Chemical Reaction Kinetics and Theories Explained

Collision Theory

For a reaction to occur, the molecules of the reacting substances must collide. Any factor that increases the collision frequency of such collisions, or increases the probability that these collisions occur with appropriate orientation and possess sufficient energy, will increase the reaction speed.

Reaction Rate ($ ext{V}_{ ext{reaccion}}$): Number of collisions $\times$ time / Molecules of reagents.

Effective Collision

An effective collision requires:

• Appropriate orientation.
• Sufficient energy.

Activation Energy

The Activation Energy ($ ext{E}_{ ext{A}}$) corresponds to the maximum energy reached in the intermediate state of the system, known as the activated complex state. It is the energy needed to move from the reactant state to the transition state.

Reactants must overcome this activation energy barrier to become products, even if the reaction is exothermic.

Factors Influencing Reaction Speed

Reaction speed is the rate at which reactants are transformed into products.

• Nature of the reactants.
• Reagent concentration: Increased concentration leads to increased speed.
• Contact surface between the reactants: Increased surface area leads to increased reaction rate.
• System Temperature ($ ext{T}$): Increased temperature leads to increased speed.
• Presence of catalysts: Catalysts speed up the reaction without being consumed.

Types of Chemical Reactions

1. Synthesis (Combination): $\text{H}_2 + \text{O}_2 \Rightarrow \text{H}_2\text{O}$ (Reversible: Decomposition)
2. Replacement (Single Displacement): $\text{Zn} + 2\text{HCl} \Rightarrow \text{ZnCl}_2 + \text{H}_2$
3. Double Displacement: $\text{Ca}(\text{OH})_2 + 2\text{HCl} \Rightarrow \text{CaCl}_2 + 2\text{H}_2\text{O}$
4. Neutralization: Reaction where an acid reacts with a base to yield salt and water.

Properties of Acids

• Electrically conductive.
• Sour taste.
• Change the color of indicators: litmus reddens and phenolphthalein becomes colorless.
• React with some metals.
• Decompose carbonates with the evolution of $\text{CO}_2$.
• React with bases to form salts.

Base Properties

• Conductors of electricity.
• Oily to the touch and bitter taste.
• Color changes of indicators: litmus paper turns blue and phenolphthalein turns red.
• Metal oxides, hydroxides, and carbonates behave as bases in an aqueous solution.
• React with acids to form salts.

Arrhenius Theory

Acid (Arrhenius)

A substance that can dissociate in an aqueous solution, yielding $\text{H}^+$ ions.

• Strong acids: $\text{H}_2\text{SO}_4$, $\text{HNO}_3$, $\text{HClO}_3$
• Weak acids: $\text{H}_2\text{S}$, $\text{CH}_3\text{COOH}$

Strong acids dissociate completely; weak acids dissociate partially.

Base (Arrhenius)

A substance containing an $\text{OH}$ group capable of dissociating in an aqueous solution, yielding $\text{OH}^-$ ions.

• Strong bases: $\text{NaOH}$, $\text{KOH}$, $\text{Ca}(\text{OH})_2$
• Weak bases: $\text{Fe}(\text{OH})_3$

Bronsted and Lowry Theory

ACID: Anything that gives up a proton ($\text{H}^+$).

BASE: Anything capable of accepting a proton ($\text{H}^+$).