Rate & Extent of Chemical change

• Rate of chemical reactions tells us how quickly reactants are converted into products.
• Collision Theory explains how reactions happen and why they happen at different rates.
• Particle collisions

  1. Particles must collide for chemical reactions to happen
  2. These collisions must happen with enough energy
  3. Activation energy is the minimum amount of energy
• Rate of reaction can be increased by...

  Increasing the frequency (number) of collisions

  Or

  The energy of reactant particles.
• We can increase the rate of a reaction...

  • Increase the frequency of collisions
  • Increase the energy of reacting particles.
• Key factors that affect the rate of chemical reactions

  1. Concentration of dissolved reactants
  2. Pressure of gas reactants
  3. Temperature
  4. Surface area of solid reactants
  5. Catalysts
• Catalysts...

  Increase the rate of chemical reactants by lowering the activation energy.
• Pros and info on Catalysts

  • Have high surface area
  • Not used up during chemical reactions- can be reused
  • Often come as powders, pellets and fine gauzes.
• Rate of reaction = change in y ÷ change in x
• According to collision theory, how can we increase the rate of a reaction?

  Increase the frequency of collisions

  Increase the energy of reacting particles
• To calculate rates of reactions using graphs, we must draw a tangent to the curve and work out its gradient.
• Increasing the concentration or pressure increases the frequency of collisions, which means that the reaction rate is faster.
• Increasing the temperature of a reaction causes the frequency of collisions and the energy of reactant particles to increase, meaning that a higher percentage of the particles will have energy in excess of the activation energy.
• Some chemical reactions can proceed in both forwards and backwards directions. We describe these reactions as reversible.
• In the reversible reactions the forwards reactions is exothermic and the backwards reaction is endothermic.
• When reversible reactions happen within a closed system, a dynamic equilibrium will eventually be reached.
• A closed system is a system where reactants and products cannot be added or removed.
• If the reaction is at dynamic equilibrium, it means that the rates of the forwards and backwards reactions are equal.
• At equilibrium, both forward and backward reactions still happen, but the concentrations of reactants and products remain constant. So, we describe the equilibrium as dynamic.
• Le Chatelier's principle is used to predict the outcome of any changes imposed on a closed system at equilibrium.
• If temperature is decreased, the position of equilibrium will shift in the exothermic direction.
• If temperature is increased, the position of equilibrium will shift in the endothermic direction.
• Pressure changes only affect reactions that involve gases.
• Changing the concentration of any reactant or product takes the system out of equilibrium. The system will react to try to restore the equilibrium.
• • Increase pressure
  • Position of equilibrium shifts in direction that yields fewest gas molecules
• • Decrease pressure
  • Position of equilibrium shifts in direction that yields most gas molecules
• Required Practical

  Hydrochloric acid+ sodium thiosulphate causes solution to go cloudy, making the cross at the bottom of the conical flask disappear. We have to measure the time for the cross to disappear and repeat this at different temperatures.