Mass-Action Expressions (Q) Chemistry Tutorial

Key Concepts

• Mass-action expression is also known as the reaction quotient, or as the concentration fraction.
• Mass-action expression (reaction quotient or concentration fraction) is given the symbol Q
• For a given reversible reaction:

  aA + bB ⇋ cC + dD

  the mass-action expression (Q) is defined as

  Q =	[C]c[D]d [A]a[B]b

  
  where the square brackets, [], refer to the concentration of that species in a certain phase.
• The mass-action expression (Q) can have any value.

Please do not block ads on this website.
No ads = no money for us = no free stuff for you!

Writing the Mass-Action Expression, Q

By agreement, reactants are defined as on the left of a chemical equation and products on the right:

Each term used in the mass-action expression, Q, is the concentration of the species raised to the power of its stoichiometric coefficient:

The mass-action expression, Q, is written in the form:

By general agreement:

• the numerator of the mass-action expression (above the line) contains the product terms multiplied together
• the denominator of the mass-action expression (below the line) contains the reactant terms multiplied together

Worked Example: Reaction in Gas Phase

Question: Write the mass-action expression for the following reaction:

2CO_2(g) ⇋ 2CO_(g) + O_2(g)

Step 1: Determine which are the products and which are reactants for this reaction:

Remember: if there is no number in front of the chemical formula for a particular species in the equation, then it is understood that the stoichiometric coefficient is in fact 1.

Step 2: Write a term for each species in the form of its concentration raised to the power of its stoichiometric coefficient:

Note: any number raised to the power of 1 is simply equal to that number, that is, 2¹ = 2, 1056¹ = 1056, etc
So, [O_2(g)]¹ = [O_2(g)]

Step 3: Write the term for the numerator by multiplying together the terms for each product:

Step 4: Write the term for the denominator by multiplying together the terms for each reactant:

Step 5: Write the mass-action expression, Q, by dividing the numerator by the denominator:

Worked Example: Reaction in Aqueous Solution

Question: Write the mass-action expression for the following reaction:

Fe³⁺_(aq) + SCN^-_(aq) ⇋ Fe(SCN)²⁺(aq)

Step 1: Determine which are the products and which are reactants for this reaction:

Remember: if there is no number in front of the chemical formula for a particular species in the equation, then it is understood that the stoichiometric coefficient is in fact 1.

Step 2: Write a term for each species in the form of its concentration raised to the power of its stoichiometric coefficient:

Note: any number raised to the power of 1 is simply equal to that number, that is, 2¹ = 2, 1056¹ = 1056, etc
So, [Fe³⁺_(aq)]¹ = [Fe³⁺_(aq)]
and [SCN^-_(aq)]¹ = [SCN^-_(aq)]
and [Fe(SCN)²⁺_(aq)]¹ = [Fe(SCN)²⁺_(aq)]

Step 3: Write the term for the numerator by multiplying together the terms for each product:

Step 4: Write the term for the denominator by multiplying together the terms for each reactant:

Step 5: Write the mass-action expression, Q, by dividing the numerator by the denominator: