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What are the units of the rate constant for a third order reaction? A mol dm⁻³ s⁻¹ B mol⁻¹ dm³ s⁻¹ C mol² dm⁻³ s⁻¹ D mol⁻² dm³ s⁻¹ - AQA - A-Level Chemistry - Question 33 - 2020 - Paper 3
Question 33
What are the units of the rate constant for a third order reaction? A mol dm⁻³ s⁻¹ B mol⁻¹ dm³ s⁻¹ C mol² dm⁻³ s⁻¹ D mol⁻² dm³ s⁻¹
Worked Solution & Example Answer:What are the units of the rate constant for a third order reaction? A mol dm⁻³ s⁻¹ B mol⁻¹ dm³ s⁻¹ C mol² dm⁻³ s⁻¹ D mol⁻² dm³ s⁻¹ - AQA - A-Level Chemistry - Question 33 - 2020 - Paper 3
Step 1
Units of the rate constant for a third order reaction
Answer
For a third order reaction, the general rate equation can be expressed as:
Where:
- Rate has units of concentration per unit time, which is mol dm⁻³ s⁻¹.
Given that the concentration of reactant A is raised to the power of 3, the units of k (the rate constant) need to compensate for this.
Thus, the units for the rate constant k can be derived as follows:
However, to express k in a more usable form we simplify the expression:
This means the correct answer is:
D mol⁻² dm³ s⁻¹