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  • Free Lessons
    • Thermodynamics >
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      • How Long will it Take to Decay?
      • What Order is this Reaction?
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      • Ideal Gas Law
      • Density of Gases
      • Ideal Gas Law and Changes in P, V, T
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      • HCl + Alkene (Adding across a double bond)
      • Reaction of OH with Alkyl Halide
      • What is Regioselectivity?
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      • Average Atomic Mass
      • Solve for Isotopic Abundance
      • Limiting Reagents
      • Percent Yield
      • Actual Yield and Percentage Yield
      • Percent Composition
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      • What's in an Atom?
      • Quantum Numbers
      • Pauli, Aufbau, Hund
      • Light: E h ν λ
      • Energy Levels of Hydrogen
      • Energy Levels of Non-Hydrogen Atoms
    • Organic Naming >
      • Naming Organic Molecules
      • How to Name Amines
      • How to Name Amides
    • Lewis Structures
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Zero-Order Reactions


Zero-order reactions aren't common, but they're included with lots of chemistry courses because the math behind them is straightforward.  Most examples of zero-order reactions are thermal decompositions - where a chemical just breaks apart because there's so much heat - because it isn't the reactant concentration controlling the reaction, it's the temperature!

The Rate Equation

Zero-order reactions mean the exponent on the reactant concentration is 0:
Picture
Notice how the whole rate expression simplifies to Rate = k.  This type of reaction doesn't depend on reactant concentration!

How Concentration Changes with Time

Just like with first- and second-order reactions, you can integrate the rate equation to determine how the concentration of the reactant changes with time.  If you know a little calculus (antiderivatives) you'll be able to follow the integration that I've put on the right.  If you can't, just skip to the second-last line.

If the reaction is zero-order, a graph of [A] vs. t will give a line!  Remember (from grade 9) how y=mx+b is the equation of a line?  Well here, y is [A], the reactant concentration.  x is t, the time elapsed.  m (the slope) is -k and b (the y-intercept, where t=0) is [A]o, the initial reactant concentration.
Picture
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Created by Nathan Oldridge (aka ChemistNATE).  All lessons here are Public Domain.