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Factors that affect reaction rate  Concentration of REACTANTS  Temperature (kinetic energy of molecules)  Presence of a catalyst  Physical state of.

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Presentation on theme: "Factors that affect reaction rate  Concentration of REACTANTS  Temperature (kinetic energy of molecules)  Presence of a catalyst  Physical state of."— Presentation transcript:

1 Factors that affect reaction rate  Concentration of REACTANTS  Temperature (kinetic energy of molecules)  Presence of a catalyst  Physical state of reactants (surface area)

2 Higher concentration = more collisions

3 Ways over (or around) the barrier  Temperature increases reaction rate by increasing the fraction of molecules which have sufficient energy to jump the barrier  A catalyst is a way to remove, or at least lower the barrier. A catalyst acts to increase the chemical reaction, but is not consumed itself during the reaction

4 Catalysts modify the pathway  Addition of a chlorine catalyst increases the rate of decomposition of ozone into O 2 – the reason for the destructive influence of these compounds in the atmosphere  Although two barriers are present, both are smaller than the one without the catalyst, and the reaction proceeds more rapidly

5 Without catalysts, there would be no life at all, from microbes to humans  ENZYMES are biological catalysts  Most enzymes are proteins – large molecules  Have correct shape to bring reactant molecules together in correct orientation

6 Natural selection produces prodigious catalysts  Green lines are the amino acid backbone of the enzyme  The white molecule is the fatty acid  Red are iron atoms

7 Catalysts can be solids, liquids or gases

8 The catalyst shape can direct certain products: zeolite molecular sieve Reactant molecules Product molecules

9 Mathematical treatment of reaction rate  In the reaction 2N 2 O 5 = 4NO 2 + O 2 We describe the rate according to the form Rate = k[N 2 O 5 ] n  k is the rate constant  n is the order of reaction with respect to N 2 O 5  NB: n is not the same as the coefficient in the equation!  Rate is normally measured by monitoring growth in concentration of a product, or decline in concentration of a reactant

10 More than one reactant  A + B = products Rate = k[A] m [B] n m and n are individual orders for the reactants A and B. The overall order of reaction = m + n

11 Strategies for determining orders of reaction  Method of initial rates  At t = 0, product concentration is zero  Vary concentration of one reactant, keeping the other(s) constant  Reaction is second order in NO and first order in O 2

12 Effect of temperature: activation energy  As temperature increases rate increases  Arrhenius expression: E A is activation energy  Plot of ln k vs 1/T is constant (T in Kelvin)

13 Activation energy using two temperatures  Measure ratio of rates at two temperatures (Rate is proportional to k)


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