For The reaction: A + B ? C + D, the rate law was found by experiment to be: rate=k[B]2. What is the effect of doubling the concentration of B?

Question

The rate would be doubled.

The rate would be one quarter as great.

The rate would be four times as great.

The rate would be unchanged.

The rate would be cut in half.

Answer 1

You can solve that by plugging numbers in and checking the outcome.

 

 

Let [B₁] = 3M and [B₂] = 6M (that's twice [B₁])

 

so, 

 

rate₁ = k[B₁]² = k(3)² = 9k

 

rate₂ = k[B₂]² = k(6)² = 36k

 

36/9 = 4... so rate₂ = 4 rate₁

 

A more formal approach is to say [B₁] = x and [B₂] = 2x

 

rate₂   =   k(2x)²   = 4kx²   = 4..... so rate₂ = 4 rate₁

rate₁        k(x)²         kx²