Reactions & Equilibrium (College Board AP® Chemistry)
Study Guide
Written by: Fallon
Reviewed by: Stewart Hird
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
Examiner Tips and Tricks
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
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?
Equilibrium was established between 20 seconds and 80 seconds because the rate of disappearance of Br2 remained constant during this period
Equilibrium was established between 140 seconds and 250 seconds because the concentration of Br2 remained constant after 250 seconds
Equilibrium was not established within 300 seconds as only 74% of the Br2 molecules were converted to HBr molecules within this time period
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
Examiner Tips and Tricks
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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