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Chemistry · Glossary

What is Rate of reaction, rates of formation and disappearance?

Also known as: rate of reaction · rate of formation · rate of disappearance

Definition 8.1 University Chemistry — Year 1 · Chapter 8 — Chemical Kinetics: Rate Laws

For a reaction 0=∑iνiAi0 = \sum_i \nu_i\mathrm{A}_i taking place in a closed reactor of constant volume VV, the rate of reaction is

v=1V ⁣dξ ⁣dt=1νi ⁣d[Ai] ⁣dt(any i),v = \frac{1}{V}\frac{\dd\xi}{\dd t} = \frac{1}{\nu_i}\frac{\dd[\mathrm{A}_i]}{\dd t} \quad\text{(any } i\text{)},

in mol L−1 s−1\mathrm{mol}\,\mathrm{L}^{-1}\,\mathrm{s}^{-1}. The rate of formation of a product is  ⁣d[Ai]/ ⁣dt\dd[\mathrm{A}_i]/\dd t; the rate of disappearance of a reactant is − ⁣d[Ai]/ ⁣dt-\dd[\mathrm{A}_i]/\dd t.

Examples

Example 8.3 (Decomposition of dinitrogen pentoxide)

For 2 NX2OX5→4 NOX2+OX2\ce{2N2O5 -> 4NO2 + O2}: v=−12 ⁣d[NX2OX5] ⁣dt=14 ⁣d[NOX2] ⁣dt= ⁣d[OX2] ⁣dtv = -\frac12\frac{\dd[\ce{N2O5}]}{\dd t} = \frac14\frac{\dd[\ce{NO2}]}{\dd t} = \frac{\dd[\ce{O2}]}{\dd t}. Nitrogen dioxide appears four times as fast as dioxygen, and dinitrogen pentoxide disappears twice as fast as dioxygen appears.

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