Table Of Acid Dissociation Constants

In this explainer, nosotros will larn how to write equations for the dissociation constants of acids and bases and calculate their values.

When comparison the strengths of acids and bases, we ofttimes employ qualitative terms similar "stronger" and "weaker." A stronger acid will be able to protonate the cohabit base of a weaker acid.

We too have descriptions of acids and bases similar "weak" and "strong," which relate to their reactions with water. A stiff acid dissociates completely in water, while a weak acid dissociates only partially.

These terms are useful for making simple judgements, but they are not that useful in calculations. Such terms also fail to distinguish between similar types of acids. Are all potent acids equally acidic? And are all strong bases every bit basic? It is difficult to tell without an objective measurement of acerbity or basicity.

We tin utilise acid dissociation constants and base dissociation constants to provide a quantitative measurement of the forcefulness of an acrid or a base.

A dissociation constant is a blazon of equilibrium constant. An equilibrium constant is a number that expresses the natural balance signal of an equilibrium under certain conditions, even when the initial concentrations are different.

In the case of acids and bases, we ofttimes utilize water as our reference substance. A substance that gives protons to water is an acid.

We tin write a chemical reaction to show the dissociation of an acid in h2o:

We tin can simplify this reaction past substituting the hydrogen ion in identify of the hydronium ion while also removing water:

The reaction quotient for this procedure tin can be expressed in terms of the concentrations of , , and :

Once at equilibrium, the reaction caliber is the same as the acid dissociation constant:

Definition: Acid Dissociation Abiding, 𝐾_𝑎

The acid dissociation constant is the equilibrium constant for the reaction of an acid with water, where the acid, , dissociates into and ions.

The preceding equation includes 3 concentration terms, only it can be simplified to include but two. We can make the assumption that considering any 1 molecule has to make one anion as it dissociates and makes i hydrogen ion . The previous reaction quotient can be simplified to the following equation:

We will now use 1 of these equations to determine for one representative weak acid. We will consider ethanoic acrid hither because it is a relatively simple molecule that has the chemical formula. We will consider the state of affairs where a 0.1 M solution of ethanoic acid has a value of G.

The following chemical equation describes the equilibrium that is established when ethanoic acid partially dissociates in water. The chemical equation has ethanoic acid on its left-hand side and and ethanoate ions on its right-hand side.

The acid dissociation constant for this equilibrium reaction can exist expressed in terms of reactant and production concentrations:

The equation has 3 concentration terms, but it tin can be simplified to include but two. We tin can assume that at the point of equilibrium. This enables us to state that is equal to divided by . The value of for ethanoic acrid can be calculated with the following equation:

Nosotros now take to insert the and values to determine . Information technology was previously stated that . It was also stated that . The value for this reaction can therefore exist calculated if we make the denominator equal to 0.one K and the numerator equal to the square of M:

The event of this adding is

It is important to realize here that we made an unstated assumption while we were determining the value of for ethanoic acrid. Nosotros assumed that the equilibrium concentration of a weak acid is equal to its starting concentration. This assumption is generally assumed to be fair because weak acids undergo relatively little dissociation when they are immersed in water.

As with any equilibrium constant, the greater the value of , the closer the position of equilibrium to the products. Nosotros also consider the forcefulness of an acrid based on the concentration of hydrogen ions in solutions of the acid and express this using pH.

Equation: pH in terms of Hydrogen Ion Concentration

The equation for pH in terms of hydrogen ion concentration is

The higher the value of for an acrid, the greater the hydrogen ion concentration in solutions of the acid. If one acid has a higher than another, it is considered more acidic.

This result might seem strange every bit a pure acid will not have whatsoever gratuitous ions. However, we can use every bit a measure of how acidic a solution an acid would produce. This allows us to talk about the acidity of an acid without talking near a solution.

The following table compares the values of some representative weak acids. The acids are listed in terms of decreasing the value of their acrid dissociation constants. This sentence could be rephrased to country that the acids are listed in terms of decreasing acidity. Information technology is clear that sulfurous acid tends to be more acidic than lots of other weak acids like carbonic acid and boric acid .

Acid	Chemical Formula	Acid Dissociation Constant (M)
Sulfurous acrid
Hydrofluoric acrid
Nitrous acid
Benzoic acid
Carbonic acid
Boric acid

Example one: Identifying the Strongest Acid of a Set Using 𝐾_𝑎 Values

The given table shows the values for a pick of acids. Which acid is the strongest?

Acid	Chloroethanoic acid	Benzoic acid	Lactic acid	Hydrofluoric acid
(mol/Fifty)

Answer

The acid dissociation constant for a given acrid is the equilibrium constant for the reaction of the acid with water. This tin exist expressed as the dissociation of the acrid in aqueous solution:

The tabular array shows iv acids and their values in units of moles per litre (mol/L or Grand). An acid that dissociates more than another will have a higher value.

The more an acrid dissociates, the higher the amount of produced when information technology is added to water.

The college the concentration of in a solution of acid, the greater the acerbity of the solution.

Pace by footstep, this leads to the conclusion that acids with higher values are those that produce the most-acidic solutions (at the same total concentration of acid).

We can put the acids in order of their values:

Of these four acids, the one with the highest value is chloroethanoic acid. This means that chloroethanoic acid is the strongest acid of the ready.

We tin do a similar matter with bases. A substance that removes protons from h2o is a base:

We cannot simplify this reaction in the same way nosotros did with an acid, so we tin move onto the reaction quotient:

Remember that nosotros exercise not include water in the reaction quotient because it is a liquid.

At equilibrium, the reaction caliber is the same as the base dissociation constant:

Definition: Base of operations Dissociation Constant, 𝐾_𝑏

The base dissociation abiding is the equilibrium constant for the reaction of a base of operations with water, where the base, , reacts with to produce and ions.

It is advisable to determine for an ethanoate ion solution here because we have already adamant for an ethanoic acid solution. We will consider the situation where a 0.1 K solution of ethanoate ions has a of M.

The following chemical equation describes the equilibrium that is established between ethanoate ions and water:

It is important to note here that 1 ethanoic acrid molecule is generated every time a hydrogen ion breaks from a water molecule and pairs with 1 ethanoate ion. This inference allows us to make up one's mind using just two concentration values.

The base dissociation constant for this equilibrium reaction can be expressed in terms of reactant and production concentrations:

This equation has three concentration terms, just it can be simplified to include just ii. We can assume here that considering one water molecule produces one hydroxide ion when information technology donates a hydrogen ion to one ethanoate ion. The simplified equation has the term as its denominator and the square of the term as its numerator:

We can utilise the known and values to determine for the ethanoate ion solution. The following equation shows the result of putting the and values into the simplified equation:

The consequence of this adding is

We fabricated an unstated assumption here while we were determining for the ethanoate ion solution. We assumed the spontaneous dissociation of water molecules is negligible. This is considered to be a reasonable assumption. At least a few water molecules in any aqueous solution should spontaneously dissociate, merely the vast majority should not.

In a similar mode to acrid dissociation constants, the greater the value of a base of operations dissociation constant, , the closer the position of equilibrium to the products. We also consider the strength of a base by the concentration of hydroxide ions in solutions of the base, and nosotros express this using pOH (or using pH, where at ).

The higher the value of for a base of operations, the greater the hydroxide ion concentration in solutions of the base. If one base of operations has a higher than another, it is considered more basic.

Nosotros employ equally a mensurate of how basic a solution a base of operations would produce. This allows united states to talk about the basicity of a base without talking about a solution.

Instance 2: Calculating the Concentration of OH⁻ Ions in a Solution of Pyridine given Its 𝐾_𝑏

What is the concentration of ions in a 0.15 mol/Fifty solution of pyridine? The value for pyridine is mol/L. Requite your answer to 2 decimal places.

Answer

Pyridine is an organic chemical compound with a chemic formula of . Its structure is shown below:

Pyridine is basic. It can react with water to produce ions:

is the symbol for a base dissociation constant. The expression for of pyridine tin can be written equally shown:

In a solution of pyridine, h2o is a liquid (information technology is the solvent), so we do not include it in the expression.

Nosotros do non really demand to know anything about pyridine to answer this question. All we need to know is the general expression for :

When a base reacts with water, 1 ion is produced for each produced. Therefore, and

A possible fault here is to assume that refers to the starting concentration of base (in this example, 0.15 M); however, this is really the concentration of unreacted base at equilibrium.

On the other hand, the value of of pyridine is very low ( One thousand), indicating that the reaction favors the reactants highly.

The equilibrium concentration of base and the starting concentration of base are going to be about identical, so nosotros tin can brand the approximation

Therefore,

We tin rearrange this to give

We can insert the values from the question to go our terminal answer:

In scientific notation, to 2 decimal places, the concentration of ions in a 0.fifteen M solution of pyridine is K.

Instance 3: Identifying the Weakest Base in a Gear up from Their 𝐾_𝑏 Values

The power of a base to accept protons could exist deduced from its base dissociation constant . The base dissociation constant of some bases is listed in the table below.

Base Name	A	B	C	D	Due east

Which of the bases listed in the table is the weakest?

Answer

A base dissociation constant describes this equilibrium:

We ignore water when amalgam the expression because it is a liquid.

A base with a high value volition produce more basic solutions than a base with a depression value (at the same full concentration).

Therefore, a weak base of operations will accept a low value.

We can put the bases in club:

The weakest base of operations of the set is the one with the lowest value, which is E.

Since the expressions for and both have two concentration terms in the numerator and one in the denominator, nosotros can assign them units of M (or mol/50 or mol/dm³ ). Formally, equilibrium constants do non have units (they are dimensionless); all the same, they are often presented as if they have units because of the simplifications made at this level. It is therefore common to come across values of given with either no units or units of K:

Key Points

Acid dissociation constants and base dissociation constants are equilibrium constants for the reactions of acids and bases with water.
The greater the value of an acid'south , the stronger the acid.
The greater the value of a base's , the stronger the base.
The full general grade of is
The general grade of is