Collision Theory and Factors Affecting Reaction Rates

This page explores collision theory and how various factors influence the rate of chemical reactions.

Collision theory states that chemical reactions occur when particles collide with sufficient energy. The minimum energy required for a reaction is called the activation energy.

Factors that increase reaction rates:

1. Higher temperature - more frequent and energetic collisions
2. Greater concentration/pressure - more particles available to collide
3. Larger surface area - more exposed surface for collisions
4. Catalysts - provide an alternate reaction pathway with lower activation energy

Definition: Activation energy is the minimum amount of energy that particles must have to react when they collide.

Highlight: Catalysts increase reaction rates by lowering the activation energy, not by changing the overall energy change of the reaction.

Example: Increasing the concentration of sodium thiosulfate in the cross disappearance experiment will make the reaction happen faster, as there are more particles available for collision.

Reversible Reactions and Dynamic Equilibrium

This section covers reversible reactions and the concept of chemical equilibrium, crucial for understanding rates of reaction and equilibrium GCSE revision.

Key points:

• In reversible reactions, products can react to reform the original reactants
• Equilibrium is reached in a closed system when forward and reverse reaction rates are equal
• Le Chatelier's Principle describes how systems at equilibrium respond to changes

Effects on equilibrium:

• Increasing temperature shifts equilibrium towards the endothermic reaction
• Increasing reactant concentration shifts equilibrium towards products
• Increasing pressure shifts equilibrium towards fewer moles of gas

Definition: Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction in a closed system.

Highlight: At equilibrium, the concentrations of reactants and products remain constant, but reactions continue to occur in both directions.

Example: In the Haber process for ammonia production, increasing pressure shifts the equilibrium towards the side with fewer gas molecules (the product side), increasing ammonia yield.

Alkanes and Combustion

This section covers alkanes and their combustion reactions, important for Quantitative Chemistry AQA GCSE questions.

Alkanes:

• Saturated hydrocarbons with single bonds between carbon atoms
• General formula: CnH2n+2
• First four members: methane, ethane, propane, butane (MEPB)

Combustion reactions:

• Complete combustion produces carbon dioxide and water
• Incomplete combustion produces carbon monoxide (toxic) and water

Definition: Saturated hydrocarbons contain the maximum number of hydrogen atoms possible for each carbon atom.

Example: Methane (CH4) combustion: CH4 + 2O2 → CO2 + 2H2O

Highlight: Carbon dioxide produced during combustion can be detected using limewater, which turns cloudy.

Vocabulary: Covalent bonds are shared electron pairs that hold atoms together in molecules.