Acid, Bases, and Salts Notes PDF

Chapter 2 Acid, bases and salts Acids  Acids is defined as the one which produces hydrogen ions in water. For Example, Sulphuric Acid, Hydrochloric Acid etc.  They give sour taste.  Acids turn blue litmus to red. This is used as confirmation test for the prese...

Chapter 2 Acid, bases and salts Acids  Acids is defined as the one which produces hydrogen ions in water. For Example, Sulphuric Acid, Hydrochloric Acid etc.  They give sour taste.  Acids turn blue litmus to red. This is used as confirmation test for the presence of acid.  When acids react with metals, gases are evolved. Classification of Acids Acids are classified as Organic Acids and Mineral Acids. Acids which are derived from plants and animals, they are known as Organic Acids. For Example, Citric Acid from fruit. Mineral acids are inorganic acids such as Sulphuric Acid. They are dangerous to be used, so need more precautions. Acids are also classified as Strong Acids or Weak Acids. Strong acid is an acid, that completely dissociates into ions in aqueous solutions. For Example, Sulphuric Acid, Hydrochloric Acid. Weak acid is the one which does not dissociate completely into ions in aqueous solutions. For Example, Acetic Acid. Acids can also be as Dilute Acid and Concentrated Acids. The one which has low concentration of acids in aqueous solution, they are known as Dilute Acids whereas the one which has high concentration of acids in aqueous solution, are known as Concentrated Acids. It is advisable to add acid to water and not vice versa because large amount of heat is released if water is added to acid. This released heat is large enough to cause harm. Acids can also be classified based on number of hydrogen ions. Monoprotic acid is the one which gives one mole of hydrogen ions per mole of acid, such as HCl. Diprotic Acid is the one which produces two mole of hydrogen ions per mole of acid. For Example, H2SO4. Bases  Bases are the one which produces hydroxide ions in aqueous solutions. Bases which are water soluble they are known as Alkalis.  They turn red litmus to blue.  They have a bitter taste.  They also produced carbon-dioxide when reacted with carbonates.  They also evolved hydrogen gas when bases react with metals. Classification of Bases Bases are classified as Strong Base and Weak Base. Strong base is the one which dissociates completely into its ions in aqueous solution. For Example, NaOH. Weak base is the one which does not dissociate completely into its ions in aqueous solutions. For Example, Ammonium Hydroxide, NH4OH Bases are also classified as Dilute Base and Concentrated Base. The solution which has low concentration of base in aqueous solution is defined as Dilute Base whereas the one which has high concentration of base in aqueous solution is known as Concentrated Base. Similarity between Acids and Bases  Both acids and base react with water. They produce ions in water  Both acids and bases acts as electrolytes, so are good conductors of electricity.  Both of them changes the colour of the litmus paper. Reactions with Acids 1. Reaction of Acid with Metal Acid + Metal → Salt + Hydrogen gas Mg + H2 SO4 → H2 + Mg SO4 2. Reaction of Acid with Carbonates Na2 CO3 (s) + 2 HCl (aq) → 2NaCl (aq) + H2O(l) + CO2(g) 3. Reaction of Acid with Bicarbonates NaHCO3 (s) + HCl (aq) → NaCl(aq) + H2O (l) + CO2 (g) Reactions of Bases 1. Reaction with Metals Base reacts with metals and produce hydrogen gas. 2NaOH + Zn → Na2 → Na2ZnO2 + H2 2. Reaction with Acids Base reacts with acids to form salts. For Example, KOH + HCl → KCl + H2O 3. Reaction with Non-metallic Oxides Base reacts with non-metallic oxides to form salt and water. 2NaOH + CO2 → CO2 → Na2CO3 + H2O Diluting Acids and Bases -  If concentrated acid (or base) is mixed with water, concentration of hydronium ions () decreases. So the acid or base becomes dilute.  The phenomena of changing concentrated acid / base into dilute acid / base (respectively) is an exothermic reaction.  Always add concentrated acid/base to water; not water to concentrated acid/base.  When concentrated acid is added to water, heat is released gradually. This heat is absorbed by large amount of water to increase its temperature.  When water is added to concentrated acid, heat is released suddenly. This heat vaporizes small amount of water to steam. This steam is splits out with drops of concentrated acid which can damage our body. So always add concentrated acid to water, not water to concentrated acid.  Same is true with base also. Reactions of Acids and Bases - Neutralization Reaction -An acid neutralizes a base when they react with each other and respective salt and water are formed. Since in the reaction between acid and base both neutralize each other, hence it is also known as neutralization reaction. Example:  Sodium chloride and water are formed when hydrochloric acid reacts with sodium hydroxide (a strong base).  In similar way, calcium chloride is formed along with water when hydrochloric acid reacts with calcium hydroxide (a base).  Sodium sulphate and water are formed when sulphuric acid reacts with sodium hydroxide (a base).  In similar way, when nitric acid reacts with sodium hydroxide, sodium nitrate and water are formed. Note: The most important thing in whole process is that H+ and OH- ions combine to form H2O. Reaction of acids with metal Oxides -Metal oxide is a chemical compound that contains oxygen and one metal in its chemical formula. For example, copper oxide (CuO), sodium oxide (Na2O), magnesium oxide (MgO). Acids give water along with respective salt when they react with a metal oxide. Metal + acid Respective salt + Water Example :  Water and sodium chloride are formed when hydrochloric acid reacts with sodium oxide. Na2O + 2HCl (aq) 2NaCl + H2O  Water and magnesium chloride are formed when hydrochloric acid reacts with magnesium oxide. MgO + 2HCl MgCl2 + H2O  Water and copper sulphate are formed when copper oxide reacts with sulphuric acid. CuO + H2SO4 (aq) CuSO4 + H2O  Water and copper chloride are formed when copper oxide reacts with hydrochloric acid. CuO + 2HCl (aq) CuCl2 + H2O Reaction of bases with non - metal Oxides -Non - metal oxide is a chemical compound that contains oxygen and one non - metal in its chemical formula. For example, carbon dioxide (CO2), sulphur dioxide (SO2). When a base reacts with non-metal oxide both neutralize each other resulting respective salt and water are produced. Base + non – metal oxide Respective salt + water Example :  Sodium hydroxide gives sodium carbonate and water when it reacts with carbon dioxide.  Calcium hydroxide gives calcium carbonate and water when it reacts with carbon dioxide. Note :  When an acid reacts with a metal oxide, both neutralize each other. In this reaction, respective salt and water are formed as we saw in neutralization reaction. So it can be said that metal oxide are basic in nature.  Similarly when a base reacts with a non – metal oxide, both neutralize each other. In this reaction, respective salt and water are formed. So it can be said that non - metal oxide are acidic in nature. Reaction of acids with metal - Acids give hydrogen gas along with respective salt when they react with a metal. Example:  Hydrogen gas and zinc chloride are formed when hydrochloric acid reacts with zinc metal.   Hydrogen gas and sodium chloride are formed when hydrochloric acid reacts with sodium metal.  Hydrogen gas and iron chloride are formed when hydrochloric acid reacts with iron.  Hydrogen gas and zinc sulphate are formed when zinc metal reacts with sulphuric acid Note: Acid is not stored in metallic containers as acids and metals can easily react. Reaction of Base with Metals - When base reacts with metal, it produces salt and hydrogen gas. Example:  Sodium hydroxide gives hydrogen gas and sodium zincate when reacts with zinc metal.  Sodium aluminate and hydrogen gas are formed when sodium hydroxide reacts with aluminium metal. Note : Not all metals do this type of reaction. Reaction of Carbonates and Bicarbonates with Acids -Acids give carbon dioxide gas and respective salts along with water when they react with carbonates. Carbonate + acid Salt + carbon dioxide + water Examples:  Hydrochloric acid gives carbon dioxide gas, sodium chloride along with water when reacts with sodium carbonate.  Hydrochloric acid gives carbon dioxide, magnesium chloride and water when it reacts with magnesium carbonate.  Hydrochloric acid gives carbon dioxide gas, calcium chloride and water when it reacts with calcium carbonate.  Sulphuric acid gives calcium sulphate, carbon dioxide gas, calcium sulphate and water when it reacts with calcium carbonate.  Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium carbonate.  Nitric acid gives sodium nitrate, water and carbon dioxide gas when it reacts with sodium carbonate. Reaction of Acid with Bicarbonates -Acids give carbon dioxide gas, respective salt and water when they react with bicarbonate. Acid + bicarbonate Salt + carbon dioxide + water Examples:  Hydrochloric acid gives carbon dioxide, sodium chloride and water when it reacts with sodium bicarbonate.  Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium bicarbonate. Sodium bicarbonate is also known as sodium hydrogen carbonate / baking soda / baking powder / bread soda or bicarbonate of soda. Note:  In general, if we use weak acids, reactions will be slow and if we use strong acids, reactions will be fast.  Generally bases do not react with carbonates and bicarbonates. So here we are not interested to see the reactions of bases with carbonates and bicarbonates. 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 olfactory indicators. For e.g., clove, vanilla, onion. Natural indicators: Turmeric, litmus (obtained from lichen) Synthetic indicators: Methyl orange, phenolphthalein. S.No. Name of the Indicator Colour Change With Acid Colour Change with Base A. Blue litmus solution To red No change B. Red litmus solution No change To blue C. Turmeric No change To red D. Methyl orange To red To yellow E. Phenolphthalein (colourless) No change To pink pH : Strength of an acid or base can be determined using a pH scale. It is a scale to measure the hydrogen ion concentration in a solution. The p stands for ‘potenz’, it is a German word which means power.  If pH is equal to 7, means the solution is neutral.  If pH is greater then 7 means alkaline solution.  If pH is less then 7 means the solution is acidic. Importance of pH  Human body works at a pH of about 7.4.  Stomach has a pH of about 2 due to presence of hydrochloric acid in it. It is needed for the activation of pepsin protein required for protein digestion.  When we eat food containing sugar, then the bacteria present in our mouth break down the sugar to form acids. This acid lowers the pH in the mouth.  Tooth decay starts when the pH of acid formed in the mouth falls below 5.5. This is because then the acid becomes strong enough to attack the enamel of our teeth and corrode it. This sets in tooth decay. The best way to prevent tooth decay is to clean the mouth thoroughly after eating food.  Many animals and plants protect themselves from enemies by injecting painful and irritating acids and bases into their skin. o When honey bee stings a person, it injects an acidic liquid into the skin. Rubbing with mild base like baking soda solution on the stung area of the skin gives relief. o When a wasp stings, it injects an alkaline liquid into the skin. Then rubbing with a mild acid like vinegar on the stung area of the skin gives relief.  Soil pH and plant growth: Most of the plants grow best when the pH of the soil is close to 7. If the soil is too acidic or basic, the plants grow badly or do not grow at all. The soil pH is also affected by the use of chemical fertilisers in the field. Chemicals can be added to soil to adjust its pH and make it suitable for growing plants. If the soil is too acidic then it is treated with materials like quicklime or slaked lime. If the soil is too alkaline then alkalinity can be reduced by adding decaying organic matter.