Rate of Reactions

1. Definition of Rate of Reaction:

The rate of a chemical reaction is a measure of how fast a reactant is used up or how fast a product is formed in a given time. Rate of Reaction = Change in amount of reactant or product / Time taken

It is usually measured in:

• g/s (grams per second) for solids,
• cm³/s for gases,
• mol/s for moles.

2. Factors Affecting the Rate of Reaction:

Several factors affect how quickly a reaction takes place. These include:

i. Concentration (for solutions):

• Increasing the concentration of reactants increases the number of particles per unit volume.
• This leads to more frequent collisions between reacting particles.
• Hence, the rate of reaction increases.

ii. Temperature:

• Increasing temperature gives particles more kinetic energy.
• Particles move faster and collide more often and with more energy.
• This increases the chance of successful collisions.
• Therefore, the rate of reaction increases with temperature.

iii. Pressure (for gases):

• Increasing pressure compresses gas particles into a smaller volume.
• This increases the number of collisions per second.
• So, higher pressure = faster rate of reaction (for gaseous reactants).

iv. Surface Area (for solids):

• When a solid is broken into smaller pieces or powdered, its surface area increases.
• More particles are exposed for collisions.
• This increases the rate of reaction.

Example: Powdered calcium carbonate reacts faster with HCl than a single large piece.

v. Catalyst:

• A catalyst is a substance that speeds up a reaction without being chemically changed or used up.
• It provides an alternative pathway with lower activation energy.
• This increases the number of successful collisions.

Example: Manganese dioxide (MnO₂) acts as a catalyst in the decomposition of hydrogen peroxide.

3. Experiment: Rate of Reaction Between Metal and Acid

Reaction:

Mg (s)+2HCl(aq)→MgCl₂(aq)+H₂(g)

Procedure:

• Add a strip of magnesium ribbon into dilute hydrochloric acid in a conical flask.
• Use a gas syringe or upturned measuring cylinder to collect the hydrogen gas produced.
• Measure the volume of gas produced at regular intervals (e.g., every 10 seconds).

Observations:

• The volume of hydrogen gas increases over time.
• The rate of reaction is fastest at the beginning and slows down as the acid is used up.

Rate Calculation:

Use the formula: Rate=Volume of H₂ gas collected/Time taken

4. Collision Theory:

• According to collision theory, a reaction occurs when reacting particles collide with enough energy.
• This energy must be equal to or greater than the activation energy.
• The activation energy is the minimum energy needed for a reaction to occur.

Key Points of Collision Theory:

• More frequent collisions = faster rate.
• Higher energy collisions = more successful reactions.
• Higher temperature increases both the frequency and the energy of collisions.
• Catalysts lower the activation energy, so more collisions result in a reaction.

Summary Table:

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