Acids, Bases And Salts (Properties)

Understanding The Chemical Properties Of Acids And Bases

Acids and bases are fundamental classes of chemical compounds with distinct properties that play crucial roles in chemistry and everyday life. Understanding their reactions is key to many chemical processes.

Acids And Bases In The Laboratory

In a laboratory setting, acids and bases are commonly encountered and used for various purposes, from synthesis to analysis. They are typically identified by their characteristic reactions.

How Do Acids And Bases React With Metals?

Acids Reacting with Metals:

Acids react with active metals (metals above hydrogen in the reactivity series) to produce hydrogen gas and a metallic salt.

General Reaction:

$$\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen gas} (\text{H}_2)$$

Example:

Zinc reacting with dilute hydrochloric acid:

$$Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$$

Bases Reacting with Metals:

Most bases do not react with metals. However, amphoteric metals (metals that can react with both acids and bases, such as Zinc, Aluminum, and Lead) can react with strong bases to produce hydrogen gas and a metallic salt (often a complex salt).

Example:

Zinc reacting with sodium hydroxide solution:

$$Zn(s) + 2NaOH(aq) + 2H_2O(l) \rightarrow Na_2[Zn(OH)_4](aq) + H_2(g)$$

(Sodium tetrahydroxozincate(II))

How Do Metal Carbonates And Metal Hydrogencarbonates React With Acids?

Reaction with Metal Carbonates: Acids react with metal carbonates to produce a salt, carbon dioxide gas, and water.

General Reaction:

$$\text{Acid} + \text{Metal Carbonate} \rightarrow \text{Salt} + \text{Carbon dioxide} (\text{CO}_2) + \text{Water} (\text{H}_2\text{O})$$

Example:

Hydrochloric acid reacting with calcium carbonate:

$$2HCl(aq) + CaCO_3(s) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$$

Reaction with Metal Hydrogencarbonates (Bicarbonates): Acids react with metal hydrogencarbonates to produce a salt, carbon dioxide gas, and water.

General Reaction:

$$\text{Acid} + \text{Metal Hydrogencarbonate} \rightarrow \text{Salt} + \text{Carbon dioxide} (\text{CO}_2) + \text{Water} (\text{H}_2\text{O})$$

Example:

Hydrochloric acid reacting with sodium hydrogencarbonate:

$$HCl(aq) + NaHCO_3(s) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g)$$

Observation: The release of carbon dioxide gas is observed as effervescence (fizzing).

How Do Acids And Bases React With Each Other?

Neutralization Reaction: Acids and bases react with each other in a process called neutralization. In this reaction, the acid and base neutralize each other's properties.

General Reaction:

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

Example:

Sodium hydroxide (a base) reacting with hydrochloric acid (an acid):

$$NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$$

Mechanism: In aqueous solutions, this reaction is essentially the combination of hydrogen ions ($H^+$ from the acid) and hydroxide ions ($OH^-$ from the base) to form water.

$$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$

Reaction Of Metallic Oxides With Acids

Metallic Oxides: Oxides of metals are generally basic in nature.

Reaction: Basic metallic oxides react with acids to produce a salt and water, similar to the reaction of a base with an acid.

General Reaction:

$$\text{Acid} + \text{Metallic Oxide} \rightarrow \text{Salt} + \text{Water}$$

Example:

Hydrochloric acid reacting with copper(II) oxide:

$$2HCl(aq) + CuO(s) \rightarrow CuCl_2(aq) + H_2O(l)$$

Reaction Of A Non-metallic Oxide With Base

Non-metallic Oxides: Oxides of non-metals are generally acidic in nature (acidic oxides).

Reaction: Acidic non-metallic oxides react with bases to produce a salt and water, similar to the reaction of an acid with a base.

General Reaction:

$$\text{Base} + \text{Non-metallic Oxide} \rightarrow \text{Salt} + \text{Water}$$

Example:

Calcium hydroxide (a base) reacting with carbon dioxide (an acidic oxide):

$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$$

Note: This reaction is also observed when carbon dioxide is passed through limewater (calcium hydroxide solution), causing it to turn milky due to the formation of insoluble calcium carbonate.

What Do All Acids And All Bases Have In Common?

While acids and bases exhibit diverse chemical properties, their behavior in aqueous solutions points to common underlying chemical species responsible for these properties.

What Happens To An Acid Or A Base In A Water Solution?

Arrhenius Definition of Acids and Bases: Svante Arrhenius proposed a definition based on the behavior of substances in water:

Acids: An acid is a substance that dissociates in water to produce hydrogen ions ($H^+$). In reality, hydrogen ions are highly reactive and combine with water molecules to form hydronium ions ($H_3O^+$). So, an acid increases the concentration of $H_3O^+$ ions in aqueous solution.
Bases: A base is a substance that dissociates in water to produce hydroxide ions ($OH^-$). So, a base increases the concentration of $OH^-$ ions in aqueous solution.

Commonality in Acids:

When an acid dissolves in water, it dissociates to release $H^+$ ions, which then readily form $H_3O^+$ ions:

$$HA(aq) \rightarrow H^+(aq) + A^-(aq)$$ $$H^+(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq)$$

Therefore, all acids produce hydrogen ions (or hydronium ions) when dissolved in water. This commonality is responsible for the characteristic acidic properties, such as tasting sour, turning blue litmus red, and reacting with active metals to produce hydrogen gas.

Commonality in Bases:

When a base dissolves in water, it either dissociates to release $OH^-$ ions (if it's an ionic hydroxide like $NaOH$) or reacts with water to produce $OH^-$ ions (if it's a base like ammonia, $NH_3$).

Dissociation of Ionic Hydroxides:

$$BOH(aq) \rightarrow B^+(aq) + OH^-(aq)$$

Reaction of Ammonia with Water:

$$NH_3(g) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$$

Therefore, all bases produce hydroxide ions when dissolved in water. This commonality is responsible for the characteristic basic properties, such as tasting bitter, feeling slippery, turning red litmus blue, and reacting with acids.

Summary of Common Properties (Arrhenius Perspective):

Acids have in common: The ability to produce $H_3O^+$ ions in water.

Bases have in common: The ability to produce $OH^-$ ions in water.

How Strong Are Acid Or Base Solutions?

The strength of an acid or base refers to the extent to which it dissociates or ionizes in water to produce $H_3O^+$ or $OH^-$ ions, respectively. This extent of ionization is crucial in determining the properties of the solution.

Importance Of pH In Everyday Life

pH Scale: The pH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It is related to the concentration of hydronium ions ($H_3O^+$).

Definition: pH is defined as the negative logarithm (base 10) of the hydronium ion concentration:

$$pH = -\log_{10}[H_3O^+]$$

Similarly, pOH is defined as:

$$pOH = -\log_{10}[OH^-]$$

And the relationship between pH and pOH at 25°C is:

$$pH + pOH = 14$$

pH Values and Acidity/Basicity:

Acidic Solution: pH < 7 (high $[H_3O^+]$, low $[OH^-]$)
Neutral Solution: pH = 7 ( $[H_3O^+] = [OH^-]$ )
Basic Solution: pH > 7 (low $[H_3O^+]$, high $[OH^-]$)

Strength of Acids and Bases:

Strong Acids: Acids that dissociate almost completely in water, producing a high concentration of $H_3O^+$ ions. Examples: HCl, $HNO_3$, $H_2SO_4$. They have very low pH values.
Weak Acids: Acids that dissociate only partially in water, producing a lower concentration of $H_3O^+$ ions. Examples: Acetic acid ($CH_3COOH$), Carbonic acid ($H_2CO_3$). They have pH values greater than strong acids but still below 7.
Strong Bases: Bases that dissociate almost completely in water, producing a high concentration of $OH^-$ ions. Examples: $NaOH$, $KOH$. They have very high pH values.
Weak Bases: Bases that react with water only partially to produce $OH^-$ ions. Examples: Ammonia ($NH_3$). They have pH values lower than strong bases but still above 7.

Importance of pH in Everyday Life:

Human Body: Blood pH must be maintained within a very narrow range (around 7.35-7.45) for proper functioning. Enzymes and biological processes are pH-sensitive.
Agriculture: Soil pH affects nutrient availability and plant growth.
Food and Beverages: The acidity of foods (e.g., citrus fruits, vinegar) affects taste and preservation. Many beverages are acidic.
Cleaning Products: Many cleaning agents are either acidic (e.g., toilet bowl cleaners) or basic (e.g., oven cleaners, soaps).
Environment: Acid rain can significantly lower the pH of lakes and rivers, harming aquatic life.
Industry: pH control is critical in many industrial processes, such as chemical manufacturing and water treatment.

More About Salts

Salts: Salts are ionic compounds formed from the reaction of an acid and a base (neutralization reaction). They are typically formed by the cation of a base and the anion of an acid.

Family Of Salts

Definition: A family of salts consists of salts that have a common cation or a common anion.

Examples:

Family of Sodium Salts: $NaCl$, $Na_2SO_4$, $NaNO_3$, $CH_3COONa$. All have the common cation $Na^+$.
Family of Chloride Salts: $NaCl$, $KCl$, $MgCl_2$, $AlCl_3$. All have the common anion $Cl^-$.

Significance: Understanding salt families helps in predicting their properties and in designing synthetic routes. For example, salts in the same family often exhibit similar solubility trends.

Ph Of Salts

Formation of Salts: Salts are formed from the reaction of an acid and a base.

pH of Salt Solutions: The pH of a salt solution depends on the strength of the parent acid and base from which the salt was formed.

Salt of Strong Acid and Strong Base: These salts are neutral. Neither the cation nor the anion undergoes hydrolysis (reaction with water). The solution has a pH of 7 (at 25°C).

Examples: $NaCl$ (from $NaOH$ + $HCl$), $KNO_3$ (from $KOH$ + $HNO_3$), $Na_2SO_4$ (from $NaOH$ + $H_2SO_4$).

Salt of Weak Acid and Strong Base: These salts are basic. The cation is derived from a strong base and is neutral. The anion is derived from a weak acid and undergoes hydrolysis, producing hydroxide ions ($OH^-$). The solution has a pH greater than 7.

Hydrolysis: $A^-(aq) + H_2O(l) \rightleftharpoons HA(aq) + OH^-(aq)$

Examples: Sodium acetate ($CH_3COONa$, from $NaOH$ + $CH_3COOH$), Potassium carbonate ($K_2CO_3$, from $KOH$ + $H_2CO_3$).

Salt of Strong Acid and Weak Base: These salts are acidic. The cation is derived from a weak base and undergoes hydrolysis, producing hydronium ions ($H_3O^+$). The anion is derived from a strong acid and is neutral. The solution has a pH less than 7.

Hydrolysis: $B^+(aq) + H_2O(l) \rightleftharpoons B(aq) + H_3O^+(aq)$

Examples: Ammonium chloride ($NH_4Cl$, from $NH_3$ + $HCl$), Copper(II) sulfate ($CuSO_4$, from $Cu(OH)_2$ + $H_2SO_4$).

Salt of Weak Acid and Weak Base: The pH of the solution depends on the relative strengths of the weak acid and the weak base.

If $K_a > K_b$ (acid is stronger than the base), the solution is acidic.
If $K_a < K_b$ (base is stronger than the acid), the solution is basic.
If $K_a \approx K_b$, the solution is nearly neutral.

Examples: Ammonium acetate ($CH_3COONH_4$, from $NH_3$ + $CH_3COOH$).