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D3.7 · Relate overall reaction rate to elementary reaction steps

Learn to relate overall reaction rate to elementary reaction steps through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Energy Changes and Rates of Reaction

How a sequence of particle-level events produces an observable reaction rate

Some reactions appear to happen quickly, while others take longer. At the particle level, a reaction may involve more than one event: particles first form a temporary substance, which then reacts again. Chemists call each individual event an elementary reaction step. In this lesson, you will connect those steps to the overall reaction and explain how the slow step affects the overall rate. First recall that reaction rate describes how quickly reactants are used or products are formed. A common unit is moles per litre per second, written as mol L−1 s−1\mathrm{mol\,L^{-1}\,s^{-1}}.

What you will learn

  • Define an elementary reaction step and an intermediate.
  • Combine elementary steps to find the overall reaction.
  • Relate the rate of a sequence of steps to its slow step.
  • Distinguish an overall reaction equation from a proposed sequence of steps.

1. From observable change to elementary steps

You can observe a reaction through changes such as a colour fading, a gas forming, or a solid appearing. Those observations show that substances are changing, but they do not reveal every particle event that leads to the change.
At the particle level, reacting particles must take part in events that rearrange their atoms. A proposed sequence of these events is called a reaction mechanism. For this expectation, focus on the elementary steps in that sequence and how their rates relate to the overall reaction.
An elementary reaction step is one individual particle-level event in a mechanism. Its equation represents that event directly. For example, in the symbolic step A+B→C\mathrm{A + B \rightarrow C}, one particle of A reacts with one particle of B to form one particle of C. The letters are placeholders, not chemical formulas.
The number of particles required for an elementary event matters. A step written with two reactant particles requires those particles to take part in that event. Do not assume that the overall reaction equation describes one such event. An overall equation can summarize several steps.
  • An elementary step represents one particle-level event.
  • A mechanism is a proposed sequence of elementary steps.
  • The overall equation summarizes the net chemical change.

2. Adding steps to obtain the overall reaction

To find the overall reaction from a sequence of steps, add the step equations and cancel substances that appear on both sides. These substances are made in one step and used in another, so they do not remain in the net equation.
A substance formed in one step and consumed in a later step is an intermediate. It participates in the mechanism, but it cancels when the steps are added. Therefore, an intermediate does not appear in the overall equation.
Consider the symbolic sequence A→I\mathrm{A \rightarrow I} followed by I+B→C\mathrm{I + B \rightarrow C}. Adding the equations gives A and I on the reactant side, and I, B, and C on the product side. Cancel I once from each side. The net change is from A and B to C.
This cancellation is bookkeeping for the sequence. It does not mean that I is absent from the particle-level events. It means that I is not used up overall: it is produced and then consumed. In a real chemical equation, the same method must preserve each element and the net charge.
A+B→C\mathrm{A + B \rightarrow C}
  • Add the elementary-step equations and cancel species found on both sides.
  • An intermediate appears in the steps but not in the net reaction.
  • Check that the resulting equation conserves atoms and charge when actual substances are used.

3. Connecting step rates to the overall rate

In a sequence of steps, the product of one step may need to form before a later step can occur. If one step is much slower than the others, it limits how quickly the sequence can proceed. This slow step is often called the rate-determining step. It is the step that most strongly controls the overall reaction rate in the proposed sequence.
A useful comparison is a process with several stations arranged in a line. The whole process cannot move faster than the station that handles items most slowly. In a reaction sequence, the slow step similarly holds back the formation of the final product. The other steps still occur; they do not make the overall process faster than the limiting step.
This comparison applies to the steps in a proposed mechanism. The overall reaction equation alone does not tell you which steps occur or which is slow. A mechanism must be supported by evidence; do not treat any convenient sequence as proven just because its steps add to the observed net equation.
Rate language also needs care. Reaction rate describes change in concentration over time. It is not the same as the amount of substance present. A large concentration does not, by itself, tell you the overall rate or identify a slow step. Here, the key connection is qualitative: when a sequence has a much slower step, that step limits the overall progress.
  • The much slower step in a sequence limits the overall progress.
  • The rate-determining step is the step that most strongly controls the overall rate.
  • An overall equation by itself does not identify a mechanism or its slow step.

4. A reliable way to reason about a mechanism

Read a proposed sequence in order. Identify which substance is formed in one step and used in a later step. That substance is an intermediate if it cancels from the net equation. Then identify any step explicitly described as much slower than the others. Relate that step to the overall rate without claiming that the other steps stop.
Keep the level of the claim matched to the information given. If a question says one step is much slower, you can say that it limits the overall rate. If the relative speeds are not given, do not guess which step is slow. Likewise, do not infer a detailed mechanism solely from the overall equation.
The symbolic steps in the worked example use placeholders to focus on the reasoning. They are not a claim about a particular real chemical reaction, and they do not require a rate calculation. This is appropriate when the information supplied is the sequence and the relative speed of its steps.
  • Cancel intermediates to determine the net reaction.
  • Use the stated relative step speeds to identify the limiting step.
  • Do not claim more about a mechanism than the information supports.

Worked example

Finding the net reaction and its rate-controlling step

A proposed reaction sequence has two steps: X→Y\mathrm{X \rightarrow Y} is much slower than Y+Z→W\mathrm{Y + Z \rightarrow W}. Find the overall reaction, identify the intermediate, and relate the slow step to the overall rate.
  1. Add the steps
    Place the reactants from both steps on the left and the products from both steps on the right. The repeated Y appears on both sides because the first step produces it and the second step uses it.
    X+Y+Z→Y+W\mathrm{X + Y + Z \rightarrow Y + W}
  2. Cancel the intermediate
    Cancel one Y from each side. Y is the intermediate: it participates in the sequence but is absent from the overall reaction.
    X+Z→W\mathrm{X + Z \rightarrow W}
  3. Relate the slow step to the rate
    The first step is stated to be much slower. It limits how quickly the sequence can proceed, so it controls the overall rate in this proposed mechanism. The second step can consume Y, but it cannot make final product form faster than the slow first step supplies material to the sequence.
Answer: The overall reaction is X+Z→W\mathrm{X + Z \rightarrow W}. Y is the intermediate. The slow step, X→Y\mathrm{X \rightarrow Y}, limits the overall rate.
Check: Y cancels because it is produced and then consumed. X and Z are used overall, while W is formed overall.

Common mistakes and how to avoid them

Including an intermediate in the overall reaction.
Correction: Add the steps and cancel a species that is produced in one step and consumed in another.
Assuming the overall reaction equation is one elementary event.
Correction: The overall equation gives the net change. A mechanism may contain several elementary steps.
Calling a step rate-determining without information about its speed.
Correction: Identify the rate-controlling step only when the relative step speeds or other suitable evidence are provided.
Saying the other steps do not occur because one step is slow.
Correction: The steps still occur in the sequence. The much slower step limits how quickly the whole sequence proceeds.

Lesson summary

  • An elementary step represents one particle-level event in a proposed mechanism.
  • Add the steps and cancel substances that are both formed and consumed to obtain the overall reaction.
  • A substance that cancels in this way is an intermediate.
  • When one step is much slower than the others, it limits the overall reaction rate.
  • The net equation alone does not reveal the steps or establish which step is slow.

Check your understanding

Question 1

For the steps P→Q\mathrm{P \rightarrow Q} and Q+R→S\mathrm{Q + R \rightarrow S}, which substance is the intermediate?
  1. P
  2. Q
  3. R
  4. S
Show answer and explanation
Q
Q is formed in the first step and used in the second, so it cancels from the net equation.

Question 2

A sequence has one step that is much slower than all the others. What does this tell you about the overall rate?
  1. The slow step limits the overall progress.
  2. The fastest step determines the overall rate.
  3. The overall reaction must stop permanently.
  4. The overall equation must contain the intermediate.
Show answer and explanation
The slow step limits the overall progress.
The sequence cannot proceed overall faster than its much slower step.

Question 3

What can you conclude from an overall reaction equation alone?
  1. It identifies every elementary step.
  2. It proves which step is slowest.
  3. It gives the net change, but not necessarily the mechanism.
  4. It shows that no intermediate is involved.
Show answer and explanation
It gives the net change, but not necessarily the mechanism.
An overall equation summarizes the net change. More information is needed to establish a mechanism or its slow step.

Key terms

Elementary reaction step
One individual particle-level event in a reaction mechanism.
Reaction mechanism
A proposed sequence of elementary steps that describes how a reaction occurs.
Intermediate
A substance formed in one step and consumed in a later step, so it cancels from the overall equation.
Rate-determining step
The step that most strongly controls the overall reaction rate, often because it is much slower than the other steps.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation D3.7. It is a study resource, not an official curriculum publication.

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