Chem- Chemical Reactions

• Collision Theory


  For a chemical reaction to occur, molecules must collide in the right orientation with enough energy.
• Activation energy is the energy needed by particles for a reaction to occur.
• The factors that affect the rate of a chemical reaction are concentration, temperature, surface area and catalyst.
• Rates of Reaction- Concentration

  Increasing the concentration increases the number of particles in a given volume. The more particles in a given volume, the more frequently collisions occur. The more frequent collisions the faster the rate of reaction.
• Rates of Reaction- Temperature

  Increasing the temperature increases the kinetic energy of the particles. Particles moving faster will collide more often and more frequent collisions will result in a faster rate of reaction. More molecules will also have sufficient energy to react, resulting in a faster rate of reaction.
• Rates of Reaction- Surface Area

  Only molecules at the surface of a solid can react. Increasing the surface area increases the number of molecules able to react.
• Rates of Reaction- Catalyst


  A catalyst will increase the rate of a reaction because the alternative pathway will have a lower activation energy. This means that more collisions will be successful as more molecules will collide with sufficient energy.
• Equilibrium reactions do not go to completion.
• At equilibrium, there is a mixture of reactants and products.
• A chemical equilibrium is a dynamic equilibrium.
• Dynamic Equilibrium

  • reactions are always occuring
  • The rate of the forward reaction is equal to the rate of the reverse reaction
• An equilibrium reaction is a reversible reaction, which is indicated in an equation by a double-headed arrow.
• When a system is at equilibrium there are no observable changes.
• The equilibrium constant is a measure of how far the reaction had gone towards the products. A large value of Kc means that the concentration of products is high (Kc>1). A small value of Kc means that the concentration of reactants is high (Kc<1).
• Equilibrium Systems

  • Closed
  • No observable change in colour, pressure, temperature and ratio of reactants to products
  • Both reactants and products are present together
  • The forward and reverse reactions are occurring at equal rates
• The value of Kc at any temperature can be calculated if the concentrations of the reactants and products are known.
• Le Chatelier's Principle


  Any change made to a system at equilibrium results in a shift of the equilibrium in the direction that minimises the change.
• Effect of Concentration on a System at Equilibrium

  Increasing the concentration causes the equilibrium to move to minimise the effect of the change until a new equilibrium is reached (with the same Kc value). Removing products will cause the equilibrium to move towards the right (to counter the change, more products are formed).
• Effect of Pressure on a System at Equilibrium

  At any given temperature and pressure one mole of gas occupies the same volume. If the pressure is increased, the equilibrium will move to reduce the pressure (Kc value is not changed).
• Effect of Temperature on a System at Equilibrium

  Depends on whether the reaction is endothermic or exothermic. For example, increasing the temperature of an exothermic reaction will force the reaction towards the reactants, and increasing the temperature of an endothermic reaction will force the reaction towards the products.
• A change in temperature affects the value of Kc
• A catalyst has no effect on the equilibrium position of a system at equilibrium. The rate of reaction increases so a system will reach equilibrium more quickly.
• Acids

  • taste sour
  • turn litmus paper or universal indicator red
  • neutralise bases
  • react with metals
  • react with carbonates
  • pH below 7
• Bases

  • taste bitter
  • feel soapy
  • turn litmus paper or universal indicator blue
  • neutralise acids
  • react with fats to form soap
  • pH above 7
• A solution is acidic if there are more hydrogen ions than hydroxide ions.
• Strong acids fully dissociate whereas weak acids only partly dissociate
• For strong acids, the concentration of the acid equals the concentration of hydronium ions. For weak acids, the concentration of the acid does not equal the concentration of hydronium ions.
• Hydrochloric acid, sulfuric acid and nitric acid are strong acids.
• Ethnic acid, methanoic acid and propanoic acids are weak acids.
• Sodium hydroxide and potassium hydroxide are strong bases
• Ammonia is a weak base
• An acid is a proton donor
• A base is a proton acceptor
• Species that can act as an acid or a base are known as amphiprotic.
• Solutions conduct electricity when they have ions present in solution. The ability of a solution to conduct increases as the concentration of ions increases.
• Strong acids and strong bases are good conductors of electricity because they dissociate fully.
• Weak acids and bases are poor conductors of electricity because they don't dissociate completely,
• Some salts can produce acidic or basic solutions. Acidic or basic salts react with water to form hydronium ions or hydroxide ions.