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By the end of this section, you will be able to:
  • Write equations representing changes in concentration and pressure for chemical species in equilibrium systems
  • Use algebra to perform various types of equilibrium calculations

We know that at equilibrium, the value of the reaction quotient of any reaction is equal to its equilibrium constant. Thus, we can use the mathematical expression for Q to determine a number of quantities associated with a reaction at equilibrium or approaching equilibrium. While we have learned to identify in which direction a reaction will shift to reach equilibrium, we want to extend that understanding to quantitative calculations. We do so by evaluating the ways that the concentrations of products and reactants change as a reaction approaches equilibrium, keeping in mind the stoichiometric ratios of the reaction. This algebraic approach to equilibrium calculations will be explored in this section.

Changes in concentrations or pressures of reactants and products occur as a reaction system approaches equilibrium. In this section we will see that we can relate these changes to each other using the coefficients in the balanced chemical equation describing the system. We use the decomposition of ammonia as an example.

On heating, ammonia reversibly decomposes into nitrogen and hydrogen according to this equation:

2 NH 3 ( g ) N 2 ( g ) + 3 H 2 ( g )

If a sample of ammonia decomposes in a closed system and the concentration of N 2 increases by 0.11 M , the change in the N 2 concentration, Δ[N 2 ], the final concentration minus the initial concentration, is 0.11 M . The change is positive because the concentration of N 2 increases.

The change in the H 2 concentration, Δ[H 2 ], is also positive—the concentration of H 2 increases as ammonia decomposes. The chemical equation tells us that the change in the concentration of H 2 is three times the change in the concentration of N 2 because for each mole of N 2 produced, 3 moles of H 2 are produced.

Δ [ H 2 ] = 3 × Δ [ N 2 ]
= 3 × ( 0.11 M ) = 0.33 M

The change in concentration of NH 3 , Δ[NH 3 ], is twice that of Δ[N 2 ]; the equation indicates that 2 moles of NH 3 must decompose for each mole of N 2 formed. However, the change in the NH 3 concentration is negative because the concentration of ammonia decreases as it decomposes.

Δ [ NH 3 ] = −2 × Δ [ N 2 ] = −2 × ( 0.11 M ) = −0.22 M

We can relate these relationships directly to the coefficients in the equation

2 NH 3 ( g ) N 2 ( g ) + 3 H 2 ( g ) Δ [ NH 3 ] = −2 × Δ [ N 2 ] Δ [ N 2 ] = 0.11 M Δ [ H 2 ] = 3 × Δ [ N 2 ]

Note that all the changes on one side of the arrows are of the same sign and that all the changes on the other side of the arrows are of the opposite sign.

If we did not know the magnitude of the change in the concentration of N 2 , we could represent it by the symbol x .

Δ [ N 2 ] = x

The changes in the other concentrations would then be represented as:

Δ [ H 2 ] = 3 × Δ [ N 2 ] = 3 x
Δ [ NH 3 ] = −2 × Δ [ N 2 ] = −2 x

The coefficients in the Δ terms are identical to those in the balanced equation for the reaction.

2 NH 3 ( g ) N 2 ( g ) + 3 H 2 ( g ) −2 x x 3 x

The simplest way for us to find the coefficients for the concentration changes in any reaction is to use the coefficients in the balanced chemical equation. The sign of the coefficient is positive when the concentration increases; it is negative when the concentration decreases.

Questions & Answers

The reaction of aceto nitrile with propane in the presence of the acid
Explain this paragraph in short
Manish Reply
What is solid state?
Manish Reply
What is chemical reaction
Manish
transforming reactants to product(s)
Andre
process
Andre
Example of Lewis acid
Chidera Reply
Example of Lewis acid
Chidera
Chlorine
Mikidad
Anything with an empty orbital... the hydrogen ion is the most common example. BH3 is the typical example, but any metal in a coordination complex can be considered a Lewis acid.
Eszter
okay thanks
Jovial
aluminium and sulphur react to give aluminium sulfide.How many grams of Al are required to produce 100g of aluminium sulphide
Soni Reply
aluminium and sulphur react to give aluminium sulphide how many grams of Al are required to produce 100g of aluminium sulphide?
Soni
aluminium and sulphur react to give aluminium sulphide how many grams of Al are required to produce 100g of aluminium sulphide?
Soni
2Al+3S=Al2S3
galina
m(Al)=100×27×2/150=36g
galina
150 comes from?
Soni
thank you very much
Soni
molar mass of Al2S3
galina
150.158
thiru
Why can't atom be created or destroyed
Jacaranda Reply
matter simply converts to pure energy
explain how to distinguish ethanol from a sample of ethanoic acid by chemical test
Alice Reply
explain how ethanol can be distinguished from ethanoic acid by chemical test
Alice
Using a suitable experiment, describe how diffusion occurs in gases.
Melody Reply
what is electrolytes?
charity Reply
substance which splits into ions during melting or dissolving
galina
on passing electric current though electrode
Kv
what is a radical
Jacob Reply
State that use law of partial pressure in a gas jar containing a gas and water what is the total pressure composed of 272cm^3 of carbon (iv) oxide were collected over water at15°c and 782mmHg pressure. calculate the volume of the dry gas at stp(SVP of water at 15°c is 12mmHg)
Aminat Reply
was Dalton's second postulate"atoms of the same kind have have similar/same mass and size" Or " the one mentioned in B here?
Maureen Reply
what was name of the Greek philosophers
Duku Reply
which of HF and HI is more acidic
Sahr Reply
HF is
HF
Duku
HI is stronger than HF (greater size of I courses greater length of bond)
galina
HI is a stronger acid due to less efficient orbital overlap. HF will react with with glass and extract calcium from bones, but those hazards are not because it's a stronger acid, but because it contains fluorine.
Eszter
hi
Victoria
hello
Jovial
hello
Asha
hi
Andre
hi
MO
hello
nanmya
hi
Hadiza
they are noble gases
Adum Reply

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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