentration curves each level off to form a plateau.

The gradient of the graph reflects the rate of reaction.

The product curve is steep (has a higher gradient) at the start of the reaction. This indicates that the reaction is faster.

As the reaction proceeds, the gradient of the product curve gets gentler as the reaction rate declines.

The reaction rate for a given reaction is rarely constant throughout the reaction.

When the reaction is almost complete, the product curve starts to plateau as the concentrations change more slowly.