So here I will be clearing some misconceptions that students generally make while studying Chemical Kinetics. But it is not totally their fault as they are taught that way.

FIRST MISCONCEPTION
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OK.......So tell me how do you define rate in chemical kinetics.....????

Students normally answer rate of a reaction as rate of change of CONCENTRATION with respect to TIME....
That is........... Rate = d(C)/dt

Now this definition in one's mind actually has no basic meaning and also this is TOTALLY WRONG conception. First let me tell you why it has no meaning and then i will tell why it is wrong.
While mentioning the rate one must mention what rate he/she is telling about.....
By this i mean that in a reaction like following,

aA + bB --> pP + sS

has actually four rates,

(-rA) = Rate of decompostion of reactant A (minus sign denotes reactantant and that it is decomposing that is getting exhausted and getting converted to products)
(-rB) = Rate of decompostion of reactant B
(+rP) = Rate of formation of product P
(+rS) = Rate of formation of product S

and all af them are related by their stoitiometric coefficients as follows.....
(-rA)/a = (-rB)/b = (+rP)/p = (+rS)/s
So just defining rate of a reaction requires proper specification that about which species you are defining the rate. All the above four are rates of the same reaction but defined with respect to specific species.

Now I say that Rate = d(C)/dt is totally WRONG.......

The ACTUAL DEFINITION is........ Rate of A = (1/VA)*d(NA)/dt
Where VA ---------> Volume of A at present
and NA -------------> Number of moles of A at present

NOW REMEMBER IF AND ONLY IF "VA" is CONSTANT then.........

VA can go within the differential term and,
Rate of A = d(NA/VA) / dt = d(CA) / dt

As we know concentration of A = CA = Number of moles of A per unit volume of A
So only for "VA" being constant your definition is true otherwise not.
So you should also know that for GAS PHASE REACTIONS there is always a change in volume of the GASES and thus your expression is never valid.
But your expression is always valid for LIQUID PHASE reactions where there is practically no change in volume. But that expression is not a correct one. When you learn you must learn the generalized concept and my definition is valid Universally and has no restrictions. Hope misconception is cleared.

If you like this then tell me i will come up with more in depth detailed and far easy and normal misconceptions that most mediocre students have while preparing for IIT.