Kinetics, Equilibria and Redox

Kinetics, Equilibria and Redox

1. Rates of Reaction

   • Maxwell-Boltzmann
     • Speed of particles in a gas vary
       • Shown on distrubution graph
     • Area under curve is equal to total number of molecules / particles
       • The peak of curve is the most likely energy of a particle
         • Average energy is right of the peak
           • No particles have zero energy
   • Collision Theory
     • To react, particles collide
       • Particles only react when they collide if:
         • (1) Collide in the correct direction
           • (2) Collide with a certain amount of kinetic energy
     • Activation Energy (Ea)
       • Minumum kinetic energy needed for reaction to occur
         • Energy needed to break bonds
   • Factors Effecting Rate
     • Temperature
       • Increases number of molecules with have activation energy
         • Shifts curve to the right with a larger area having activation energy
         • Also increases the frequency of successful collisions in the reaction
     • Concentration
       • Increases frequency of more successful collision, increases rate
     • Pressure
       • Does the same as concentration only in gases
     • Catalysts
       • Increases rate by providing an alternative route for the reaction that has a lower activation energy
         • Catalysts are chemically unchanged
       • Increases the area of curve with correct activation energy without moving the curve itself
   • Measuring Rate
     • Rate is the change in conc. of product or reactant over time
     • Timing how long it takes for precipitate to form
       • Subjective - relies on a persons judgement
     • Measuring the decrease in mass
       • Easy and accurate
     • Measuring the volume of gas given off
       • Very accurate

2. Equilibrium Constant

   • Kc is the Equilibrium Constant
     • • Concs of A,B,C,D given in mol/dm^3
         • Whatever value for a,b,c,d units to the power of these values
   • Calculations
     • Calculating Equili concs
       • conc = mole / vol
         • Use kc to work conc
           • Use Kc to work out conc in mixture
             • Enter values into the Kc
               • Check molar ratio, 1:1 concs equal
                 • Rearrange Kc equation
   • Temperature changes Kc
     • Kc only valid for one temperature
       • If a temperature rise increases product
         • Kc will rise
           • Catalysts don't affect Kc

3. Redox

   • Oxidation States (OS)
     • Total number of electrons donated or accepted by a element
       • Uncombined elements and diatomic have OS of 0
         • OS of ion or compound ion is the same as its charge
           • Sum of OS in a neutral compounds is 0
             • Oxygen is nearly always -2
               • Hydrogen is nearly always +1
   • Electrons transfered is a redox reaction
     • OILRIG (of electrons)
       • Oxidising agent accepts electrons itself is reduced
         • Reducing agent donates electrons itself is oxidised
   • Half Equations
     • Combine half equations to form redox equations
       • Balance equations by CHO and equal electrons

4. Reversible Reactions

   • Resersible reactions can reach dynamic equilibrium
     • This is when the reaction is at equilibrium
       • Where forward and backwards reactions are at the same rate
         • And the concentrations are equal and constant
     • Only in closed systems
       • (nothing added, nothing lost)
   • Le Chatelier's Principle
     • A reaction at equilibrium will shift to oppose any change
       • Concentration
         • E.g. Increase conc. of reactant, shifts to make more product
       • Pressure
         • Only involving gases
           • Increase pressure, shifts to the side with the least moles
             • to reduce pressure
       • Temperature
         • Increase temp, shift to endothermic side (+ delta H)
       • Catalysts
         • No effect on position of equilibrium, doesnt increase yield either
           • Faster to reach equilibrium
       • Reactions in industry
         • Balancing high rates with high reaction costs
           • High pressure increases rate
             • Increasing the pressure too much is expensive to run / maintain
           • High temperatures increase rate
             • Low temperatures may be needed to oppose exothermic reactions for high yield
               • But need a good rate to make product in a good time
                 • Compromise is required

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