What is First Order Reaction?

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Grade Level:

Class 12

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Definition

What is it?

A first order reaction is a chemical reaction where the rate of the reaction depends only on the concentration of one reactant. This means if you double the amount of that one reactant, the reaction rate will also double.

Simple Example

Quick Example

Imagine you are making a cup of chai, and the speed at which the sugar dissolves depends only on how much sugar you put in. If you put in twice the sugar, it takes twice as long (or dissolves twice as fast, depending on how you measure 'speed'). The amount of milk or tea leaves doesn't affect how fast the sugar dissolves. This is like a first order reaction.

Worked Example

Step-by-Step

Let's say a reaction A -> Products is first order with respect to A. The rate constant (k) is 0.05 per second. If the initial concentration of A is 0.2 M, what is the initial rate of reaction?

1. Identify the given values: Rate constant (k) = 0.05 s^-1, Initial concentration of A ([A]0) = 0.2 M.
---2. Recall the rate law for a first order reaction: Rate = k * [A].
---3. Substitute the given values into the rate law: Rate = 0.05 s^-1 * 0.2 M.
---4. Calculate the product: Rate = 0.01 M s^-1.
---5. The initial rate of reaction is 0.01 M per second.

Why It Matters

Understanding first order reactions is crucial in fields like medicine for drug dosage, as our body metabolizes many medicines following this rule. In environmental science, it helps predict how quickly pollutants break down. Engineers use it to design chemical processes efficiently, impacting careers in biotechnology and material science.

Common Mistakes

MISTAKE: Thinking the rate depends on all reactants | CORRECTION: For a first order reaction, the rate depends ONLY on the concentration of ONE reactant, raised to the power of one.

MISTAKE: Confusing the order of reaction with the stoichiometry (coefficients) in the balanced equation | CORRECTION: The order of reaction must be determined experimentally, it is not necessarily the same as the coefficient in the balanced chemical equation.

MISTAKE: Incorrectly using units for the rate constant (k) | CORRECTION: For a first order reaction, the unit of k is typically 'per time' (e.g., s^-1, min^-1), not M s^-1.

Practice Questions

Try It Yourself

QUESTION: If a first order reaction has a rate constant of 0.1 s^-1 and the concentration of the reactant is 0.5 M, what is the rate of the reaction? | ANSWER: Rate = k * [A] = 0.1 s^-1 * 0.5 M = 0.05 M s^-1

QUESTION: For a first order reaction, if the initial concentration of reactant X is 0.4 M and the rate of reaction is 0.08 M min^-1, what is the rate constant (k)? | ANSWER: k = Rate / [X] = 0.08 M min^-1 / 0.4 M = 0.2 min^-1

QUESTION: A first order reaction takes 20 minutes for 50% completion. What is its rate constant (k)? (Hint: Use the half-life formula for first order reactions) | ANSWER: For a first order reaction, t(1/2) = 0.693 / k. So, k = 0.693 / t(1/2) = 0.693 / 20 min = 0.03465 min^-1

MCQ

Quick Quiz

Which of the following statements is true for a first order reaction?

The rate is independent of reactant concentration.

The rate is directly proportional to the concentration of one reactant.

The rate is proportional to the square of the reactant concentration.

The unit of the rate constant is M s^-1.

The Correct Answer Is:

For a first order reaction, the rate is directly proportional to the concentration of one reactant raised to the power of one. Options A, C, and D describe other reaction orders or incorrect units.

Real World Connection

In the Real World

Many everyday processes follow first order kinetics. For example, the decay of radioactive isotopes used in medical imaging (like in PET scans) or in carbon dating of ancient artifacts follows first order kinetics. Also, how quickly medicines like paracetamol are removed from your body by your liver often follows a first order reaction.

Key Vocabulary

Key Terms

RATE OF REACTION: How fast reactants are used up or products are formed | RATE CONSTANT (k): A proportionality constant relating the rate of reaction to the concentrations of reactants | CONCENTRATION: The amount of a substance in a given volume | HALF-LIFE (t1/2): The time required for the concentration of a reactant to reduce to half its initial value

What's Next

What to Learn Next

Next, you should explore 'Second Order Reactions' and 'Zero Order Reactions'. Understanding these will help you compare different reaction types and fully grasp how reaction rates are influenced by reactant concentrations, which is super important for advanced chemistry.