Composite Rate Constant 

Composite rate constant

Composite rate constant is the sum of two rate constants at equilibrium. It is denoted by Kc. It is written as

Kc = kf +kb eq.14.1

For opposing reactions, we know that at equilibrium, rate of forward reaction equals to the rate of backward reaction.

Rf = Rb

kf (a - xe) = kb (xe)         eq.13.3

In previous topic (opposing reactions) we found that slope =  . So, Kc = slope

or 

We can say that slope is the composite rate constant. From the straight line equation of 1st order opposed by 1st order opposing reaction having no initial concentration of products 

 eq.14.2

Click Here To Read More Articles

Click Here

Graphical representation of eq.14.2 will be

In the previous topic we found the value of kf and kb which is

and           kb = slope - kf  

The equation for kb can also be written as

kb + kf = slope         or         Kc = kf + kb        again eq.14.1 

Now we can substitute the values of kf and kb in eq.14.1 . For that we will further substitute kb.

kb = slope - kf

Putting value of kf and then simplifying

                    

eq.14.3

now putting the values of kb and kf in eq.14.1 we got

    Kc = slope        final equation

It is to be noted that rate composite and equilibrium rate constants are different. From eq.13.3

kf (a - xe) = kb (xe) 

where . So,   eq.14.4

So, Keq is the ratio is rate constants while Kc is sum of rate constants.

Click Here To Read More Articles

Click Here

• Opposing Reactions
• Experimental Methods For Determining Order Of Reaction
• CHEMICAL KINETICS (Introduction)