Sodium cyanide occurs as a white crystalline solid that is soluble in water. It is a salt of hydrogen cyanide and sodium hydroxide. It is also known as Cyanide of sodium or Cyanobrik or Cyanogran. It is highly poisonous as it interferes with various metabolic processes inside the body.
In an aqueous solution, it dissociates to form cyanide anions and sodium cations. Sodium cyanide is manufactured through the Castner process in which sodium amide reacts with carbon at high temperatures.
The chemical reaction is written as under :
NaNH2 + C → NaCN + H2
So, Is NaCN an acid or base? Sodium cyanide is a base. It is actually a basic salt formed by a reaction between a strong base, sodium hydroxide (NaOH), and a weak acid, hydrogen cyanide (HCN). We already know that the salt of a weak acid and strong base is basic in nature. Therefore, NaCN is basic. Also, it has a pH greater than 7, between 10-11, and the Ka value of 4.9 X 10-10, again indicating its alkalinity.
In this article, we will study the basicity of Sodium Cyanide (NaCN) and its fundamentals.
Why is NaCN a Basic Salt?
Sodium cyanide is prepared by the direct reaction between sodium hydroxide and hydrogen cyanide.
The reaction equation for the preparation of sodium cyanide is written as:
HCN (acid) + NaOH (base) —-> NaCN (salt) + H2O
Such reactions in which an acid reacts with a base for the formation of salts is known as neutralization reaction and the nature of the salt produced as a result of such reactions depends upon the strength of the acid and base used as reactants.
The simple rule for determining the nature of the product of neutralization reactions is as follows:
Strong Acid + Weak Base = Acidic salt; pH < 7; For example – NH4Br, NH4NO3, etc.
Weak Acid + Strong Base = Basic salt; pH > 7; For example – NaCN, Na2CO3, etc.
Strong Acid + Strong Base = Neutral salt; pH = 7; For example – NaCl, KCl, etc.
We already know that HCN is a weak acid while NaOH is a strong base, therefore, NaCN which is formed as a product of their neutralization reaction is a basic salt.
Why is NaCN not an Acidic Salt?
An acid is defined as a compound that produces protons in an aqueous solution.
They turn blue litmus red and should have a pH value below 7. A substance can be categorized as an acid or base based on the following theories:
• Bronsted-Lowry Theory: Also known as the proton theory it was given by Johannes Bronsted and Thomas Lowry in 1923.
This theory states that acids are substances that have the capacity to transfer their protons to other molecules.
• Lewis Theory: It was suggested by G.N. Lewis in 1923. As per this theory acids are the acceptors of unshared electron pairs.
• Arrhenius Theory: It was introduced by Svante Arrhenius in 1887. According to the theory, the substances that dissociate to form hydrogen ions are known as acids.
Now taking into consideration the postulates of these theories let us Look at the dissociation equation for sodium cyanide:
NaCN —> Na+ + CN-
It is clear from the above reaction equation that the NaCN molecules dissociate to form Na+ ions which are not capable of reacting further and CN- ions that would rather accept a proton or give away a lone pair of electrons.
Hence, sodium cyanide molecules on dissociation are neither capable of producing hydrogen ions or protons nor do they accept the unshared electrons.
Therefore, sodium cyanide cannot be called an acid.
A very similar compound NaCl, check out if NaCl is basic or acidic, or neutral.
Why is the aqueous solution of NaCN basic?
The molecules of sodium cyanide dissociate into sodium and cyanide ions as seen in the reaction equation mentioned in the previous section.
Here, let us recall the rules for the conjugate acids and bases:
A strong acid always produces a weak conjugate base.
A weak acid always produces a strong conjugate base
A strong base always produces a weak conjugate acid
A weak base always produces a strong conjugate acid
Now again looking at the sodium cyanide molecules, we know they are prepared from a strong base, sodium hydroxide, and a weak acid, hydrogen cyanide.
Therefore, the Na+ produced is the weak conjugate acid of the strong base and therefore, does not hydrolyze further and hence, does not have any effect on the pH of the solution.
However, CN- ion is a strong conjugate base of a weak acid (HCN), and therefore, it hydrolyses further by accepting a proton from water molecule thereby causing the release of OH- ions in water.
The chemical equation is written as follows:
CN- + H2O —–> HCN + OH-
These hydroxyl ions are responsible for the basicity of the aqueous solution of sodium cyanide.
Is NaCN considered a strong or weak base?
Technically, NaCN is salt and should not be categorized as an acid or a base. It is actually a product of the neutralization reaction between an acid and a base.
In the case of salt, we must remember the ground rule which was mentioned in the earlier section.
A salt formed by the reaction between a strong base and a weak acid always has a pH greater than 7 and is, therefore, basic.
Hence, sodium cyanide is a basic salt.
However, if only pH and Ka values are taken as the criteria for deciding the strength of sodium cyanide as a base then it is a strong basic compound as it has a pH value much above 7, between 10 to 12, and the Ka value of 4.9 X 10^-10.
pH of NaCN
The pH value of a substance is used as an indicator of its nature and strength as an acid or base.
The pH scale runs from 1 to 14 on which acids are the substances having pH value lower than 7 while bases have a pH value above 7.
The neutral substances for example water are said to have a pH value of 7.
Now let us calculate the pH of 0.1 M solution of NaCN. The Ka value of the solution is given as 4.9 X 10-10. The chemical equation is written as:
CN- + H2O —> HCN + OH-
We know that, Ka X Kb = 10-14
Therefore, Kb = 10-14/4.9 X 10-10
= 2.04 X 10-5
Now, calculating the concentration of OH- ions using the above chemical equation, we know that,
Kb = ([HCN] [OH-]) / [CN-]
Hence, 2.04 X 10-5 = x2/0.1 – x
Therefore, x = 1.43 X 10-3 = [OH-]
Now, we know that, pOH = -log [OH-]
= -log (1.43 X 10-3)
Also, pH + pOH = 14
pH = 14 – 2.84 = 11.15
Therefore pH of 0.1 M NaCN solution is 11.15 which is far above 7 indicating that it is a basic salt.
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A few important properties of sodium cyanide are given below:
• It is a salt of sodium hydroxide and hydrogen cyanide.
• It is denoted by the chemical formula NaCN.
• It has a molecular weight of 49.0072 gm/mol.
• It has a density of 1.59 gm/cm3.
• It is a white crystalline solid but appears colorless in an aqueous solution.
• It has a smell like that of faint almonds.
• The value of melting and boiling point of sodium cyanide is 563.7 °C and1496 °C, respectively.
• It is soluble in water as well as other solvents like methanol, ethanol, ammonia, etc.
• It has a refractive index of 1.452.
• It is a highly poisonous compound.
• It is not combustible.
• It is corrosive for some metals such as aluminum.
• It is an alkaline salt with a pH greater than 7.
• In a NaCN molecule, the sodium and cyanide ions are present in a 1:1 ratio.
• It is deliquescent i.e. it absorbs water from the atmosphere and turns it into liquid.
Some of the significant uses of sodium cyanide are listed below:
• Cyanide has a great affinity for gold due to which it is used in the extraction of gold. The reaction is written as:
4Au + 8NaCN + O2 + 2H2O —–> 4Na[Au(CN)2] + 4NaOH
• Due to its strong nucleophilic nature cyanide is used in the preparation of nitriles including pharmaceuticals.
• Owing to its highly poisonous nature it is also used by entomologists for their collecting jars.
• Sodium cyanide is also used for testing the functioning of chemoreceptors where it is used as a test reagent.
• It is also used in the manufacturing of dyes.
• Being toxic it is also used as a rodenticide.
• It is used in cleaning metals and also in manufacturing the electroplating solution.
• Sodium cyanide is used in making hydrocyanic acid.
• It is also used as a chelating agent.
• Sodium cyanide is also used as a pesticide or insecticide in agricultural fields.
Sodium cyanide is a basic salt formed as a result of a neutralization reaction between a strong base, sodium hydroxide, and a weak acid, hydrogen cyanide.
It does not fit into the definition of acids prescribed by any of the theories of acids and bases as it neither produces protons or hydrogen ions nor it accepts unshared pairs of electrons.
The aqueous solution of NaCN is basic as it produces hydroxyl ions due to the reaction between cyanide ions and water.
The pH and Ka values for a 0.1 M solution of NaCN are 11.15 and 4.9 X 10-10, respectively.
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