Acids and Bases Notes

Acids and Bases

An acid is a substance that produces $H^+$ ions when dissolved in water. Non-metal oxides such as $CO<em>2$, $NO</em>x$, and $SO<em>2$ are acidic because they react with water to form acids like $H</em>2CO<em>3$, $HNO</em>2$, $HNO<em>3$, $H</em>2SO<em>3$, and $H</em>2SO_4$. All acids are soluble in water.

A base is a substance that reacts with acids and neutralizes them. Metal oxides such as $CaO$, $Na<em>2O$, $NaOH$, and $Ca(OH)</em>2$ are bases. Not all bases are soluble in water; only a few are. Soluble bases are called alkalis.

An alkali is a substance that produces $OH^-$ ions when dissolved in water.

Physical Properties of Acids and Alkalis

Acids:

• Turn blue litmus paper red.

• Have a sour taste.

• Have a pH less than 7. The stronger the acid, the lower the pH number.

• Turn universal indicator red if strong, and orange or yellow if weak.

Alkalis:

• Turn red litmus paper blue.

• Have a bitter taste.

• Feel soapy on the skin.

• Have a pH greater than 7. The stronger the base, the greater the pH number.

• Turn universal indicator violet if strong, and blue if weak.

Strong and Weak Acids and Alkalis

Strong Acid: A substance that ionizes completely in water to produce many $H^+$ ions. Examples include hydrochloric acid (HCl) found in the stomach, sulfuric acid ($H<em>2SO</em>4$) in car batteries, and nitric acid ($HNO_3$) for making fertilizers.

Weak Acid: A substance that ionizes partially in water to produce a low concentration of $H^+$ ions. An example is ethanoic acid ($CH_3COOH$) present in vinegar.

Strong Alkali: A substance that ionizes completely in water to produce many $OH^-$ ions. Examples include sodium hydroxide ($NaOH$) and calcium hydroxide ($Ca(OH)_2$).

Weak Alkali: A substance that ionizes partially in water to produce few $OH^-$ ions. An example is ammonia ($NH_3$).

pH and the pH Scale

pH is a measure of the concentration of $H^+$ ions in a solution. pH is inversely proportional to acidity; when acidity increases, pH decreases, and vice versa. At $25^\circ C$, pH values range between 0 and 14. This range is called the pH scale.

The pH scale is used to compare the acidity and basicity of solutions.

Indicators

An indicator is a substance that changes color when added to acids or bases, giving different colors in acidic and basic solutions.

Universal Indicator

A universal indicator is a mixture of various indicators that gives different colors at different pH numbers. Each color indicates a specific pH value on the pH scale. Unlike individual indicators, a universal indicator shows how acidic or alkaline a solution is and distinguishes between the strengths of acids and alkalis.

Example: If a substance X gives a yellow color with a universal indicator, its pH value is approximately between 5 and 6 indicating a weak acid.

Hazard Symbols

Acids and bases in the lab are labeled with hazard symbols to indicate potential dangers.

Chemical Properties of Acids

• Acids react with metals above hydrogen in the reactivity series to produce salt and hydrogen.

• Acids react with alkalis to produce salt and water.

• Acids react with metal carbonates to produce salt, water, and carbon dioxide.

Chemical Properties of Alkalis

• Bases react with acids to form a salt and water (neutralization reaction).

Uses of Acids

• Hydrochloric acid (HCl): in the stomach.

• Sulfuric acid ($H<em>2SO</em>4$): in car batteries and making fertilizers.

• Nitric acid ($HNO_3$): for making fertilizers and explosives.

Uses of Alkalis

1. Sodium hydroxide is used in the manufacture of paper, soap, and ceramics.

2. Calcium hydroxide (slaked lime) is added to soils or lakes to reduce acidity. Lime water is used in the test for carbon dioxide.

3. Indigestion tablets contain bases like magnesium oxide or calcium carbonate to neutralize excess stomach acid.

4. Bee stings (acidic) can be neutralized by a weak alkali such as dilute ammonia solution.

Acids and Bases at Home

Neutralization

Neutralization is the reaction between acids and bases to produce salt and water.

Uses of Neutralization:

• Farmers use lime (calcium oxide) to neutralize acidic soils.

• Antacid tablets containing bases (magnesium hydroxide, magnesium carbonate) neutralize excess stomach acid.

• Wasp stings (alkali) can be neutralized with a dilute weak acid (lemon juice or vinegar).

• Bee stings (acidic) can be neutralized with a dilute weak alkali (ammonia solution, baking powder solution).

• Baking powder (sodium hydrogen carbonate and tartaric acid) is used to raise cakes.

Reactions of Acids and Making Soluble Salts

All reactions of acids produce a salt.

1. Reactions of acids with reactive metals:

   Acid + reactive metal → salt + hydrogen

   The type of salt produced depends on the acid used.

   • Hydrochloric acid (HCl) produces metal chloride (e.g., $MgCl_2$).

   • Nitric acid ($HNO<em>3$) produces metal nitrate (e.g., $Ca(NO</em>3)_2$).

   • Sulfuric acid ($H<em>2SO</em>4$) produces metal sulfate (e.g., $ZnSO_4$).

   • Ethanoic acid ($CH<em>3COOH$) produces metal ethanoate (e.g., $(CH</em>3COO)_2Ba$).

   Example:

   $2HCl(aq) + Mg(s) \rightarrow MgCl<em>2(aq) + H</em>2(g)$

   Ionic equation:

   $2H^+ + Mg \rightarrow Mg^{2+} + H_2$

2. Reactions of acids with metal oxides/hydroxides (neutralization):

   Acid + alkali → Salt + water

   Example:

   $2HCl(aq) + MgO(s) \rightarrow MgCl<em>2(aq) + H</em>2O(l)$

   $H<em>2SO</em>4(aq) + CuO(s) \rightarrow CuSO<em>4(aq) + H</em>2O(l)$

   $2HNO<em>3(aq) + Mg(OH)</em>2(s) \rightarrow Mg(NO<em>3)</em>2(aq) + 2H_2O(l)$

3. Reactions of acids with metal carbonates and hydrogen carbonates:

   Acid + metal carbonate → salt + water + carbon dioxide

   Example:

   $2HCl + ZnCO<em>3 \rightarrow ZnCl</em>2 + H<em>2O + CO</em>2$

   $CuCO<em>3(s) + 2HNO</em>3(aq) \rightarrow Cu(NO<em>3)</em>2(aq) + H<em>2O(l) + CO</em>2(g)$

   Acid + metal hydrogen carbonate → salt + water + carbon dioxide

   $2HCl(aq) + 2NaHCO<em>3(aq) \rightarrow 2NaCl(aq) + H</em>2O(l) + CO_2(g)$

Reactivity Series

Comparing Reactions of Metals with Strong and Weak Acids

Practical Investigation

Fair comparison requires same volume, same concentration of acids, and same mass, same particle size of solid metal.

Reactions of Acids with Ammonia

Acids react with ammonia to produce fertilizers.

$HCl + NH<em>3 \rightarrow NH</em>4Cl$ (ammonium chloride)

$HNO<em>3 + NH</em>3 \rightarrow NH<em>4NO</em>3$ (ammonium nitrate)

$H<em>2SO</em>4 + 2NH<em>3 \rightarrow (NH</em>4)<em>2SO</em>4$ (ammonium sulfate)

$H<em>3PO</em>4 + 3NH<em>3 \rightarrow (NH</em>4)<em>3PO</em>4$ (ammonium phosphate)

Preparing Soluble Non-Alkali Salts

1. Reacting Acid with a MAZIT Metal (Mg, Al, Zn, Fe, Sn)

1. Add excess metal to dilute acid, heat, and stir until fizzing stops.

2. Filter off unreacted metal.

3. Heat filtrate to the crystallization point.

4. Cool slowly, then collect crystals by filtration.

5. Dry crystals between filter papers.

2. Reacting Acid with Insoluble Metal Oxide/Hydroxide (CuO, PbO, ZnO, Zn(OH)2, CaO)

1. Add excess insoluble metal oxide or hydroxide to dilute acid, heat, and stir until a solid remains.

2. Filter off unreacted metal oxide/hydroxide.

3. Heat filtrate to the crystallization point.

4. Cool slowly, then collect crystals by filtration.

5. Dry crystals between filter papers.

3. Reacting Acid with Insoluble Metal Carbonate (CuCO3, PbCO3, ZnCO3, CaCO3)

1. Add excess insoluble metal carbonate to dilute acid, stir until fizzing stops.

2. Filter off unreacted metal carbonate.

3. Heat filtrate to the crystallization point.

4. Cool slowly, then collect crystals by filtration.

5. Dry crystals between filter papers.

Important Notes

• Use metal powder rather than lumps for a faster reaction (larger surface area).

• Use excess metal to ensure all acid is neutralized.

• Heating and stirring speeds up the reaction.

• Filtering removes unreacted metal.

• Slow cooling obtains crystals (rapid cooling results in powder).

Example: Magnesium Sulfate from Magnesium and Sulfuric Acid

$Mg + H<em>2SO</em>4 \rightarrow MgSO<em>4 + H</em>2$

(Mg(s) + 2H+ (aq) \rightarrow Mg2+(aq) + H_2(g))

Add excess magnesium to sulfuric acid, heat and stir until fizzing stops. Filter to remove unreacted Mg. Heat filtrate to crystallization point. Cool and collect crystals by filtration. Dry crystals between filter papers.

Example: Copper Chloride Crystals from Copper(II) Oxide and Hydrochloric Acid

$CuO(s) + 2HCl(aq) \rightarrow CuCl<em>2(aq) + H</em>2O(l)$

O2- (s) + 2H+(aq) \rightarrow H2O(l) or CuO(s) + 2H+(aq) \rightarrow Cu2+(aq) + H2O(l)

Add excess CuO to hydrochloric acid, heat and stir until no more reacts. Filter to remove unreacted CuO. Heat filtrate to crystallization point. Cool and collect crystals by filtration. Dry crystals between filter papers.

Titration Method

A lab technique that measures the exact volume of an acid needed to neutralize a measured volume of an alkali, or vice versa.

Uses of Titration

• Prepare soluble alkali metal salts.

• Find the unknown concentration of an acid or an alkali.

Preparing Soluble Alkali Salts by Titration

Examples: LiI, NaCl, KBr, CsNO3, Rb2SO4, NH4Cl

Example: Sodium Sulfate Crystals

Using a pipette, transfer 25 cm3 of NaOH(aq) to a conical flask, then add 3 drops of phenolphthalein indicator. From a burette, add dilute H2SO4(aq) to the conical flask until the indicator just changes color with one drop “Colour changes from pink to colourless”. Record the added volume of the acid. Repeat the titration without an indicator (to obtain pure crystals) using same volumes, then, evaporate solution to crystallization point. Leave the solution stand to cool and separate the crystals. Dry the crystals between filter papers or in a warm place.

Preparing Insoluble Salt (Precipitation Method)

1. Mix two solutions of soluble salts, forming a precipitate.

2. Filter to separate the precipitate from the solution as residue.

3. Wash residue with distilled water to remove traces of soluble salt.

4. Dry residue between filter papers.

Choosing Suitable Reagents

Most suitable reagents are a nitrate salt of the cation and a potassium or sodium salt of the anion.

Example: Silver Bromide from Silver Nitrate and Sodium Bromide

$AgNO<em>3(aq) + NaBr(aq) \rightarrow NaNO</em>3(aq) + AgBr(s)$

Add silver nitrate solution to sodium bromide solution, a cream precipitate, AgBr forms. Filter to obtain the precipitate, AgBr(s), as residue. Wash the residue with distilled water to remove traces of the soluble salt NaNO3(aq). Dry between filter paper or in an oven at low temperature.