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Acid strength can be defined as the measure of an acid’s ability to lose its H+ ions. Acid strength is symbolised by the formula HA. Some of the common properties of acids are that they have a pH lower than 7, turn blue litmus paper red, have a sour taste and react with alkalis to produce salts.
- A few examples of strong acid are Hydrochloric Acid (HCl), Nitric Acid (HNO3), Sulphuric Acid (H2SO4). The dissociation of a strong acid is complete in a solution, except when in the most concentrated solutions. Thus, HA = H+ + A-.
- One example of Weak acid is Acetic Acid (CH3COOH). A weak acid is known to be partly dissociated with the acid which is dissociated and its undissociated product in equilibrium with one another. Thus, HA ⇌ H+ + A-.
- Different acids come with different acid strengths. An acid which has a greater degree of dissociation starts to behave as a stronger acid.
- Thus, the weaker the HA bond’s strength, the stronger will be the acid.
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Key Terms: Acid Strength, Acids, Bases, Dissociation, Acid-Base Pairs, pKa, Glacial Acetic Acid, Oxidation, H-A Bond, Weak Acids, Strong Acids, pH Value
What is Acid Strength?
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Acid Strength can be defined as:
| “The measure of the ability of an acid to lose the H+ ion it has.” |
- A strong acid's dissociation in solution is nearly complete, except for its most concentrated solutions. Hydrochloric acid, perchloric acid, nitric acid, and sulfuric acid are examples of strong acids.
- In the presence of both the undissociated acid and its dissociation products in the solution, a weak acid partially dissociates. For instance, acetic acid. The substituent effects help to determine the strength of a weak organic acid. The oxidation state of the atom to which the proton may connect also affects the strength of an inorganic acid.
- Acid strength is affected by the solvent. For example, hydrogen chloride (HCl) is a strong acid in an aqueous solution, but it is a weak acid when dissolved in glacial acetic acid.
Strength of Acids
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Acid Strength Order
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The order of Acidic strength, when comparing the elements in the same periodic table’s group, of the H-A bond is considered more essential in determining the acidity than the polarity.
With the size of A increasing, while descending a group, the H-A bond strength decreases down, causing the acid strength to rise. For instance, the acid strengths of hydrides present in group 17 of the period table increase in the following order:
| HF < HCl < HBr < HI |
pH and Acid-Base Ratio Calculation
Factors Determining Acid Strength
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As previously stated, different acids have varying acid strengths. A stronger acid has a higher degree of dissociation. Let's look at the things that influence the acid's strength. The degree of dissociation of an acid is determined by the two criteria listed below.
- Strength of H-A bond
- Polarity of H-A bond
In general, the weaker the H-A bond, the stronger the acid. Similarly, the stronger the acid, the higher the polarity of the H-A bond. Both factors make it easier for acid molecules to dissociate into H+ and A-, increasing acidity.
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Strong and Weak Acids
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Depending upon the dissociation of HA bonds, acids can be classified as strong and weak acids.
Strong Acids
A strong acid dissociates because of a chemical reaction as shown below:
| HA + S \(\rightleftharpoons\) SH+ + A− |
Here, S stands for a solvent molecule, such as water or DMSO, to the point where the concentration of the undissociated species HA is too low to be detected.
For practical purposes, a strong acid can be separated. Acid is an example of a strong acid.
HCl → H+ + Cl− (in aqueous solution)
A strong acid is defined as an acid with a pKa value less than or equal to around -2. This is explained by the levelling effect, which is caused by the extremely high buffer capacity of solutions with a pH of 1 or below.
Strong and Weak Acids
Weak Acids
When dissolved in a solvent, a weak acid is a chemical that partially dissociates. There is an equilibrium in the solution between the acid, HA, and the dissociation products.
HA \(\rightleftharpoons\) H+ + A-
Solvent, such as water, is omitted from this formula when the concentration is unchanged by the method of Acid Dissociation. The strength of a weak acid can also be evaluated by acid dissociation. Thus, the acid strength formula for weak acids can be shown as:
| \(K_{a}=\frac{\left[H^{+}\right]\left[A^{-}\right]}{[H A]} \) |
The pH of a weak acid solution is determined by both its Ka value and its concentration. Ethanoic acid and Hypophosphorous acid are two examples of weak acids. Because it may lose two protons and react with two molecules of a base, an acid like ethanedioic acid (HOOC–COOH) is said to be dibasic. Similarly, H3PO4 is a tribasic phosphoric acid.
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Factors Affecting Acid Strength
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Some of the factors that affect the acid strength include:
- One of the factors that affect the acid strength is the H-A bond’s strength. Thus, it can be said that the weaker the bond is going to be, the lesser energy will be needed to break it, causing the acid to be strong.
- The H-A bond’s polarity can affect its acid strength. Assuming that the bond is highly polar, the proton potentially leaves the molecule more readily, thus making it a strong acid.
- Considering that we compare the acid strengths of elements present in the same row, more emphasis is given to the polarity of the H-A bond.
- Acid strength is also affected by the atomic size of A. With the atom growing in size, the bond starts to get weaker. Therefore, acid strength increases.
Also Read: Acid, Bases and Salt important questions
Things to Remember
- The ability of an acid to lose its H+ ion is measured by its acid strength.
- Hydrochloric acid, perchloric acid, nitric acid, and sulfuric acid are some strong acids examples.
- The weaker the H-A bond is, the stronger the acid is going to become. However, the stronger the acid, the higher the polarity of the H-A bond.
- A strong acid can be defined as an acid with a pKa value less than or equal to around -2. This is explained by the levelling effect caused by the extremely high buffer capacity of solutions with a pH of 1 or below.
- The pH of a weak acid solution is determined by both its Ka value and its concentration. Ethanoic acid and hypophosphorous acid are two examples of weak acids.
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Sample Questions
Ques. Is Acetic Acid a Weak or Strong Acid? Why? (3 marks)
Ans. Acetic acid is known to be a weak acid.
Acetic acid, CH3COOH, is a weak acid because it is largely present in a solution as entire CH3COOH molecules, with only minor amounts of H+ and CH3COO ions. CH3COOH (aq)\(\rightleftharpoons\)H+(aq) +CH3COO−(aq) Acetic acid has an acid dissociation constant (Ka) of 1.76 x 105. This essentially means that the ratio of CH3COOH concentration to H+ and CH3COO ion concentration is 1:1.76105:1.
→ {[H+] [CH3COO−] / [CH3COOH]} =1.76 × 10−5
This also suggests that ethanoic acid is weak, as strong ions ionize practically completely.
Ques. What decides whether an acid is strong or weak? (1 mark)
Ans. Strong acids have ions in the solution, implying that the bonds that hold H and A together are weak. Strong acids easily break down into ions. Weak acids exist in the form of molecules in solution with ions. As a result, the bonds that connect H and A must be robust.
Ques. Is it possible for a strong acid and its conjugate base to function as a buffer? (1 mark)
Ans. A buffer is a solution made up of a weak acid and its conjugate base, or a weak base and its conjugate acid. Buffers control the pH by reacting with any added acid or base. As a result, when a strong acid and its conjugate base are present, the conjugate base cannot operate as a buffer.
Ques. HBR vs HF, which is the stronger acid? Why? (2 marks)
Ans. The acidity of HBr is higher.
The number of H+ atoms present in a solution determines its acidity.
The amount of H+ atoms present is proportional to the degree of dissociation a molecule undergoes in the solution. The electronegativity (how much they desire that sweet electron) of the atoms within the molecules determines dissociation.
Florine is the stingiest atom we've found, desiring all of the electrons it can obtain, and once she has them, she won't let go lightly. Bromine, on the other hand, is more likely to share, allowing it to dissociate more quickly.
Ques. Why is phenolphthalein ineffective as a titrator of a weak base against a strong acid? (1 mark)
Ans: Because, as the names suggest, a titration's equivalence point will result in an acid salt. The acid's stronger acidity overcomes the base's basicity. As a result, it will occur with a pH lower than 7. At this pH, phenolphthalein is colourless, and it will be colourless at pH 8 before you reach the end. You'll need something with a distinct colour change near the pH of neutralization.
Ques. What characteristics do strong acids have? (4 marks)
Ans. An acid substance dissociates into its ions in a solvent to form H+ ions, according to the Arrhenius Acid-Base hypothesis. The term "strong acid" refers to an acid that has entirely dissociated into its ions in a solvent, resulting in H+ ions. HCL, H2SO4, HNO3, HBr, HI, HCLO4, and other strong acids are examples.
- Acids with a high acid ionization value (Ka) have a high acid ionization value.
- When a solution is diluted, the acid strength does not change; only the concentration or PH does.
- Strong acids have a lower ph because they have more H+ ions.
- Metals react more violently with strong acids.
- Strong acids have a lot of conductivity.
- Ionic or extremely polar bonding is seen in strong acids.
- Polyprotic acids are strong acids.
- Acids that can lose more than one proton are known as polyprotic acids.
- Except for carbonic and sulfuric acids, all mineral acids are strong and inorganic.
- The equilibrium of strong acids is to the right.
Ques. What causes the heat of neutralization between a strong acid and a strong base to be constant? (3 marks)
Ans. For a strong acid and a strong base, the usual heat of neutralization is constant. The strong acid HCl, for example, is already ionized as H+ and Cl- ions in a polar solvent like water (default). Strong bases, such as NaOH, are also ionized as OH- ions and Na+ ions.
The Na+ and Cl- ions remain as ions in the new mixture when mixed in standard concentrations, while the H+ and OH- ions combine to produce water. H+ and OH- combine to form H2O. Because the quantities of H+ and OH- are similar in standard concentrations, the creation of water is exothermic and releases the same amount of heat for any strong acid and base.
Ques. Which metals are unaffected by powerful acids? (1 mark)
Ans. Weak acids have little effect on gold, silver, or copper. Gold is a non-reactive metal that reacts with only a few acids. Silver and copper are the same way. Platinum behaves similarly to gold, with aqua regia being the acid with which it reacts the most. Strong hydrofluoric acid, for example, is extremely reactive with most metals. Acid chemistry is a distinct field of study.
Ques. List the differences between hydra acids and oxyacids? (3 marks)
Ans. Hydra acid is a type of acid found in nature.
- It doesn't have any oxygen atoms in it.
- These are hydrogen and halogen binary compounds.
- It's called hydra acid because it dissolves in water and produces acid.
- H2Se, HCL, and H2S are among the examples.
Oxyacid:
- Are made up of oxygen, hydrogen, and one other element
- At least one hydrogen atom is linked to oxygen here.
- For example, HClO4, H2SO4, and so on.
Ques. Why is H3PO4 such a weak acid? Show its chemical structure. (5 marks)
Ans. The chemical structure of H3PO4 is:
When we examine this structure, we can see that phosphoric acid is weak due to two fundamental factors.
Phosphorus does not have enough electronegative properties. A proton must be able to easily fall off for H3PO4 to be a strong acid. There must be a pulling force that changes electron density away from the H atoms linked to the lone-pair-possessed oxygens so that they can readily convert to H+ ions for a proton to fall off. Phosphorus cannot do this, but nitrogen can, which is why HNO3 is so powerful.
There are insufficient free O atoms to remove electrons from the OH bonds. O atoms are extremely electronegative, and they can move the electron density away from hydrogens on their own. In compounds like HNO3, there is one OH bond for every two N-O bonds. Along with the increased electronegativity of N compared to P, the extra oxygen in HNO3 makes it a strong acid, whereas H3PO4 is weak.
When H3PO4 loses a proton, it generates H2PO4-, which lacks enough resonance stabilization to be useful. A molecule's stability is proportional to the number of resonance structures it has. The higher the likelihood for acid to lose a proton and convert it into its conjugate base, the more stable the conjugate base is. H2PO4- has two resonance structures, whereas NO3- has three (which you can draw out if you like), therefore HNO3 is more willing to lose a proton than H3PO4.
Because of these three factors, H3PO4 is classified as a weak acid rather than a strong acid.
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