Is HCN a Potassium Hydroxide?

Hydrocyanic acid, commonly known as Formonitrile or Prussic acid, is a colourless vapour or liquid with a subtle almond scent. The molar mass of this substance is 27.0253 g/mol.

It’s a flammable liquid that’s made commercially by combining ammonia, methane, and air with a platinum catalyst. It has a melting point of 13.4 degrees Celsius and a boiling point of 25.6 degrees Celsius. It’s a one-carbon molecule made composed of a methine group and a nitrogen atom linked by a triple bond.

Is HCN a powerful acid, then? HCN is a weak acid since it does not entirely dissociate in water and only produces a tiny number of H+ ions. Ionization is difficult because the cyanide ion (CN-) bonds firmly to its proton. The acid dissociation constant for HCN is 6.2 X 10-10, and the logarithmic constant is 9.2, although the Ka value for strong acids reaches infinity, and the pKa value must be less than 2. The 0.1M HCN solution has a pH of 5.2, indicating that it is a mild acid.

HCN is a chemical that is used to make nylons, polymers, and fumigants. When given intravenously, sodium nitroprusside (Na2[Fe(CN)5NO)2H2O) has been shown to reduce blood pressure.

HCN is extremely toxic and has a short half-life. It is a systemic toxin that works by inhibiting the activity of cytochrome oxidase, which limits the use of oxygen for cellular functions.

Long-term exposure can lead to respiratory arrest and death. HCN exposure, on the other hand, is restricted to industrial settings or cigarette smoke and combustion products. It is not normally thought to be corrosive.

Acids, both strong and weak

Strong acid is an acid that almost entirely dissociates or ionises in an aqueous solution. They have a high proclivity for losing a proton. This proton generates the hydronium ion when it comes into contact with a water molecule:

HA   +   H2O   ——>    H3O+   +    A-

Strong acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), and others. Although most powerful acids are corrosive, some superacids are an exception.

The acid dissociation constant (Ka) of strong acids is large, but the logarithmic constant is low (pKa). The Ka and pKa values for HCl, for example, are 1.3 X 106 and – 6.3, respectively.

In a solution, weak acids, on the other hand, only partially ionise. Dissociation capability is used to measure their strength.

Weak acids have a difficult time losing their proton and even tend to take it back from the solution, hence their reaction is represented as:

HA (aq)   +   H2O      <=======>       H3O+   +   A-

This may also be seen in the instance of HCN, where the CN- ion is so strongly connected with its proton that when it comes into touch with a proton in a solution, it repairs the HCN molecule, preventing complete dissociation of the acid in the solution.

Acetic acid (CH3COOH), Formic acid (HCOOH), and other weak acids are examples.

Why is HCN considered a weak acid?

The dissociation equation for HCN in an aqueous solution can be written as follows:

Is HCN a Strong Acid?

Is HCN a Strong Acid

Hydrocyanic acid is also known as Formonitrile or Prussic acid is a colorless gas or liquid that has the faint smell of bitter almonds. It has a molar mass of 27.0253 g/mol.

It is a flammable liquid and is commercially produced by reacting ammonia with methane, and air over a platinum catalyst. It has a melting point of −13.4°C and a boiling point of 25.6°C. It is a one-carbon compound that consists of a methine group connected with a nitrogen atom through a triple bond.

So, is HCN a strong acid? HCN is a weak acid as it does not completely dissociate in a solution and furnishes a very small amount of H+ ions in a solution. The cyanide ion (CN-) binds strongly to its proton making the ionization difficult. The value of acid dissociation constant for HCN is 6.2 X 10-10 and that of the logarithmic constant for HCN is 9.2 while for strong acids the Ka value reaches infinity and the pKa value shall be below 2. The pH of 0.1M HCN solution is 5.2 further indicating it to be a weak acid.

HCN is used in the manufacturing of nylons, plastics, and fumigants. Sodium nitroprusside (Na2[Fe(CN)5NO]·2H2O) when administered intravenously is proven to lower blood pressure.

HCN is highly poisonous in nature and acts very rapidly. It is a systemic poison that acts through inhibition of cytochrome oxidase activity which prevents utilization of oxygen for cellular activities.

Prolonged exposure may even result in death due to respiratory arrest. However, the exposure to HCN is limited to industrial situations or cigarette smoke and combustion products. Normally, it is not considered corrosive.

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Strong and Weak Acids

The acid that almost completely dissociates or ionizes in an aqueous solution is referred to as strong acid. They have a strong tendency to lose a proton. This proton together with water molecule forms hydronium ion:

HA   +   H2O   ——>    H3O+   +    A-

A few examples of strong acids are Hydrochloric Acid (HCl), Sulfuric acid (H2SO4), nitric acid (HNO3), etc. Most strong acids are corrosive in nature but some superacids are an exception to this.

Strong acids have a high value for acid dissociation constant (Ka) and a low value for logarithmic constant (pKa). For example, the Ka and pKa value for HCl is 1.3 X 106 and – 6.3, respectively.

Weak acids, on the other hand, only partially ionize in a solution. Their strength is estimated from dissociation capacity.

Weak acids do not easily lose their proton and even tend to take it back from the solution, therefore their reaction is written as:

HA (aq)   +   H2O      <=======>       H3O+   +   A-

This can also be seen in the case of HCN where CN- ion is so closely associated with its proton that in a solution it reforms the HCN molecule as soon as it comes in contact with a proton hence, not allowing complete dissociation of the acid in the solution.

Few other examples of weak acids are Acetic acid (CH3COOH), Formic acid (HCOOH), etc.

Why is HCN a Weak Acid?

Looking at the dissociation equation for HCN in an aqueous solution, it can be represented as:

HCN  +   H2O  <=====>   H3O+   +    CN-

H3O+ is the conjugate acid, while CN- is the conjugate base generated when the acid dissociates. The reaction is reversible, as indicated by the double arrow.

The cyanide ion is an excellent base that readily accepts the proton, rebuilding the acid molecules that had previously been ionised in the solution.

Furthermore, the carbon atom in CN- is sp hybridised (triple bond) and the nitrogen atom is just mildly electronegative, supporting HCN’s status as a weak acid.

In addition, the cyanide ion is isoelectric. Because the negative charge in CN- is concentrated closer to the carbon atom, it bonds to the proton more firmly, making dissociation more difficult.

What causes acids to be weak or strong?

An acid’s ionisation in an aqueous solution is determined by its polarity, which is determined by the distribution of electrons in a chemical bond.

The polarity of a molecule is determined by the difference in electronegativity of the atoms that make up that molecule, as we already know.

Due to the separation of charges on the chemical bond, the greater the difference in electronegativity of two atoms, the stronger the polarity of a molecule.

When hydrogen is combined with an electronegative atom, it has a tiny positive charge.

The hydrogen ion tends to dissociate from the molecule as the electron density around it decreases, i.e. the electronegativity difference increases, making it a strong acid.

When the electronegativity difference between the hydrogen ion and the other related ion is smaller, as it is in the case of HCN, the hydrogen ion is more difficult to dissociate from the molecule, resulting in the production of strong acids.

Furthermore, because the hydrogen ion is so small, the strength of the bond, and hence the acid’s strength, is determined by the size of the other connected atom.

The potential of the bond drops as the size of the other atom rises, making it easier for hydrogen ions to dissociate from the molecule, resulting in a strong acid.

Acid strength is determined by a number of factors.

• Electronegativity: The polarity of a bond is affected by the electronegativity of the two atoms involved in its creation.

The greater the electronegativity difference, the greater the polarity that allows for ion dissociation, which is a feature of strong acids.

Chlorine, for example, is strongly electronegative, attracting the electrons involved in the chemical interaction towards itself in the case of hydrochloric acid.

As a result, when hydrogen ion is dissolved in an aqueous solution, it easily attaches itself to the water molecule, generating hydronium ion.

• Electrical Charge: A species’ electrical charge influences its acidity since removing a proton from a positively charged entity is simple, possible in the case of a neutral molecule, and challenging in the case of a negatively charged molecule.

• Atomic radius: As previously stated, atomic radius is quite important.

The chemical connection between two atoms joined to form a molecule weakens as the distance between their atomic radiuses grows, resulting in facile ion dissociation and the formation of strong acids.

• Solvent: When it comes to non-aqueous solutions, acidity and basicity have different meanings.

Acetic acid, for example, totally dissociates in liquid ammonia, making it a powerful acid.

The strength of an acid, on the other hand, is normally measured in an aqueous solution.

• Equilibrium: After the dissociation of acid in an aqueous solution (to the degree possible), equilibrium is reached with the conjugate base of acid.

The equilibrium is more skewed towards the product in the case of strong acids.

The conjugate base of a strong acid is known to be a strong base, whereas the conjugate base of a weak acid is known to be a weak base.

Even a strong acid’s conjugate base, on the other hand, is weaker as a base than water.

• Logarithmic Constant (Ka) and Acid Dissociation Constant (Ka) (pKa)

The degree to which the acid has been ionised in the solution is represented by the acid dissociation/ionization constant.

It is a quantitative measure of the strength of an acid in a solution and the equilibrium constant. The acid’s strength is reflected in the numerical value of Ka.

The Ka value of weak acids is lower than that of strong acids, and vice versa. Strong acids, such as HCl and H2SO4, virtually entirely ionise in water, resulting in a Ka value of infinity.

It’s expressed as a fraction of equilibrium concentrations (mol/L).

Ka is frequently represented in terms of the logarithmic constant (pKa), whose value is -log10 (Ka).

The higher the pKa value, the less dissociation and thus the weaker the acid.

The pKa value of strong acids is usually less than -2, while the pKa value of weak acids is usually between -2 and 12. It’s a measurement of how much an acid in a solution has dissociated.

Calculating the Ka and pKa values for acids

As previously stated, an acid’s strength is determined by its ability to give up a proton or generate a hydronium ion in an aqueous solution.

Strong acids readily contribute hydrogen ions, whereas weak acids only partially dissolve in a solution.

The strength of an acid is represented by the numerical value of Ka and pKa. It is computed as follows for monoprotic acid:

Ka = [A-][H+] / [HA] Or Ka = [A-][H3O+] / [HA]

pH, which is a measure of the concentration of free hydrogen ions in a solution, can also be used to calculate Ka.

It has the following representation:

pH = -log [H+] or pH = -log [H30+]

The preceding statement can be rephrased as,

[ H30+] = 10-pH

When the molar concentration of an acid solution is known, the pH can be determined, and the Ka value for the solution may be derived using the equation above.

Percentage dissociation can also be determined as follows:

% Dissociation = ([ A- (aq)]) / [HA (aq)]  *  100


• HCN is a strong acid because it does not dissociate completely in water.

• Protons in strong acids are easily given away in an aqueous solution, but protons in weak acids are more tightly bound.

• The strength of any acid is determined by a variety of factors. Electronegativity, atomic size, electrical charge, solvent, and equilibrium are the terms used to describe them.

• Acidity or acid strength can be expressed in Ka or pKa values, which are computed using the following formula:

Ka = [A-][H+] / [HA]  or  Ka = [A-][H3O+] / [HA]

Read more: MO Diagram, Molecular Geometry, Hybridization, Polarity, and F2 Lewis Structure

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.


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