Description of the process and the correct equation of the hydrolysis of sodium acetate
How the hydrolysis of sodium acetate takes place
Sodium acetate is a normal salt that is the product of the reaction of neutralization between sodium hydroxide and acetic acid. By its physical properties, it is a white crystal hygroscopic powder.
It exists in the form of highly soluble trihydrate CH₃COONa·3H₂O
How to obtain sodium acetate
Not only an alkali can be used to obtain this salt – an analogous product is obtained in the reaction of sodium carbonate or bicarbonate with acetic acid:
CH₃COOH + NaHCO₃ = CH₃COONa + CO₂ + H₂O
This is the most popular method for obtaining the substance known as “slaking soda of vinegar” – in this way pure sodium acetate can be obtained, which crystallizes when water evaporates.
Sometimes the compound is also obtained according to a typical metabolic reaction:
(CH₃COO)₂Zn + Na₂S = 2CH₃COONa + ZnS
Sodium acetate can be obtained in two stages, and in the first one only organic substances take part (they do not break down into ions in the solution, which means that they are weak electrolytes):
C₂H₅OH + CH₃COOH = CH₃COOC₂H₅ + H₂O (formation of ethyl acetate in the presence of sulfuric acid as a dehydrating agent)
CH₃COOC₂H₅ + NaOH = CH₃COONa + C₂H₅OH (formation of sodium acetate and ethanol under the impact of an alkali).
The properties of reagents used to obtain sodium acetate determine the properties of the salt. When sodium acetate is dissolved in water, a solution may be obtained with an alkaline reaction of the medium, as a consequence of hydrolysis (or solvolysis) – the breakdown of a substance on contact with water, with the formation of new compounds. A detailed description of the process of hydrolysis of CH₃COON is required, as this salt has certain special features.
How hydrolysis of sodium acetate takes place
As sodium acetate is a single-base salt, it only hydrolyzes by one degree (it forms one series of compounds on hydrolysis – an alkali and acid). A substance is formed by a strong base (alkali) NaOH and a weak acetic acid CH₃COOH.
As vinegar dissociates little, acid weakly breaks down into ions. In a solution it exists in the form of a symmetrical dimer – a compound consisting of two vinegar molecules.
Hydrolysis takes place with the weak ion – in this case it is the negatively charged acetate ion CH₃COO⁻.
The correct reaction equation of the hydrolysis of sodium acetate looks as follows:
- in molecular form: CH₃COONa + H₂O = CH₃COOH + NaOH;
- in ionic form the reaction can be portrayed as follows: CH₃COO⁻ + Na⁺ + H₂O = CH₃COOH + Na⁺ + OH⁻;
- in abbreviated ionic form: CH₃COO⁻ + H₂O = CH₃COOH + OH⁻.
In this equation it is clear that an anion is hydrolyzed, and because of the presence of an OH-group in the solution its medium is alkaline.
As potassium and sodium hydroxides and salts do not have fundamental differences in their properties, the process of obtaining and hydrolysis of potassium acetate (or calcium acetate) is fully analogous to the one applied for sodium salt:
CH₃COOK + H₂O = CH₃COOH + KOH;
(CH₃COO)₂Ca + 2H₂O = 2CH₃COOH + Ca(OH)₂.
The special nature of sodium acetate crystal hydrates
Sodium acetate trihydrates (CH₃COONa·3H₂O) easily move to a flux – gentle heating is sufficient for this. Melting takes place unusually, as the dissolution of salt in water takes place, which was the component part of the structure of the crystal hydrate (if a drop of phenolphthalein is added, it turns crimson, as when CH₃COON is dissolved, an alkaline medium is created as a consequence of hydrolysis). On cooling, a white ice-like substance is formed (the experiment of “hot ice” is based on this).
Here’s how it works:
At boiling temperature of the water bath, solid sodium acetate with a small addition of water (without excess) forms a rich solution, which on cooling becomes saturated, but crystallization of the substance does not take place, as in the case with other salts. Crystals begin to settle only when a “seed” is put in – several crystals of solid sodium acetate. If a small amount of water is not added to the water before heating, crystallization immediately takes place.
Anhydrous sodium acetate also hydrolyzes in water. It can be obtained by heating at a temperature of around 320 °C in a porcelain bowl. The salt must be dehydrated carefully, without exceeding a heating temperature of 324 °C, as the salt may break down (and hydrolysis will take place according to a different equation):
2CH₃COONa = Na₂CO₃ + (CH₃)₂CO (acetone)
Click here for safe experiments with sodium acetate.
What reactions take place in a solution of sodium acetate
Usually on an analysis of the behavior of solutions of sodium acetate or potassium acetate, only the hydrolysis of salt is examined. But this is not the only reverse process that takes place in the solution – for example, dissociation of substances takes place – their breakdown into ions in the water (in hydrolysis this is only calculated for sodium acetate dissolved in water and the strong sodium alkali)
CH₃COONa = CH₃COO⁻ + Na⁺
NaOH = Na⁺ + OH⁻
It should also be taken into account that to a small degree, acetic acid is also subject to dissociation (for a solution with a molar concentration of 1 mol/l dissociation comes to 0.4%):
CH₃COOH = H⁺ + CH₃COO⁻
Sodium acetate has found wide application in industry – this salt is used as a food additive, a conserving agent, a buffer solution for stabilization of a reaction in the pH medium (especially in biochemistry), and a component of chemical heating pads.