Rate of Reaction - Practice Questions & MCQ

It is defined as "The rate of change of concentration of a reactant or a product per unit time" that is it is nothing but it is the per unit mole of rate of decomposition of any reactant or formation of product.

As rate of reaction varies greatly with time, so generally, average reaction rate and instantaneous reaction rates are used. Average rate ways to is the rate at some particular instant of time and is equal to the time rate of change of the active mass of any of the reactants or any of the products.

Change in the concentration of reactants or products per unit time is called average reaction velocity. If Δc is the change in the concentration of reactants and product in Δt time, then:

As average reaction rate fails to predict rate at a particular moment of time so we use instantaneous rate which is equal to small change in concentration (dx) during a small interval of time (dt). It is given as dx/dt.

• dx/dt = tanӨ = slope of curve
• It can be written for any of the reactant or the product in terms of stoichiometric coefficients V_j which is negative for reactants and positive for products as follows:
  
  
  
  
  
  
  
  
  
  
  
• For the reactants, negative sign indicates decrease of concentration and for products positive sign indicates increase in concentration.
• For a reversible reaction at dynamic equilibrium, net reaction rate is always zero as:
  

There are various factors on which the rate of reaction depends:

• Nature of reactant and product: 
  • For ionic reactants reaction rate is fast as activation energy is zero for them. For example:
    
  • Molecules have slow reaction rate due to need of more activation energy. For example:
    
• Physical state of reactants: Rate also changes with physical state.
  Gaseous states > Liquid states > Solid states
• Pressure: For gaseous reactants rate varies with pressure just like concentration.
  
• Surface Area: Greater the surface area, faster is the rate of reaction due to more number of active sites.
  

• Temperature: Rate of reaction increases with the increase of temperature as it increases the number of effective collisions. It is observed that for every 10^oC rise in temperature -dx/dt or rates become double to triple as the number of molecules with every greater than activation energy becomes two-three times.
  
  The value of temperature coefficient lies in between 2-3. In case we increase the temperature by more than 10^oC the above relation can be given as:
  
  
  
  
  
  
   
• Catalyst: It increases the rate of a reaction by decreasing the activation energy by accepting a new alternative smaller path for the reaction. It is reverse in case of negative catalyst to that of positive catalyst. Catalysts are more effective in 'Solid powdered form' due to larger surface area, that is, more active sites.
  
• Intensity of light: Rate of photochemical reactions depends upon intensity of light radiations.
  
• Concentration of reactants: Rate increases with the increase of concentration as due to more number of reactants there are more collisions.