IB Chemistry Topic R2.2.1 Rate of Reaction Revision Notes (SL/HL)

IB Chemistry Topic R2.2.1 Rate of Reaction Revision Notes (SL/HL) | RevisionDojo

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The Rate of Reaction: Definition, Expression, and Graphical Determination

Definition

Rate of reaction

The rate of reaction measures how quickly reactants are consumed or products are formed during a chemical reaction. It is defined as:

The change in concentration of a reactant or product per unit time.

In mathematical terms, the rate of reaction is expressed as:

$$\text{Rate} = \frac{\Delta[\text{Reactant or Product}]}{\Delta t}$$

Where:

$\Delta[\text{Reactant or Product}]$ is the change in concentration (in $\text{mol dm}^{-3}$).
$\Delta t$ is the time interval over which the change occurs (in seconds).

Hint

If the concentration of a reactant decreases, $\Delta[\text{Reactant}]$ will be negative.
However, since reaction rates are always expressed as positive values, we take the absolute value.

Example

Consider the reaction: $$4 \text{NH}_3(g) + 5 \text{O}_2(g) \rightarrow 4 \text{NO}(g) + 6 \text{H}_2\text{O}(g)$$
Over a period of 5 seconds, the concentration of $\text{NO}(g)$ increases from $0$ to $6.0 \times 10^{-3} \, \text{mol dm}^{-3}$.
The rate of reaction with respect to $\text{NO}(g)$ is: $$\text{Rate} = \frac{\Delta[\text{NO}]}{\Delta t}$$ $$= \frac{6.0 \times 10^{-3} \, \text{mol dm}^{-3}}{5 \, \text{s}} = 1.2 \times 10^{-3} \, \text{mol dm}^{-3} \, \text{s}^{-1}$$
To find the rate of reaction with respect to $\text{O}_2(g)$, we use the stoichiometric relationship.
For every 5 moles of $\text{O}_2$ consumed, 4 moles of $\text{NO}$ are produced.
Thus: $$\text{Rate with respect to } \text{O}_2 = \text{Rate with respect to } \text{NO} \times \frac{4}{5}$$ $$=1.2 \times 10^{-3} \times \frac{4}{5} = 9.6 \times 10^{-4} \, \text{mol dm}^{-3} \, \text{s}^{-1}$$

Determining Reaction Rates Graphically

In experiments, the concentration of a reactant or product is often measured at regular time intervals.
This data is plotted on a graph, with time on the x-axis and concentration on the y-axis.
The resulting curve provides valuable insights into the reaction rate.

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The Rate of Reaction: Definition, Expression, and Graphical Determination

Definition

Rate of reaction

The rate of reaction measures how quickly reactants are consumed or products are formed during a chemical reaction. It is defined as:

The change in concentration of a reactant or product per unit time.

In mathematical terms, the rate of reaction is expressed as:

$$\text{Rate} = \frac{\Delta[\text{Reactant or Product}]}{\Delta t}$$

Where:

$\Delta[\text{Reactant or Product}]$ is the change in concentration (in $\text{mol dm}^{-3}$).
$\Delta t$ is the time interval over which the change occurs (in seconds).

Hint

If the concentration of a reactant decreases, $\Delta[\text{Reactant}]$ will be negative.
However, since reaction rates are always expressed as positive values, we take the absolute value.

Example

Consider the reaction: $$4 \text{NH}_3(g) + 5 \text{O}_2(g) \rightarrow 4 \text{NO}(g) + 6 \text{H}_2\text{O}(g)$$
Over a period of 5 seconds, the concentration of $\text{NO}(g)$ increases from $0$ to $6.0 \times 10^{-3} \, \text{mol dm}^{-3}$.
The rate of reaction with respect to $\text{NO}(g)$ is: $$\text{Rate} = \frac{\Delta[\text{NO}]}{\Delta t}$$ $$= \frac{6.0 \times 10^{-3} \, \text{mol dm}^{-3}}{5 \, \text{s}} = 1.2 \times 10^{-3} \, \text{mol dm}^{-3} \, \text{s}^{-1}$$
To find the rate of reaction with respect to $\text{O}_2(g)$, we use the stoichiometric relationship.
For every 5 moles of $\text{O}_2$ consumed, 4 moles of $\text{NO}$ are produced.
Thus: $$\text{Rate with respect to } \text{O}_2 = \text{Rate with respect to } \text{NO} \times \frac{4}{5}$$ $$=1.2 \times 10^{-3} \times \frac{4}{5} = 9.6 \times 10^{-4} \, \text{mol dm}^{-3} \, \text{s}^{-1}$$

Determining Reaction Rates Graphically

In experiments, the concentration of a reactant or product is often measured at regular time intervals.
This data is plotted on a graph, with time on the x-axis and concentration on the y-axis.
The resulting curve provides valuable insights into the reaction rate.

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R2.2.1 Rate of Reaction Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

The Rate of Reaction: Definition, Expression, and Graphical Determination

Determining Reaction Rates Graphically

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The Rate of Reaction: Definition, Expression, and Graphical Determination

Determining Reaction Rates Graphically

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