كيمياء 10 - تفاعلات النحاس والبنزين

Chemical Kinetics - General Concepts

• Chemical kinetics is the study of reaction rates and mechanisms.
• It differs from thermodynamics, which focuses on initial and final states, not the pathways or time taken.

Reaction Rates

• Reaction rate is the change in concentration of reactants or products over time.
• Rate of chemical reaction is denoted by R.
• Rate = Change in concentration / Change in time
• For reactants: R = -Δ[R]/Δt
• For products: R = Δ[P]/Δt

Factors Affecting Reaction Rate

• Nature of reactants: Reaction rates vary based on the physical state (solid, liquid, gas) and chemical structure (ions, molecules, free radicals) of the reactants.
• Temperature: Increasing temperature generally increases reaction rates. A 10°C rise often doubles or triples the rate.
• Concentration: Higher concentrations of reactants typically lead to faster reactions, though this is not always the case.
• Pressure and volume: In gas-phase reactions, pressure and volume can affect reaction rates, particularly concerning the concentration of reactants on a surface.
• Catalysts: Catalysts increase reaction rates without being consumed in the process. They operate by providing an alternate pathway with a lower activation energy.
• Surface area: In heterogeneous catalysis, a larger surface area for the catalyst generally leads to increased reaction rates. Solid catalysts demonstrate this effect.
• Solvent: Solvent effects are crucial in homogeneous reactions and liquid-phase reactions.
• Presence of inhibitors: Inhibitors decrease reaction rates.

Reaction Order and Molecularity

• Reaction order: Determines how reaction rate changes with reactant concentration. It's an experimentally determined value.
• **Molecularity:**The number of molecules or ions that collide to form activated complex (or transition state). It's typically an integer (1, 2, 3). It's often the same as the overall reaction order in elementary reactions, but not always in complex reactions.
• Determining Reaction Order: The reaction order is determined via experiments or by analysing the rate law.

First-Order Reactions

• In first-order reactions, the rate is dependent on the concentration of one reactant.
• Rate law: Rate = k [A]

Zero-Order Reactions

• In zero-order reactions, the reaction rate is independent of the reactant concentration.
• Rate law: Rate = k

Second-Order Reactions

• In second-order reactions, the rate is proportional to the square of the concentration of one reactant, or the product of the concentrations of two reactants.
• Rate law: -Rate = k [A]² (if it's a second-order reaction on a single reactant)
  • Rate = k [A][B] (if it's second order in two reactants)

Pseudo-First-Order and Pseudo-Zero Order Reactions

• Reactions that appear to be first-order or zero-order despite having higher orders due to one of the reactants being significantly more concentrated.

Arrhenius Equation

• The Arrhenius equation expresses the relationship between the rate constant (k) and temperature (T).
• It relates a reaction's rate constant to the activation energy of the reaction.

Catalysts

• A catalyst changes the reaction mechanism and lowers the activation energy.
• It increases the rate without being consumed.

Types of Catalysts:

• Homogeneous catalysts: Catalysts in the same phase as the reactants.
• Heterogeneous catalysts: Catalysts in a different phase than the reactants.

Reaction Mechanisms

• Reaction mechanisms describe the steps involved in a reaction. Describing the path a reaction follows, including any intermediary species.

Complex Reactions

• Complex reactions occur via multiple steps.
• Consecutive reactions: Reactants go through intermediate(s) to form products.
• Parallel reactions: Reactants can form different products simultaneously.

Theories of Reaction Rates

• Collision theory: Reaction rate depends on the frequency and energy of collisions between reactants.
• Transition state theory: Reaction rate depends on forming a transition state.