Acids, Bases and Salts Chemical Reactions and Equations Law of Conservation

Notes For

Acids, Bases and Salts

Their definitions in term of furnishing of H+ and OH- ions, General properties, examples and uses, Concept of pH scale Importance of pH in everyday life; Preparation and uses of Sodium hydroxide, Bleaching powder, Washing soda and Plaster of Paris.

Facts that Matter

Acids : Acids are sour in taste, turn blue litmus to red, dissolve in water to release H+ ions.

E.g., vinegar, hydrochloric acid and sulphuric acid.

• Reaction with Metal

Acid + Metal → Salt + Hydrogen gas

e.g., 2HCl + Zn → ZnCl2 + H2

• Reaction with Metal carbonate

Acid + Metal carbonate → Salt + CO2 + H2O

e.g., 2HCl + Na2CO3 → 2NaCl + CO2 + H2O

• Reaction with Metal hydrogen carbonate

Acid + Metal hydrogen carbonate Salt + CO2 + H2O

e.g. HCl + NaHCO3 → NaCl + H2O + CO2

• Reaction with Metallic oxide

Acid + Metal oxide → Salt + Water

e.g. 2HCl + CuO → CuCl2 + H2O

• Acids in water:

Acids produce H+ ions when dissolved in water. H+ ions cannot exist alone. They combine with water molecule (H2O) to form H3O+ (hydronium ions). It conducts electricity.

• Decrease in H30+ ions concentration per unit volume results in formation of dilute acids.

 • It is a highly exothermic reaction.

Acids when dissolved in water release large amount of heat. If water is added to concentrated acid then the heat generated may cause the mixture to splash out and cause bums. Hence to avoid burns acid must be added drop wise into water with constant stirring. So that the heat generated spreads over in water.

Strong acids → release more H+ ions → HCl

Weak acids → releases less number of H+ ions → acetic acid

Strong base → give more OH– ions → NaOH

Weak base → gives less OH– ions → CH3COOH

Bases: Bases are bitter in taste, turns red litmus to blue and when dissolved in water releases OH– ions; e.g., NaOH and KOH.

• Reaction with metal

 Base + Metal → Salt + H2 gas

e.g., 2NaOH + Zn → Na2ZnO2 + H2

This reaction is possible only with reactive metals like sodium and potassium.

• Reaction with non-metallic oxide

 Base + Non-metallic oxide → Salt + H2O

Bases in water → Bases produce OH– ions when dissolved in water. Bases soluble in water are called alkalis. It conducts electricity.

 • Decrease in OH– ions single concentration per unit volume results in formation of dilute bases.

 • It is a exothermic reaction.

To make basic solution, base must be added drop wise into water with constant stirring, so that the heat generated spreads over in water.

Indicators: Indicators are those substances which tell us whether a substance is acidic or basic by change in colour. For e.g., litmus solution.

• Olfactory indicators: Those substances whose odour changes in acidic or basic media are called lfactory indicators. For e.g., clove, vanilla, onion.

 • Natural indicators: Turmeric, litmus (obtained from lichen)

 • Synthetic indicators: Methyl orange, phenolphthalein.

Elements are classified into two basic categories, metals and non-metals.

Before giving the definition of metal and non-metal, we must know some important terms:

(i) Malleability: Ability of an element due to which it can be beaten with hammer into thin sheets.

(ii) Ductility: Ability of an element due to which it can be drawn into wires.

(iii) Sonorous: Property of an element to produce sound when it is struck with a hard substance.

(iv) Brittleness: Property of an element to break easily into pieces on hammering or stretching.

Notes on Chemical Reactions and Equations

1. Chemical reactions- The transformation of chemical substance into a new chemical substance by making and breaking of bonds between different atoms is known as Chemical Reaction.

2. Signs of a chemical reaction- These factors denote that a chemical reaction has taken place- change of state of substance, change of color of substance, evolution of heat, absorption of heat, evolution of gas and evolution of light.

3. Chemical Equation: The representation of chemical reaction by means of symbols of substances in the form of formulae is called chemical equation. E.g. – H2 + O2 ⇒ H2O

4. Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both left and right sides of the reaction.

 

Note- According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.

5. Types of Chemical Reactions-

I. Combination- When two elements or one element and one compound or two compounds combines to give one single product.

II. Decomposition- Splitting of a compound into two or more simple products.

III. Displacement- It takes place when a more reactive metal displaces a less reactive metal.

IV. Double displacement- Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

 V. Precipitation- The insoluble compound called precipitate forms in this reaction.

 VI. Exothermic- Reactions which produce energy are called exothermic reaction. Most of the decomposition reactions are exothermic.

VII. Endothermic- Reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic.

 VIII. Oxidation: Gain of oxygen or removal of hydrogen or metallic element from a compound is known as oxidation.

IX. Reduction: Addition of hydrogen or removal of oxygen from a compound is called reduction.

 X. Redox- A chemical reactions where oxidation and reduction both take place simultaneously are also known as redox reaction.

 Eg – NaOH + HCl ⇒ NaCl + H2O

6. Rusting- When iron reacts with oxygen and moisture forms a red substance called rust.

7. Rancidity- Oils and fats when get oxidized on exposure to air show a change in taste and smell.

8. Corrosion- Metals when attacked by oxygen, water, acids, gases, present in air changes its surface which is called corrosion