APChemistryCollege BoardRevision NotesUnit 7: Equilibrium7.1 Introduction to EquilibriumReactions & Equilibrium

Reactions & Equilibrium (College Board AP Chemistry)

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Reversible & Irreversible Reactions

Irreversible reactions:

  • Proceed in one direction only
  • The reaction stops when at least one reactant is completely consumed
  • Are denoted with a forward arrow (→)
  • Example:
    • Combustion CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

Reversible reactions (and processes):

  • Can proceed in both directions
  • The reaction can continue in both directions because as the reactants react to form products, the products can react to reform the reactants
  • Are denoted with two opposing half arrows (⇌)
  • Examples:
    • Evaporation/condensation H2O (l) ⇌ H2O (g)
    • Absorption/desorption of a gas CO2 (g) ⇌ CO2 (aq)
    • Dissolution/precipitation of a salt in a saturated solution NaCl (s) ⇌ Na+ (aq) + Cl- (aq)
    • Acid-base proton transfer HCN (aq) + NH3 (aq) ⇌ NH4+ (aq) + CN- (aq)
      • In the forward reaction, the proton (H+) is transferred from HCN to NH3
    • Electron transfer in redox reactions Cd (s) + 2Ag+ (aq) ⇌ 2Ag (s) + Cd2+ (aq)
      • In the forward reaction, two electrons are removed from one Cd atom and each electron is gained by an Ag+ ion

Exam Tip

Note that reactions involving a gas are only reversible if they occur in a closed system.

Dynamic Equilibrium

  • When no observable changes occur to a system involving a reversible reaction, the reaction has reached a state of equilibrium
  • At equilibrium both reactants and products are present in the system
  • Equilibrium is dynamic, because reactant and product molecules react to form one another at the same constant rate.
  • So, the rate of the forward and reverse reactions are equal and the concentrations or partial pressures of all species remain constant
  • On a graph of concentration, partial pressure, or rate of reaction versus time the point at which equilibrium is established is the point in time in which the provided variable no longer changes

Sample rate versus time graph, partial pressure versus time graph, and particle diagram for a reversible reaction

  equilibrium-graphs-and-partial-diagram

The rate versus time graph, partial pressure versus time graph, and particle diagram for the reversible reaction N2O4(g) ⇌ 2NO2(g) showing that equilibrium is established when the rates first become equal, and the partial pressures and number of particles of each species remain constant

Worked example

2HBr (g) ⇌ H2 (g) + Br2 (g)

In an experiment, equal amounts of H2(g) and Br­2(g) were pumped into a 1.0 L sealed, rigid, previously evacuated container at a constant temperature of 350 K. The gases were allowed to react, according to the equation above. The table below shows the concentration of Br2(g) during the experiment.

Time elapsed (s)

Concentration Br2(g) (M)

0

6.21 x 10-3

20

5.11 x 10-3

50

3.91 x 10-3

80

2.71 x 10-3

140

1.97 x 10-3

250

1.64 x 10-3

300

1.64 x 10-3

450

1.64 x 10-3

 

What claim can be made about the equilibrium reaction based on the data above?

  1. Equilibrium was established between 20 seconds and 80 seconds because the rate of disappearance of Br2 remained constant during this period
  2. Equilibrium was established between 140 seconds and 250 seconds because the concentration of Br2 remained constant after 250 seconds
  3. Equilibrium was not established within 300 seconds as only 74% of the Br2 molecules were converted to HBr molecules within this time period
  4. It is not possible to determine if equilibrium was established as the concentration of HBr is not known


Answer:

  • Equilibrium is established when:
    • The rates of the forward and reverse reactions remain constant and equal
    • The concentrations of the reactants and products remain constant
  • From the data, the concentration of Br2 remains constant after 250 seconds
    • This also implies that the concentrations of H2 and HBr remain constant during this time frame as well
  • However, we do not know from the data if equilibrium was established at exactly 250 seconds into the reaction
  • The concentration could have become and remained 1.64 x 10-3 M at any point between 140 seconds and 250 seconds as no data was collected during this time
  • So based on the data, equilibrium was established between 140 seconds and 250 seconds because the concentration of Br2 remained constant after 250 seconds

Exam Tip

Remember that at equilibrium both the forward and reverse reactions continue to occur even though no observable changes take place

Note that at equilibrium the reactant concentrations or partial pressures may be greater than, less than, or equal to that of the products. The only distinguishing feature is that both the reactant and product concentrations or partial pressures must remain constant at equilibrium

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    Fallon

    Author: Fallon

    Fallon obtained a double major in chemistry and secondary education, and after graduating she taught Chemistry and Organic Chemistry for 7 years. Fallon’s passion for creating engaging classroom materials led her to pursue a career in content development. For over 3 years, Fallon has created videos, review materials, and practice questions for AP Chemistry, IGCSE, and other international exam boards.