AHA! Chemistry with Prof Bob
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    • 02 The nature of matter >
      • 0201 Atoms: Building blocks
      • 0202 Classification of matter
    • 05 Chemical reaction, chemical equations >
      • 0500 Overview, preview
      • 0501 Amount of substance, mole
      • 0502 Avogadro constant: How many?
      • 0503 Avogadro constant: Why that number?
      • 0504 Chemical formulas: What do they mean?
      • 0505 What can equations tell us?
      • 0506 Limiting reactants
    • 09 Solutions >
      • 0901 What is a solution?
      • 0902 Miscibility of liquids
      • 0903 Like dissolves like?
      • 0905 Dissolution of ionic salts in water
      • 0906 Can we predict solubilities of salts?
      • 0907 Solution concentration
      • 0908 Chemical species, speciation
      • 0909 Solutes: Electrolytes or non-electrolytes?
      • 0910 Electrolytes - strong or weak?
      • 0911 Concentrated, dilute, strong, weak
      • 0912 Species concentration vs. solution concentration
      • 0913 Weak electrolytes: Getting quantitative
    • 11 Dynamic chemical equilibrium >
      • 1100 Equilibrium: An overview
      • 1101 Visualising dynamic equilibrium
      • 1102 The jargon of equilibrium
      • 1103 Equilibrium constants
    • 22 Evidence from spectroscopy >
      • 2200 Overview, preview
      • 2201 Spectroscopy: Quantization of energies
      • 2202 Light: Wave-particle "duality"
      • 2203 UV-Visible spectroscopy
      • 2204 Beer's law
    • 27 Communicating chemistry >
      • 2700 Overview, preview
      • 2703 The jargon we use
  • TOC
  • Index
  • Teachers' area
    • T01 Communicating chemistry
    • T02 Beer's law
    • T03 Professional amnesia of the chemistry teaching professio
    • T04 Law of equilibrium
    • T05 Visusalizing dynamic chemical equilibrium
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​T04  tHE LAW OF EQUILIBRIUM


What do equilibrium constants really mean?
​

With reference to Module 1103: Equilibrium constants: Clarifying what the tell us ......

The law of equilibrium is often expressed in textbooks, and understood by students, in such a way that it implies that when a reaction mixture is at equilibrium, the function of concentrations that we call the reaction quotient, Q, has a constant value over time.

This is correct, but every imaginable function of concentrations of reactant species is constant with time in a reaction mixture at equilibrium – for the simple reason that the concentrations of all of the species are constant.

The real meaning of the law of equilbrium is much more profound that this.

What is so special about that function of concentrations that we call the reaction quotient, Q? Well, if we have many reaction vessels in which a give reaction is at equilibrium, then Q has the same value in all of them. This deeper meaning is amazing – and is not the case for any other function of concentrations.

I suggest that the shallower meaning expressed in the first paragraph is due to the way that the law of equilibrium is often stated. As is so often the case, it is all a matter of language!

The law of chemical equilibrium is often stated something like this: In a system at equilibrium, at a specified temperature, the function Q (defined) is a constant.

The words in bold are language traps for novices.

To chemists, the word system refers to a type of reaction. For example, chemists might talk about the NO2/N2O4 system – referring to all reaction vessels in which there is the reaction represented by the equation  2 NO2(g)  =  N2O4(g)

However, to most students, it is probable that the words “a system” induce visualization of one reaction vessel.

And the word constant? What does this word mean in everyday life – and presumably to most students? Yes that’s correct ….. not changing as time passes.

Scientists, however, use the word “constant” to mean the same in all cases.

So, I think there is value in expressing the law of equilibrium in a way that deals with both of these language issues in such a way as to state clearly the true meaning – by referring to multiple reaction mixtures, and to avoid using the word “constant” for the value of Q ….. as follows:

In all reaction mixtures in which a particular reaction is at equilibrium, and at the same temperature, the reaction quotient Q has the same numerical value.
​
[Where of course, Q needs to be defined in relation to the way that the chemical equation is written]
 



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  • Home
  • Modules
    • 02 The nature of matter >
      • 0201 Atoms: Building blocks
      • 0202 Classification of matter
    • 05 Chemical reaction, chemical equations >
      • 0500 Overview, preview
      • 0501 Amount of substance, mole
      • 0502 Avogadro constant: How many?
      • 0503 Avogadro constant: Why that number?
      • 0504 Chemical formulas: What do they mean?
      • 0505 What can equations tell us?
      • 0506 Limiting reactants
    • 09 Solutions >
      • 0901 What is a solution?
      • 0902 Miscibility of liquids
      • 0903 Like dissolves like?
      • 0905 Dissolution of ionic salts in water
      • 0906 Can we predict solubilities of salts?
      • 0907 Solution concentration
      • 0908 Chemical species, speciation
      • 0909 Solutes: Electrolytes or non-electrolytes?
      • 0910 Electrolytes - strong or weak?
      • 0911 Concentrated, dilute, strong, weak
      • 0912 Species concentration vs. solution concentration
      • 0913 Weak electrolytes: Getting quantitative
    • 11 Dynamic chemical equilibrium >
      • 1100 Equilibrium: An overview
      • 1101 Visualising dynamic equilibrium
      • 1102 The jargon of equilibrium
      • 1103 Equilibrium constants
    • 22 Evidence from spectroscopy >
      • 2200 Overview, preview
      • 2201 Spectroscopy: Quantization of energies
      • 2202 Light: Wave-particle "duality"
      • 2203 UV-Visible spectroscopy
      • 2204 Beer's law
    • 27 Communicating chemistry >
      • 2700 Overview, preview
      • 2703 The jargon we use
  • TOC
  • Index
  • Teachers' area
    • T01 Communicating chemistry
    • T02 Beer's law
    • T03 Professional amnesia of the chemistry teaching professio
    • T04 Law of equilibrium
    • T05 Visusalizing dynamic chemical equilibrium
  • Aha! Whatever
    • Playful dolphins
    • The University of Western Australia
    • Kings Park
  • Prof Bob?
    • Family
    • Travel
    • Perth
    • At work
  • Travelling
  • Contact
  • Blog
  • In four days for two days