MassAction Expressions (Q) Tutorial
Key Concepts
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Writing the MassAction Expression, Q
By agreement, reactants are defined as on the left of a chemical equation and products on the right:
reactants 
⇋ 
products 
aA 
+ 
bB 
⇋ 
cC 
+ 
dD 
Each term used in the massaction expression, Q, is the concentration of the species raised to the power of its stoichiometric coefficient:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
aA 
+ 
bB 
⇋ 
cC 
+ 
dD 
term for each species 
[A]^{a} 

[B]^{b} 

[C]^{c} 

[D]^{d} 
The massaction expression, Q, is written in the form:
Q = 
numerator denominator 
By general agreement:
 the numerator of the massaction expression (above the line) contains the product terms multiplied together
 the denominator of the massaction expression (below the line) contains the reactant terms multiplied together
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
aA 
+ 
bB 
⇋ 
cC 
+ 
dD 
term for each species 
[A]^{a} 

[B]^{b} 

[C]^{c} 

[D]^{d} 
numerator term 

[C]^{c}[D]^{d} 
denominator term 
[A]^{a}[B]^{b} 

massaction expression, Q = 
numerator denominator 
= 
[C]^{c}[D]^{d} [A]^{a}[B]^{b} 
Worked Example: Reaction in Gas Phase
Question: Write the massaction 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:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
2CO_{2(g)} 
⇋ 
2CO_{(g)} 
+ 
1O_{2(g)} 
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:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
2CO_{2(g)} 
⇋ 
2CO_{(g)} 
+ 
1O_{2(g)} 
terms for each species: 
[CO_{2(g)}]^{2} 

[CO_{(g)}]^{2} 

[O_{2(g)}]^{1} 
Note: any number raised to the power of 1 is simply equal to that number, that is, 2^{1} = 2, 1056^{1} = 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:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
2CO_{2(g)} 
⇋ 
2CO_{(g)} 
+ 
1O_{2(g)} 
terms for each species: 
[CO_{2(g)}]^{2} 

[CO_{(g)}]^{2} 

[O_{2(g)}]^{1} 
numerator term 


[CO_{(g)}]^{2}[O_{2(g)}] 
Step 4: Write the term for the denominator by multiplying together the terms for each reactant:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
2CO_{2(g)} 
⇋ 
2CO_{(g)} 
+ 
1O_{2(g)} 
terms for each species: 
[CO_{2(g)}]^{2} 

[CO_{(g)}]^{2} 

[O_{2(g)}]^{1} 
numerator term 


[CO_{(g)}]^{2}[O_{2(g)}] 
denominator term: 
[CO_{2(g)}]^{2} 




Step 5: Write the massaction expression, Q, by dividing the numerator by the denominator:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
2CO_{2(g)} 
⇋ 
2CO_{(g)} 
+ 
1O_{2(g)} 
terms for each species: 
[CO_{2(g)}]^{2} 

[CO_{(g)}]^{2} 

[O_{2(g)}]^{1} 
numerator term 


[CO_{(g)}]^{2}[O_{2(g)}] 
denominator term: 
[CO_{2(g)}]^{2} 




massaction expression, Q = 
numerator denominator 
= 
[CO_{(g)}]^{2}[O_{2(g)}] [CO_{2(g)}]^{2} 
Worked Example: Reaction in Aqueous Solution
Question: Write the massaction expression for the following reaction:
Fe^{3+}_{(aq)} + SCN^{}_{(aq)} ⇋ Fe(SCN)^{2+}(aq)
Step 1: Determine which are the products and which are reactants for this reaction:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
1Fe^{3+}_{(aq)} 
+ 
1SCN^{}_{(aq)} 
⇋ 
1Fe(SCN)^{2+}_{(aq)} 
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:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
1Fe^{3+}_{(aq)} 
+ 
1SCN^{}_{(aq)} 
⇋ 
1Fe(SCN)^{2+}_{(aq)} 
individual terms: 
[Fe^{3+}_{(aq)}]^{1} 

[SCN^{}_{(aq)}]^{1} 

[Fe(SCN)^{2+}_{(aq)}]^{1} 
Note: any number raised to the power of 1 is simply equal to that number, that is, 2^{1} = 2, 1056^{1} = 1056, etc
So, [Fe^{3+}_{(aq)}]^{1} = [Fe^{3+}_{(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:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
1Fe^{3+}_{(aq)} 
+ 
1SCN^{}_{(aq)} 
⇋ 
1Fe(SCN)^{2+}_{(aq)} 
individual terms: 
[Fe^{3+}_{(aq)}]^{1} 

[SCN^{}_{(aq)}]^{1} 

[Fe(SCN)^{2+}_{(aq)}]^{1} 
numerator term: 


[Fe(SCN)^{2+}_{(aq)}] 
Step 4: Write the term for the denominator by multiplying together the terms for each reactant:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
1Fe^{3+}_{(aq)} 
+ 
1SCN^{}_{(aq)} 
⇋ 
1Fe(SCN)^{2+}_{(aq)} 
individual terms: 
[Fe^{3+}_{(aq)}]^{1} 

[SCN^{}_{(aq)}]^{1} 

[Fe(SCN)^{2+}_{(aq)}]^{1} 
numerator term: 


[Fe(SCN)^{2+}_{(aq)}] 
denominator term: 
[Fe^{3+}_{(aq)}][SCN^{}_{(aq)}] 


Step 5: Write the massaction expression, Q, by dividing the numerator by the denominator:
general reaction: 
reactants 
⇋ 
products 
reaction equation: 
1Fe^{3+}_{(aq)} 
+ 
1SCN^{}_{(aq)} 
⇋ 
1Fe(SCN)^{2+}_{(aq)} 
individual terms: 
[Fe^{3+}_{(aq)}]^{1} 
+ 
[SCN^{}_{(aq)}]^{1} 
⇋ 
[Fe(SCN)^{2+}_{(aq)}]^{1} 
numerator term: 


[Fe(SCN)^{2+}_{(aq)}] 
denominator term: 
[Fe^{3+}_{(aq)}][SCN^{}_{(aq)}] 


massaction expression, Q = 
numerator denominator 
= 
[Fe(SCN)^{2+}_{(aq)}] [Fe^{3+}_{(aq)}][SCN^{}_{(aq)}] 