Reaction Rate Article Index for
Reaction 		Website Links For
Reaction 		 

Information About

Reaction Rate
  CATEGORIES ABOUT REACTION RATE
   
chemical kinetics
   
chemical engineering
   
SHOPPER'S DELIGHT
 
Shop For:  
Department
Price Minimum:
Price Maximum:
      




FACTORS INFLUENCING RATE OF REACTION


There are several factors that affect the rate of reaction:
  • '', whose result is factored into the equation by ''k''. As a rule of thumb, the reaction rate doubles for every 10 degrees Celsius increase in temperature.

  • '' of Collision increases and so therefore does the frequency of collisions having sufficient Energy to cause reaction.

  • '' Pressure '': The rate of gaseous reactions usually increases with an increase in pressure. Increase in pressure in fact is equivalent to an increase in concentration of the gas.

  • '' reacts with Chlorine in the dark, the reaction rate is very low. It can be sped up when the Mixture is put under diffused light. In bright sunlight, the reaction is explosive.

  • ''ally, and, for most basic reactions, is an Integer value.

  • ''A .

  • ''The nature of the reactants'': If a reaction involves the breaking and reforming of bonds (complex) compared to just the forming of bonds (simple) then it generally takes longer. The reactants position in the Reactivity Series also affects reaction rate.

  • ''Surface Area'': In Reactions On Surfaces , which take place during Heterogeneous Catalysis , the rate of reaction increases as the surface area does. The larger the surface area compared to the volume, the faster a reaction can take place, as more simultaneous reactions can occur.



RATE LAW

See Also: rate equation


For a Chemical Reaction ''n'' A + ''m'' B → C + D, the Rate Equation or '''rate law''' is a Mathematical Expression used in chemical kinetics to link the rate of a reaction to the concentration of each reactant and their various Orders .

: rac{d = k(T)[A ^{n'}[B]^{m'}

For a reaction between liquids, gases, or solutes, {Link without Title} stands the Concentration of X. For a reaction taking place at a boundary it would denote something like moles of X per area. When gases are involved the rate law can also be expressed in pressure units.

In this equation k(T) is the reaction rate coefficient or rate constant, although it is not really a constant, because it includes everything that affects reaction rate outside concentration: mainly temperature but also ionic strength or light irradiation.

The exponents n' and m' are called Orders and depend on the Reaction Mechanism .


TEMPERATURE DEPENDENCE

Each reaction rate coefficient k (i.e., k1 and k2) has a temperature dependency, which is usually given by the Arrhenius Equation :

: k = A e^{ - rac{E_a}{RT} }

''Ea'' is the Activation Energy and R is the Gas Constant . Since at Temperature ''T'' the molecules have energies given by a Boltzmann Distribution , one can expect the number of collisions with energy greater than ''Ea'' to be proportional to e^{ rac{-E_a}{RT}}. ''A'' is the Pre-exponential Factor or Frequency Factor .

The values for A and Ea are dependent on the reaction (so, for example, they may differ between k1 and k2). There are also more complex equations possible, which describe temperature dependence of other rate constants which do not follow this pattern.


EXAMPLE

For the reaction
: 2H_2 (g) + 2 NO(g) arr N_2 (g) + 2 H_2O (g)
The rate equation is:
: r = k {Link without Title} {Link without Title} ^2 \,
The reaction is first order in H2, as the hydrogen concentration is raised to the power of 1; it is second order in NO, according to the index.

As can be seen, the rate equation does not simply reflect the reactants - all the steps of the reaction must be looked at, this reaction has three:

# 2 NO \Leftrightarrow N_2O_2 (fast equilibrium)
#N_2O_2 + H_2 arr N_2O + H_2O (slow)
#N_2O + H_2 arr N_2 + H_2O (fast)

As reactions 1 and 3 are very rapid compared to the second, it is the slowest reaction that is reflected in the rate equation.


SEE ALSO