Rates of Reaction

All substances are made up of tiny particles that are constantly in motion. For a chemical reaction to occur, these particles must collide with each other.

However, the rate at which a chemical reaction occurs is not always the same. It depends on a number of factors that can affect whether the reacting particles collide with enough energy to initiate the reaction. These factors are:

  • Temperature
  • Concentration of reactants
  • Pressure
  • Size of the particles
  • Presence of a catalyst

Calculating the Rate of Reaction

We can look at the rate of a chemical reaction in two different ways:

1. How quickly a product forms. To calculate this, we use the equation:

Mean\:rate\:of\:reaction=\dfrac{Quantity\:of\:product\:formed}{Time\:taken}

2. How quickly a reactant is used up. To calculate this, we use the equation:

Mean\:rate\:of\:reaction=\dfrac{Quantity\:of\:reactant\:used}{Time\:taken}

Following Chemical Reactions

There are three main ways we can measure the rate of a chemical reaction:

  • Change in mass over a period of time
  • Volume of gas over a period of time
  • Time taken for a cross to be obscured when a precipitate forms.

Measuring the mass

We can measure the change in mass of a reactant or product during a chemical reaction. Although, it is easier to measure a product forming than a reactant disappearing.

A conical flask contains a reaction, with cotton wool wedged in its opening to prevent the escape of gas. The scale beneath the flask reads 96.8g.

This method might not be suitable for gases with a small relative formula mass (Mr), as the change in mass could be too small.

  • The unit of time is usually seconds, so the rate is usually measured in grams per second g/s. However, it can also be measured in grams per minute g/min.

Measuring the volume of gas

We can measure the volume of gas given off using a gas syringe or an upside-down measuring cylinder.

A labeled apparatus measures the gas released during a reaction. A gas syringe connects to a conical flask, and a tube extending into an inverted measuring cylinder in water allows gas bubbles to be released.

The rate is usually measured in cubic centimetres per second (cm³/s) or cubic centimetres per minute (cm³/min).

Measuring the time taken for a cross to disappear

We can place the reaction container on top of a black cross and measure the time taken for the mixture to become cloudy. The precipitate formed will cloud the mixture, which prevents the cross from being seen.

A labelled apparatus for an experiment calculating the rate of a reaction based off of the presence of a precipitate and the visibility of a cross. Dilute acid is poured into a transparent solution and over time the solution becomes opaque.

However, this method can be subjective, as different individuals might record varying times, disagreeing on the exact moment the solution turns cloudy.

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