For example, for the reaction $\ce{A + 2B -> C}$, we have

$$\text{rate} = -\frac{1}{2}\frac{\mathrm d[\ce{B}]}{\mathrm dt} \quad \text{and} \quad \text{rate} = k[\ce{A}]^x[\ce{B}]^y.$$ Together, this means that

$$-\frac{1}{2}\frac{\mathrm d[\ce{B}]}{\mathrm dt} = k[\ce{A}]^x[\ce{B}]^y$$

However, in many books it is also written simply that

$$-\frac{\mathrm d[\ce{B}]}{\mathrm dt} = k[\ce{A}]^x[\ce{B}]^y.$$

Which is correct?

  • 4
    $\begingroup$ Both are.$\,\!$ $\endgroup$ – Ivan Neretin Sep 20 '18 at 14:22
  • $\begingroup$ @IvanNeretin can you please elaborate? Are we taking $2k$ As $k$ ? $\endgroup$ – Fawad Sep 20 '18 at 14:26
  • 4
    $\begingroup$ Yeah, sort of. What is $k$? Some number. What is $2k$? Some number. You may use either form, as long as you know which one are you using. $\endgroup$ – Ivan Neretin Sep 20 '18 at 14:33

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