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Reaction Rates


Chemistry that deals with the speed of chemical reactions means how fast the chemical reaction occur and to what extent the reaction takes place is known as chemical kinetics. Chemical kinetics involves the measurement of the rates of chemical conversion. But the chemical reaction are normally embedded in a sequence of rate phenomena that precede and succeed the actual chemical transformation. 

In order to be certain that it is rate of chemical reaction that is being measured, one must ascertain that it is the chemical transformation that is the rate controlling step in the sequence of events associated with that reaction. In chemical reactions, the state is reached in which the concentrations of reactant and products remain constant. This is the point, the reaction is said to have reached equilibrium.

Activation Energy

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During chemical reactions, the kinetic energy of the molecules are converted into potential energy during collisions. The collisions distort and stretch the bonds; the bonds can be viewed as acting like tiny springs or rubber bands. If the energy of the collision is sufficient, bonds break, similar to the breaking of a rubber band if it is stretched too far. 

"In chemical reaction, the minimum energy needed to break bonds and initiate a reaction is known as the activation energy". No matter how many collisions occur between reactants, a reaction does not occur unless the energy of the collision is equal to the activation energy.

According to the concept of activation energy, all the molecules cannot take part in the chemical reaction. It is only a certain number of molecules that may be called active molecules which could take part in a chemical reaction. Thus the reactants do not pass directly to the products but must first acquire necessary energy to pass over an energy barrier known as the activated state or transition state.

Activation Energy and Catalysis

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In the early 1900's Haber and later Bosch discovered that the reaction did however, proceed at reasonable temperatures in the presence of osmium and subsequently iron based materials. These catalysts acted by lowering the activation energy of the reaction. In other words by interacting with the starting materials they altered the reaction pathway to one of lower energy. Catalysts do not, however, alter the equilibrium position of a reaction, which is under thermodynamic control, therefore high pressures are still neededto force the reaction, 

Catalyst are commonly defined as " A material which changes the rate of attainment of chemical equilibrium without itself being changes or consumed in the process". Catalysis chemical process in which a reaction rate is accelarated. 

Activation Energy and Catalysis

  • The positive catalyst increases the rate of reaction by lowering the activation energy.
  • Catalyst does not change the heat of reaction. It makes a reaction faster by enhancing the rate of both backward and forward reactions thereby early establishing the equilibrium.
  • Catalyst do not change the yield of products.
  • The negative catalyst retards the rate of chemical reaction by deactivating the reactant molecules.

Factors Affecting the Rate of Reaction

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The rate of reaction for a given chemical change is the speed with which the reactants disappear and the products form. It is measured by the amount of products produced or reactants consumed per unit time. Usually this is done by monitoring the concentration of the reactants or products over time as the reaction runs.

The factors that affect the rate of the reaction are 
  1. Nature of the reactants and products
  2. Concentration of the reactants
  3. Temperature of the system
  4. Pressure of the reacting system
  5. Nature of the catalyst if present
  6. Surface area of reactants 
  7. Rate of heat and mass transfer