Rates of reactions

1. What is the Rate of a Reaction?

• The rate of a reaction measures how quickly reactants are converted into products.
• Defined as the change in concentration of reactants or products per unit time.

2. Formula for Reaction Rate

$r = \frac{-\Delta[\text{Reactants}]}{\Delta t} \quad \text{or} \quad r = \frac{\Delta[\text{Products}]}{\Delta t}$

Where:

✅ $r =$ reaction rate $(mol·dm⁻³·s⁻¹)$

✅$Δ[Reactants] =$ change in concentration of reactants $(mol·dm⁻³)$

✅ $Δ[Products] =$ change in concentration of products $(mol·dm⁻³)$

✅ $Δt =$ change in time $(s)$

3. Factors Affecting Reaction Rate

✅ Concentration: Higher concentration → More frequent collisions → Faster reaction.

✅ Temperature: Higher temperature → Particles have more energy → Faster reaction.

✅ Surface Area: More exposed surface → More collisions → Faster reaction.

✅ Catalysts: Lower activation energy → Reaction occurs faster.

✅ Pressure (for gases): Higher pressure → More particle collisions → Faster reaction.

4. Exothermic vs Endothermic Reactions

Reaction Type	ΔH Value	Energy Flow
Exothermic	Negative $(-ΔH)$	Releases energy (products have less energy than reactants).
Endothermic	Positive $(+ΔH)$	Absorbs energy (products have more energy than reactants).

🔹 Example:

$H_2 + I_2 \rightarrow 2HI \quad (\Delta H < 0, \text{Exothermic})$

• Energy of products is less than that of reactants.

5. Key Takeaways

✅ Reaction rate measures how quickly reactants turn into products.

✅ It depends on factors like concentration, temperature, surface area, and catalysts.

✅ Reaction rate is calculated using concentration changes over time.

✅ Exothermic reactions release energy $(-ΔH),$ while endothermic reactions absorb energy $(+ΔH).$