Acids, Bases and Salts

Transcription

1 Acids, Bases and Salts 2 HAPTER Tips and Tricks Acids are sour in taste and bases are bitter in taste. Indicators are chemical substances which give different colours in acidic and basic solutions. If a substance gives change in odour in acidic or basic medium, then it is olfactory indicator. Indicator Acid Basic solution solution Litmus Red Blue Methyl Orange Pink Yellow Phenolphthalein olourless Pink Bromothymol blue Yellow Blue hemical properties of acids and bases (i) Acids contain hydrogen atom and give hydrogen ions (H + ) in water. (ii) Acids react with metals to give salt and hydrogen gas. (iii) Acids react with metal oxide to give salt and water. In addition carbon dioxide is evolved if metal carbonate is taken. (iv) Acids and bases react to give salt and water. This is neutralization reaction. (v) Bases are the hydroxides of metals, which give hydroxide ion (OH ) in aqueous solutions. (vi) Bases react with only some metals to give salt and hydrogen, e.g., zinc. (vii) Bases which are soluble in water are known as alkalies. (viii) Acids and Bases in water conduct electricity due to the presence of ions. ph: All water solutions contain H + and OH ions. Total of H + and OH ion concentration in aqueous solution is fixed. If H + ion cocentration is known, that of OH ions is determined. [H + ] [OH ] = ph scale is the scale for measuring hydrogen ion concentration in a solution. Its value varies from 0 to 14 for solutions very acidic to very basic. Neutral solution has ph equal to 7. ph is defined as negative logarithm of hydrogen ion concentration. The value of ph is very important in our daily life. A ph range of 6 8 is good for living organisms and plants. Strong acids give H + ions readily in aqueous solutions, e.g., hydrochloric acid, sulphuric acid, nitric acid etc. Weak acids do not give all the hydrogen atoms contained as H + ions in water solution, e.g., acetic acid, carbonic acid etc. Strong bases give OH ions readily, e.g., NaOH, KOH etc. Weak bases are ammonium hydroxide, calcium hydroxide etc. Salts are formed by combination of acids and bases. Acidic salts are formed by reaction of weak base with a strong acid, e.g., ammonium chloride, aluminium chloride. Their aqueous solution has ph less than seven. Basic salts are from strong base and weak acid, e.g., sodium acetate, sodium carbonate. Their aqueous solutions give ph greater than seven. Neutral salts are formed from combination of strong acids and strong bases, e.g., Nal, Na 2 SO 4, NaNO 3 etc. These give neutral aqueous solution with ph nearly equal to seven. A-24

2 AIDS, BASES AND SALTS A-25 Hydrated salts: Some salts crystallize with a fixed number of water molecules attached to one formula unit of salt, e.g., Blue copper sulphate (uso 4.5H 2 O), washing soda (Na 2 O 3.10H 2 O), Gypsum (aso 4.2H 2 O). Gypsum on controlled heating loses water molecules partially and give calcium sulphate hemihydrate, aso 4 H 2O or plaster of paris. alculation of ph values: If H + ion concentration in aqueous solution is 10 3 M, ph = log = 3 If OH ion concentration in aqueous solution is 10 3 M, H + ion concentration = = ph = log = 11 ommon salt is sodium chloride (Nal) commercial used variety is rock salt. Nal(aq) + H 2 O Electrolysis + O 2 + NH 3 athode Anode (Na) (l 2 ) NH 4 l + NaHO Ammonium Sodium H 2 O a(oh) 2 chloride hydrogen carbonate NaOH aol 2 (Baking soda) Sodium Bleaching heat hydroxide powder Na 2 O 3 + H 2 O + O 2 Sodium carbonate (washing soda) Formative Assessment ORAL QUESTIONS (onversation Type) 1. (a) Acids are sour in taste. Is it a way to find whether a substance is an acid or a base? (b) What is other physical test? (c) Any test with solid acid? (d) an you check the evolution of O 2 chemically? 2. (a) What are acids? (b) an presence of H + in water be estimated? How? (c) How is ph related to H + ion concentration? (d) How is ph related to strength of an acid? (e) Name one strong acid and one weak acid. 3. (a) What are salts? (b) How many types of salts are formed? (c) What are neutral salts? (d) What do you mean by acidic salts? (e) Define basic salts. (f) Give the corresponding acid and base from which sodium carbonate is formed. 4. (a) What is common salt? (b) Why does common salt become moist in rainy season? (c) How is it used as a freezing mixture? (d) Name two important laboratory chemicals prepared from common salt on large scale. 5. (a) What is washing soda? (b) Name the process by which sodium carbonate is manufactured. (c) What are the raw materials used in the preparation of washing soda? (d) Sodium carbonate is obtained from another carbonate on heating. Name it. (e) What is the commercial name of sodium bicarbonate. 6. (a) Name the substance used for bleaching cotton and wood pulp in textiles. (b) What is its chemical name? (c) How is it manufactured? (d) What is slaked lime? (e) Why does bleaching powder smell of chlorine? ORAL QUESTIONS 1. Name the acid present in lemon juice. 2. Name an antacid. 3. What name is given to an indicator which gives different odour in acidic and basic solution? 4. What is the chemical difference between washing soda and baking powder? 5. How will you obtain sodium carbonate from sodium bicarbonate?

3 A-26 NEW AGE E SIENE X 6. Name the acid present in ant sting. 7. Which of the two, curd or soap solution, give blue colour with litmus paper? 8. What happens when nitric acid is poured over egg shells? 9. What is the ideal ph of the soil for the healthy growth of a plant? 10. At what ph the mouth teeth start decaying? 11. Name the base used to reduce acidity in stomach. 12. How is ph of an acid solution affected when it is diluted? 13. Give one example of an acidic salt and one of a basic salt. 14. Give one example each of basic oxide and acidic oxide. 15. Name the gas responsible for extinguishing fire in a soda-acid fire extinguisher. 16. What contributes to the basic character of a base? 17. Out of glucose and acetic acid which one will conduct electricity in water? 18. Name the scale which is used to measure H + ion concentration in a solution. 19. What is the ph of blood? 20. What is the chemical name of the compound which has the property of hardening when mixed with water? TRUE OR FLASE 1. Ethyl alcohol ( 2 H 5 OH) produces H + ions in a solution. 2. Dilute hydrochloric acid in stomach helps in digestion. 3. Sodium sulphate is a neutral salt. 4. Brine is an aqueous solution of sodium chloride. 5. Sodium acetate is an acidic salt. 6. The electrolysis of aqueous sodium chloride gives hydrogen gas at cathode. 7. arbonated soft drink gives blue colour with litmus paper. 8. All metals react with bases to give salt and hydrogen gas. 9. Phenolphthalein is an acid-base indicator. 10. Non-metallic oxides give amphoteric solution Metal oxides and bases are similar in their reaction with acids. 12. Dry hydrochloric acid does not conduct electricity but dissolves in water. 13. Water of crystallization of a salt depends on the moisture content of air. 14. Bleaching powder is obtained when brine is passed over calcium hydroxide solution. 15. Plaster of Paris is obtained by strongly heating gypsum. QUIZ WHO AM I 1. I can roughly measure ph value from 0 to I am called antichlor and is used to remove excess chlorine from clothes when treated with bleaching powder. 3. I am a product of gypsum and is used in making chalks and fire proof materials. 4. I am a compound of calcium and can be used for disinfecting drinking water as well as for decolourisation. 5. I give different smell in acid and base solutions 6. I am an oxide capable of showing properties of both acids and bases. 7. I am a covalent compound and conducts electricity in aqueous medium. 8. I am a salt of potassium hydroxide and nitric acid. 9. I am the term used when a solid becomes liquid when exposed to moist air. 10. I am derived from tomato and turn blue litmus into red. PUZZLES 2 L 3 1

4 AIDS, BASES AND SALTS A-27 I. Across 1. Known as vinegar (6). 4. A mineral acid (12). Down 2. Acid obtained from milk (6) 3. An acid obtained from lemon (6). II. Across 1. A stone used for manufacture of sodium carbonate (9) 3. A substance that changes colour in acidbase solution (9). 2 1 I T 3 N A 4 Down 2. A gas obtained in the electrolysis of sodium chloride (8). 4. A substance that gives relief from acidity (7). SUGGESTED PROJETS/ATIVITIES 1. Prepare extract of natural materials like red cabbage leaves, flower petals, tea petals etc. and test with acid-base solutions to find which can act as indicator. 2. Different students may check ph of their urine, blood, cough etc and compare with their diet on that day and summarise what diet suits human body best. 3. Prepare extracts of different masala at home and check their ph with ph paper and draw a conclusion. 4. ompare the reaction of different acids with carbonate/bicarbonate and suggest which combination is best to construct a fire extinguisher.

5 A-28 NEW AGE E SIENE X Assignments LASS ASSIGNMENT 1 1. Give one example each of natural indicator and synthetic indicator. 4. What name is given to an indicator which gives different colour in different medium. Give examples. 2. Give an example each of acidic salt and neutral salt as well basic salt. Explain. 3. What are the salts of nitric acid called? Give one example of such a salt. 5. How is phenolphthalein used as an indicator?

6 AIDS, BASES AND SALTS A What is that reaction called when acids and bases react? What is the common product of all such reactions? 7. Name the salt formed when nitric acid reacts with ammonium hydroxide. Is it an acidic salt or a basic salt? 8. Show that sodium acetate is a basic salt. 9. A student prepared two solutions (1) dil. Hl and (ii) distilled water in separate test tubes. She forgot to label the tubes. Both solutions are colourless. Solutions of litmus, phenolphthalein and lime water are available to her. How will she distinguish between the two solutions? 10. What are strong acids and weak acids? Do both behave similarly with litmus solution.