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Reaction Kinetics - Coggle Diagram

- - - - T increases, k increases
      - Ea decreases, k increases
- - - - Catalyst and reactants are in different physical states
      - Reactants are adsorbed and products are desorbed
      - The availability of 3d and 4s electrons allows the ready exchange of electrons between the transition metal catalyst and the reactants molecules to form weak bonds. This adsorption weakens the bonds in reactant molecules, thereby lowering the activation energy, as well as increasing the surface concentration reactants. Once the products are formed, they are desorbed from the surface of the catalyst.
      - Eg. Finely divided iron powder in Haber process
      - Eg. Catalytic removal of oxides of nitrogen in exhaust gases from car engine
        
        In many countries, pollution from motorcars is being reduced by fitting a catalytic converter onto motorcar exhaust system. The exhaust gases react together on the surface of a platinum, or rhodium catalyst, and are converted into harmless nitrogen, carbon dioxide and steam
    - - Autocatalysis
        
        When a product of a reaction acts as a catalyst in the reaction
        
        The reaction starts slowly as there is no catalyst
        
        As reaction nears completion, rate decreases as reactants are gradually used up
      - Biological catalysis (enzymes)
        
        Catalyze specific chemical reactions
        
        Can be homogeneous or heterogenous but they are neither as they are colloidal in nature
        
        Highly selective and have specific activity
        
        They tend to only catalyze a specific reaction or type of reaction
        
        Sensitive to pH changes and poisons
        
        Enzymatic action
        
        Provides an alternate pathway with lower Ea
        
        Enzyme forms a complex with the substrate or substrates of the reaction
        
        At low [substrate], the active sites are not fully filled, and the rate is directly proportional to [substrate]. The reaction is first-order with respect to [substrate]
        
        At high [substrate], the active sites are fully filled, and any increase in substrate concentration will not have any effect on the reaction rate. The reaction is zero-order with respect to [substrate]
      - Same physical state
      - They take part in the reaction by being converted into an intermediate species which is subsequently consumed to form the products
      - Eg. Role of Fe2+ in the redox reaction between peroxydisulfate ions, (S2O8)2- and iodide ions, I-
        
        The uncatalyzed reaction is slow because it involves the reaction between 2 negatively charged ions. The repulsion between the 2 negatively charged ions causes the reaction to have high activation energy
        
        Step 1: 2Fe2+ (aq) + (S2O8)2- (aq) --> 2Fe3+ (aq) + 2(SO4)2- (aq)
        Step 2: 2Fe3+ (aq) + 2I- (aq) --> 2Fe2+ (aq) + I2 (aq)
        Overall: 2I- (aq) + (S2O8)2- (aq) --> I2 (aq) + 2(SO4)2- (aq)
        
        In the presence of Fe2+, the reaction proceeds via a two-step mechanism, which involves the approach of oppositely charged species. This lowers the activation energy and enhances the reaction rate
      - Eg. Catalytic role of atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide
        
        The overall reaction for the oxidation of sulfur dioxide is SO2 (g) + 1/2 O2 (g) --> SO3 (g)
        High Ea
        
        When NO acts as the catalyst
        Step 1: NO (g) + 1/2 O2 (g) --> NO2 (g)
        Step 2: NO2 (g) + SO2 (g) --> SO3 (g) + NO (g)
        
        When NO2 (g) acts as the catalyst
        Step 1: NO2 (g) + SO2 (g) --> SO3 (g) + NO (g)
        Step 2: NO (g) + 1/2 O2 (g) --> NO2 (g)
- - - - Independent of the initial concentration of the reactant
      - Half life = ln2 / k
      - (Conc at time = t / Conc at time = 0) = (1/2)^n, where n is the number of half-lives elapsed
    - - If the initial concentration of one reactant is very large, the change in concentration of the reactant during the course of the reaction is considered to be negligible, hence its concentration can be taken to be essentially constant during the reaction
      - Cases
        
        One of the reactants is in large excess
        
        The reactant is also the solvent