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Acids and bases

Acids and bases. Common acids/ bases. Acids. Most acids have the general formula HA, where A- is an anion. HA ---  H + + A - in solution. In aqueous solutions acids increase the hydrogen ion (H+) concentration. Acids are proton donors = H+

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Acids and bases

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  1. Acids and bases

  2. Common acids/ bases

  3. Acids • Most acids have the general formula HA, where A- is an anion. • HA --- H+ + A - in solution. • In aqueous solutions acids increase the hydrogen ion (H+) concentration. • Acids are proton donors = H+ • Acids are chemical compounds that have a sharp taste, a corrosive action on metals, and turn blue Litmus paper red.

  4. Properties of Acids • taste sour (don't taste them!)... the word 'acid' comes from the Latin acere, which means 'sour' • acids change litmus (a blue vegetable dye) from blue to red • their aqueous (water) solutions conduct electric current (are electrolytes) • react with bases to form salts and water • evolve hydrogen gas (H2) upon reaction with an active metal (such as alkali metals, alkaline earth metals, zinc, aluminum)

  5. Examples of Common Acids • citric acid (from certain fruits and veggies, notably citrus fruits) • ascorbic acid (vitamin C, as from certain fruits) • vinegar (5% acetic acid) • carbonic acid (for carbonation of soft drinks) • lactic acid (in buttermilk

  6. Acid = proton donor

  7. Bases • Most bases have the form BOH, where B+ is an appropriate cation. • BOH -- B+ + OH- in solution • Bases are proton acceptors = H+ • H+ + OH----- H2O • Bases are chemical compounds that, in solution, are soapy to the touch and turn red vegetable dyes blue.

  8. Properties of Bases • taste bitter (don't taste them!) • feel slippery or soapy (don't arbitrarily touch them!) • bases don't change the color of litmus; they can turn red (acidified) litmus back to blue • their aqueous (water) solutions conduct and electric current (are electrolytes) • react with acids to form salts and water

  9. Examples of Common Bases • detergents • soap • lye (NaOH) • household ammonia (aqueous)

  10. Examples of acids and bases

  11. Acid – base reactions • Greek words meaning "acid" (from the Greek οξυς (oxys) meaning "acid“ • Neutralization reactions • acid+ + base- → salt + water • 2NaOH + H2SO4 → 2 H2O + Na2SO4

  12. Acids and bases neutralize • Acids and bases can be grouped into two general types: strong and weak acids and bases. • When mixed, acids and bases neutralize one another and produce salts. These are substances with a salty taste and none of the characteristic properties of either acids or bases.

  13. Acids and bases neutralize • Acid-base reactions are universal. • When an acid and a base react in an aqueous solution the H+ and OH- ions combine to form water. These ions thus "neutralize" one another. • HCl + NaOH - NaCl + H2O

  14. pH scale

  15. pH scale

  16. pH scale : definition • The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic. • The pH scale is logarithmic and as a result, each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6.

  17. pH scale

  18. pH indicators

  19. Indicators

  20. Diagram of indicators

  21. What is an acid-base indicator? • An acid-base indicator is a weak acid or a weak base. • The undissociated form of the indicator is a different color than the iogenic form of the indicator. • An Indicator changes colour depending on the hydrogen ion concentration.

  22. Adding hydroxide ions:

  23. Adding hydrogen ions:

  24. Methyl orange You have the same sort of equilibrium between the two forms of methyl orange as in the litmus case - but the colours are different. You have the same sort of equilibrium between the two forms of methyl orange as in the litmus case - but the colours are different. You have the same sort of equilibrium between the two forms of methyl orange as in the litmus case - but the colours are different. You have the same sort of equilibrium between the two forms of methyl orange as in the litmus case - but the colours are different.

  25. Volumetric analysis

  26. Titration experiment • An acid-base titration is a method in chemistry that allows quantitative analysis of the concentration of an unknown acid or basesolution. It makes use of the neutralization reaction that occurs between acids and bases and the knowledge of how acids and bases will react if their formulas are known.

  27. Method • Title : To standardize an unknown solution of NaOH using a standard solution of HCl. • Apparatus (Equipment).

  28. Apparatus / Equipment • The key equipment used in a titration are: • Burette • White Tile - used to see a colour change in the solution • Pipette • Acid/Base Indicator (the one used varies depending on the reactants) • Erlenmeyer flask (conical flask) • Standard Solution (a solution of known concentration, a common one is aqueous Na2CO3) • Solution of unknown concentration • It is also recommended that a control is used in order to see the difference in colour between the initial and final colour

  29. Procedure • Rinse all glass ware ,the Burette, pipette and flask with distilled water. • Fill the burette with the standard solution of HCl. Open tap and note start reading (0.0 cm3) • Using the pippette measure 25 cm3 of the unknown NaOH into the conical flask. • Place 2-3 drops of the indicator into the flask. • Place flask below burette and add acid dropwiseuntill the colour of the indicator changes.

  30. Procedure... • Note final reading on the burette. • Difference between final reading and first reading will give amount of HCl used. • Wash and rinse flask and repeat titration 4-5 times till your readings are very close, not more than 0.1 cm3. • Calculate the average amount of HCl used.

  31. Calculation

  32. Calculation... • The equation of the reaction is : • HCl + NaOH -- NaOH + H2O so 1mole of HCl reacts with 1 mole of NaOH. • Volume of acid (Va)x concentration of acid (Ca) = Volume of base (Vb) x conc of base (Cb) • Ca = 0.1M, Va = average reading from titration. • Vb = 25 cm3 , Cb = unknown

  33. Calculation..... • Cb = Va x Ca / Vb • So Cb is = the concentration of NaOH.

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