Chemistry Keypoints: Acids / Basis / Salts; Acids, bases, and salts are important topics in chemistry that are essential for understanding a wide range of chemical reactions and processes.
In this note, we will cover the general characteristics and properties of acids, bases, and salts, acid/base indicators, the basicity of acids, normal, acidic, basic, and double salts, preparation of salts by neutralization, precipitation and action of acids on metals, oxides, and trioxocarbonate (IV) salts, qualitative comparison of the conductances of molar solutions of strong and weak acids and bases, the relationship between conductance and amount of ions present, pH and pOH scale and simple calculations, acid/base titrations, and hydrolysis of salts.
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Chemistry Keypoints: Acids / Basis / Salts
(a) General characteristics and properties of acids, bases, and salts:
Acids are substances that, when dissolved in water, produce hydrogen ions (H+). Bases, on the other hand, produce hydroxide ions (OH-) when dissolved in water. Salts are compounds formed from the neutralization reaction between an acid and a base. Acids are sour in taste, while bases are bitter and have a slippery feel. Salts have characteristic crystalline structures, are usually soluble in water, and have a neutral pH.
Acid-base indicators are substances that change color depending on the acidity or basicity of a solution. The basicity of an acid refers to the number of hydrogen ions it can donate, while the acidity of a base refers to the number of hydroxide ions it can donate. Normal salts are formed by the complete neutralization of an acid and a base, acidic salts are formed by the partial neutralization of an acid, basic salts are formed by the partial neutralization of a base, and double salts are formed from the combination of two or more simple salts.
Organic acids, such as ethanoic, citric, and tartaric acids, are naturally occurring acids that contain carbon atoms in their molecules. Alums are examples of double salts. Salts can be prepared by neutralization, precipitation, or the reaction of acids on metals. Oxides and trioxocarbonate (IV) salts are important examples of salts.
(b) Qualitative comparison of the conductances of molar solutions of strong and weak acids and bases, the relationship between conductance and amount of ions present:
Strong acids and bases are completely ionized in water, while weak acids and bases are only partially ionized. As a result, solutions of strong acids and bases conduct electricity more effectively than solutions of weak acids and bases. The conductance of a solution is directly proportional to the number of ions present in it.
(c) pH and pOH scale; Simple calculations:
The pH scale is a measure of the acidity or basicity of a solution and ranges from 0 to 14. A pH of 7 is neutral, a pH below 7 is acidic, and a pH above 7 is basic. The pOH scale is similar but measures the concentration of hydroxide ions instead of hydrogen ions. The pH and pOH scales are related by the equation pH + pOH = 14. Simple calculations involve using the formula pH = -log[H+] to find the pH of a solution, and pOH = -log[OH-] to find the pOH of a solution.
(d) Acid/base titrations:
An acid-base titration is a laboratory technique used to determine the concentration of an unknown acid or base by reacting it with a known amount of a standard acid or base of a known concentration. The endpoint of the titration is determined using an indicator that changes color at the equivalence point when the moles of acid and base are equal.
(e) Hydrolysis of salts: Principle Simple examples such as NH4Cl, AlCl3, Na2CO3, and CH3COONa:
Hydrolysis of salts involves the reaction of salt with water to produce an acidic or basic solution. For example, the salt NH4Cl undergoes hydrolysis to produce an acidic solution, while Na2CO3 undergoes hydrolysis to produce a basic solution. AlCl3 and CH3COONa also undergo hydrolysis. The nature of the resulting solution depends on the relative strengths of the acid and base components of the salt.
Chemistry Keypoints: Acids / Basis / Salts
(i) Distinguish between the properties of acids and bases:
Acids and bases have different properties that distinguish them from each other. Acids taste sour, turn blue litmus paper red, and react with metals to release hydrogen gas. They also have a pH of less than 7. Bases, on the other hand, taste bitter, turn red litmus paper blue, and feel slippery to the touch. They also have a pH greater than 7.
(ii) Identify the different types of acids and bases:
Acids can be classified as organic or inorganic. Organic acids are derived from living organisms and contain carbon. Examples include acetic acid, citric acid, and lactic acid. Inorganic acids, on the other hand, do not contain carbon. Examples include hydrochloric acid, sulfuric acid, and nitric acid. Bases can be classified as alkalis or non-alkalis. Alkalis are soluble in water and produce hydroxide ions when dissolved. Examples include sodium hydroxide, potassium hydroxide, and calcium hydroxide. Non-alkali bases do not produce hydroxide ions when dissolved.
(iii) Determine the basicity of acids:
The basicity of an acid is determined by the number of hydrogen ions (H+) it can donate. Acids that can donate only one hydrogen ion are known as monobasic acids. Examples include hydrochloric acid and acetic acid. Acids that can donate two hydrogen ions are known as dibasic acids. Examples include sulfuric acid and carbonic acid. Acids that can donate three hydrogen ions are known as tribasic acids. Examples include phosphoric acid and citric acid.
(iv) Differentiate between acidity and alkalinity using acid/base indicators:
Acid/base indicators are used to differentiate between acidity and alkalinity. Acidic solutions turn blue litmus paper red, while alkaline solutions turn red litmus paper blue. A universal indicator is a mixture of several indicators and can be used to determine the approximate pH of a solution.
(v) Identify the various methods of preparation of salts:
Salts can be prepared by neutralization, precipitation, and the action of acids on metals. Neutralization involves reacting an acid with a base to form salt and water. Precipitation involves mixing two solutions to form a solid precipitate, which is then filtered and dried to form a salt. The action of acids on metals involves reacting an acid with a metal to form a salt and hydrogen gas.
(vi) Classify different types of salts:
Salts can be classified as normal, acidic, basic, or double salts. Normal salts are formed by the complete replacement of the hydrogen ions of acid by metal ions or ammonium ions. Acidic salts are formed by the partial replacement of the hydrogen ions of acid by metal ions or ammonium ions. Basic salts are formed by the partial replacement of the hydroxide ions of a base by metal ions or ammonium ions. Double salts are formed by the combination of two simple salts.
(vii) Relate the degree of dissociation to the strength of acids and bases:
The degree of dissociation of an acid or base is related to its strength. Strong acids and bases dissociate completely in water, while weak acids and bases dissociate only partially. The degree of dissociation can be calculated by the equation: degree of dissociation = (amount dissociated / initial concentration) x 100%.
(viii) Relate degree of dissociation to conductance:
The degree of dissociation of an acid or base is also related to its conductance. Strong acids and bases have higher conductance than weak acids and bases because they dissociate completely, producing more ions in the solution.
(xiii) Balance equations for the hydrolysis of salts:
The hydrolysis of salts involves the reaction of a salt with water to produce an acidic or basic solution. The products of the hydrolysis reaction depend on the nature of the cation and anion in the salt and their respective acid and base strengths. The hydrolysis of salts can be classified as acidic, basic or neutral.
The general equation for the hydrolysis of a salt is:
AB + H2O ⇌ AH + BOH
- If the cation of the salt is from a weak base and the anion is from a strong acid, then the solution is acidic. For example, NH4Cl hydrolyses to form HCl and NH4OH: NH4Cl + H2O ⇌ NH4OH + HCl.
- If the cation of the salt is from a strong base and the anion is from a weak acid, then the solution is basic. For example, Na2CO3 hydrolyses to form NaOH and H2CO3: Na2CO3 + H2O ⇌ 2NaOH + H2CO3.
- If both the cation and anion are from strong acids and bases respectively, then the salt is neutral and does not undergo hydrolysis.
(xiv) Deduce the properties (acidic, basic, neutral) of the resultant solution:
The properties of a solution after a reaction depends on the nature of the reactants and products.
- If the resultant solution has a pH less than 7, it is acidic. An acidic solution has an excess of H+ ions.
- If the resultant solution has a pH greater than 7, it is basic. A basic solution has an excess of OH- ions.
- If the resultant solution has a pH of 7, it is neutral. A neutral solution has equal concentrations of H+ and OH- ions.
The determination of the pH of a solution is crucial in deducing the properties of the solution. pH can be measured using pH meters, pH indicators or calculated based on known concentrations of H+ and OH- ions.
