Rates and Equilibrium

• The steeper the slope in a graph, the faster the rate of reaction.
• Rate of reaction decreases as we have a lot of reactant molecules reacting and turning into products
• Mean rate of reaction


  Amount of product formed/reactant used DIVIDED by time
• Gradient


  Change in y/change in x
• Collision theory


  Chemical reactions that only take place when reacting particles collide with each other, collision must have sufficient energy
• What is the rate of reaction determined by?

  The frequency of the collisions/the number of successful collisions per second
• Why are reactions rapid at the start?

  High concentration/large number of reactant molecules and collisions per second
• Why does the reaction slow down?

  The number of reactant molecules decreases/smaller number of collisions per second
• What happens at the end of a reaction

  All reactant molecules have been used up/zero collisions
• We have two containers both the same size, A has more molecules than B, what happens?

  Higher concentration, more frequent collisions - faster rate of reaction
• What happens if the concentration is high?

  Faster rate of reaction - line in graph is steeper
• What happens when we increase the S.A. of a solid reactant?

  Rate increases due to a lot of collisions per second
• Smaller-sized blocks/particles or solid reactants have a greater S.A. to volume ratio than larger sized particles due to them having more particles on the surface - more collisions per second - increases rate of reaction
• Size of the particle shows us the energy of each particle
• If two particles with a small arrow/low energy collide, they won't overcome the activation energy barrier - won't collide successfully - no reaction
• If 2 particles with a big arrow/high energy collide, they will overcome the activation energy barrier - will collide successfully - reaction occurs
• If you increase the temperature of the reaction, the rate of reaction increases and the energy of particles
• More energy in particles - it moves faster - increases frequency of collisions - faster rate of reaction
• Activation Energy

  minimum amount of energy required in order for particles to react
• Catalysts


  increase the rate of reactions but ARE NOT USED UP during the reaction
• Catalysts allow us to carry out reactions quickly without needing to increase the temperature - saves money - can re-use it again
• What does rate of reaction depend on?

  Number of particles that have enough energy to cross the activation energy barrier and collide successfully
• CATALYSTS INCREASE THE RATE OF REACTION BY CREATING A DIFFERENT PATHWAY FOR THE REACTION THAT HAS A LOWER ACTIVATION ENERGY
• What happens when a catalyst is present

  Particles require less energy to cross barrier - more particles collide successfully - faster rate of reaction
• CATALYST IS NOT A REACTANT
• Reversible reactions


  Happens either way around. The double arrow means that the reaction is reversible. Can change direction by changing conditions.
• Hydrated copper sulfate (blue)

  if heated, reacts to form anhydrous copper sulfate which is white, and also produces water. Blue to -> white
• ammonium chloride ⇌ ammonia + chlorine


  If it occurred in a sealed container, no escape of reactants or products.
• Equilibrium

  Where the forward and backward reactions take place at the same rate
• Le Chatelier's Principle


  If a system/reaction is at equilibrium and a change is made to the conditions, then the system responds to counteract the change - equilibrium shifts sides
• 2NO2 ⇌ N2O4

  If we increase concentration of NO2, then reactions not at equilibrium - forward and backward reactions aren't taking place at the same rate - more N2O4'S formed until equilibrium's reached
• 2NO2 ⇌ N2O4

  If we decrease concentration of N2O4, reaction's no longer at equilibrium so more NO2 will react to form N2O4 until equilibrium's reached
• In reversible reactions, when endothermic's in the reverse reaction, energy is taken IN so temperature DECREASES
• In reversible reactions, when exothermic's in the forward reaction, energy is RELEASED so temperature INCREASES
• 2NO2 ⇌ N2O4 (forward is exo. and reverse is endo.)


  If we increase the temperature, equilibrium shifts to left side to reduce the temperature - reverse is endothermic - energy's taken IN and temperature DECREASES - amount of NO2 decreases, N2O4 increases
• Pressure affects reactions involving gases, they depend on the number of molecules
• If a reaction is at equilibrium and pressure incr. and decr., position of equilibrium shifts to the side with the smaller number of molecules always!
• If the number of molecules on both sides are same, changing pressure had no effect on position of equilibrium