4.1 - Introduction to Rate of Reaction

Fast Reactions and Slow Reactions in Daily Life

• A chemical reaction is a process in which one or more reactants are converted into one or more products.
• Example:

• Colourless lead(II) nitrate solution and colourless potassium iodide solution are the reactants.
• Yellow lead(II) iodide precipitate and colourless potassium nitrate solution are the products.
• As a reaction proceeds:
  • the quantity of reactant decreases with time;
  • the quantity of product increases with time.

Fast reaction	Slow reaction
High rate of reaction	Low rate of reaction
Happens quickly	Happens slowly
Takes a shorter time to complete	Takes a longer time to complete
Reactant decreases quickly; product increases quickly	Reactant decreases slowly; product increases slowly
Example: burning of butane gas	Example: rusting of iron

Graph interpretation

• A graph of quantity of reactant against time slopes downwards.
• A graph of quantity of product against time slopes upwards.
• A fast reaction has a steeper curve and reaches its final quantity in a shorter time.
• A slow reaction has a less steep curve and reaches its final quantity in a longer time.

Rate of Reaction

Rate of reaction is the change in the quantity of reactant or product per unit time.

Changes that can be observed or measured over a specific period of time include:

• decrease in the mass, volume or concentration of a reactant;
• increase in the mass, volume or concentration of a product;
• decrease or increase in the pressure, temperature, pH value, electrical conductivity, heat conductivity or intensity of colour of the reacting mixture;
• increase in the volume or pressure of gas released;
• increase in the height of precipitate formed.

The unit of rate follows the quantity measured and the unit of time, for example:

• mass per unit time: ${\text{g s}}^{-1}$;
• gas volume per unit time: ${\text{cm}}^3{\text{ s}}^{-1}$.

Determining the Average Rate of Reaction

The average rate of reaction is the average value for the rate of reaction that occurs in a specific time interval.

Example 1: decrease in the mass of a reactant

When $0.3\text{g}$ of magnesium tape reacts completely with excess dilute hydrochloric acid in $30\text{s}$:

Example 2: volume of hydrogen gas against time

Given the graph readings $20.00{\text{cm}}^3$ at $60\text{s}$, $30.00{\text{cm}}^3$ at $120\text{s}$ and $35.00{\text{cm}}^3$ at $180\text{s}$, after which the graph is horizontal:

Time interval	Average rate of reaction
First minute, $0$–$60\text{s}$	$\frac{20.00}{60}=0.33{\text{cm}}^3{\text{ s}}^{-1}$
First 2 minutes, $0$–$120\text{s}$	$\frac{30.00}{120}=0.25{\text{cm}}^3{\text{ s}}^{-1}$
Second minute, $60$–$120\text{s}$	$\frac{30.00-20.00}{120-60}=0.17{\text{cm}}^3{\text{ s}}^{-1}$
Third minute, $120$–$180\text{s}$	$\frac{35.00-30.00}{180-120}=0.08{\text{cm}}^3{\text{ s}}^{-1}$
Whole reaction, $0$–$180\text{s}$	$\frac{35.00}{180}=0.19{\text{cm}}^3{\text{ s}}^{-1}$

• Use the change within the stated interval, not the total quantity from time zero, when the interval does not begin at zero.
• The reaction ends when the graph first becomes horizontal. In this example, use $180\text{s}$, not the final time shown on the horizontal axis.

Rate of Reaction at a Particular Point of Time

The rate of reaction at a particular point of time, or instantaneous rate of reaction, is the rate at a specific instant.

Method

1. Mark the required point on the curve.
2. Draw a tangent that touches the curve at that point.
3. Choose two well-separated points on the tangent.
4. Form a large right-angled triangle.
5. Calculate the gradient:

Worked tangent examples

• At the 20th second:

• At the 60th second:

• At the 120th second: