Tabla de Contenidos
aA + bB ⇌ cC + dD
where a and b represent the number of relative moles of reactants A and B, and c and d represent the number of moles of products C and D respectively. Together a, b, c and d are the “stoichiometric coefficients”.
The ⇌ symbol has two half arrowheads, one pointing in each direction. It is used in equations showing reversible reactions:
- the forward reaction is the one that goes to the right
- the reverse reaction is the one that goes to the left
The reaction mixture can contain reactants and products and their proportions can be changed by altering the reaction conditions.
How does a reversible reaction work?
In addition to being converted to products, in reactive reactions products can also be converted to reactants. Both reactions, both one way and the other, can happen at the same time; this is until a relative equilibrium is established between reactants and products where the concentrations equalize.
Each reaction reaches its equilibrium point depending on the characteristics of the elements and the conditions in which that reaction is taking place. An example of a reversible reaction is:
H 2 (g) + I 2 (g) ⇌ 2HI (g)
In the reaction going to the right, hydrogen and iodide combine to form hydrogen iodide. In the reaction going to the left, hydrogen iodide decomposes into hydrogen and iodide. The two reactions can be written as:
In the direct reaction: H 2 (g) + I 2 (g) –> 2HI (g)
In the reverse reaction: 2 HI (g) –> H 2 (g) + I 2 (g)
When gaseous hydrogen and iodide are mixed in a sealed container they begin to react to form hydrogen iodide. First, only the forward reaction takes place because no HI is present. As the reaction progresses, it begins to slow down as the concentration of hydrogen and iodide decreases.
Other Examples of Reversible Reactions
Ammonium chloride
Ammonium Chloride is a white solid. Decomposes when heated, forming ammonia and hydrogen chloride. When the two gases cool enough, they react together to form ammonium chloride again. The reversible reaction can be written as:
NH 4 Cl(s) ⇌ NH 3 (g) + HCl(g)
Carbonic acid
In human blood, excess hydrogen ions (H + ) bind with bicarbonate ions (HCO 3 – ) to form carbonic acid (H 2 CO 3 ):
HCO 3 – + H + ⇌ H 2 CO 3
Since this is a reversible reaction, if carbonic acid were added to the system, some of it would be converted to bicarbonate and hydrogen ions to restore equilibrium:
H 2 CO 3 (l) + H 2 O (l) ⇌ HCO 3 – (aq) + H 3 O + (aq)
Sources
Reversible Reaction | Chemistry for Non-Majors . (2021). Retrieved July 5, 2021 from https://courses.lumenlearning.com/cheminter/chapter/reversible-reaction/
Reversible reactions – Reversible reactions – AQA – GCSE Combined Science Revision – AQA Trilogy – BBC Bitesize . (2021). Retrieved July 5, 2021 from https://www.bbc.co.uk/bitesize/guides/z32bpbk/revision/1
Chemical Reactions (article) . Khan Academy. Retrieved on February 14, 2022 from https://es.khanacademy.org/science/biology/chemistry–of-life/chemical-bonds-and-reactions/a/chemical-reactions-article .
