ADEng. Programme Chemistry for Engineers Prepared by M. J. McNeil, MPhil.

Transcription

1 ADEng. Programme Chemistry for Engineers Prepared by M. J. McNeil, MPhil. Department of Pure and Applied Sciences Portmore Community College Main Campus 1

2 LECTURE OBJECTIVES Define acid, bases, alkali, salt, acid anhydride, acidic, basic, amphoteric and neutral oxides; (proton donor or acceptor and replaceable hydrogen); investigate reaction of bases with ammonium salts; What is ph, ph scale; relate acidity and alkalinity to the ph scale; Litmus paper and universal indicator; Discuss the strengths of acids and alkalis on the basis of their completeness of ionization. Use of ph meter; Investigate the reactions of non-oxidizing acids with metals, carbonates, hydrogen carbonates and bases (reference reactions of antacids, baking powder, fire extinguisher); Investigate the reaction of bases with ammonium salts; Distinguish between acid salts and normal salts. 2

3 ???? What is an acid/base? What could you do to tell if something was an acid or a base? Are all acids dangerous? Can you drink an acid and survive? What happens if acids and bases mixed? 3

4 TASTE, FEEL AND CHEMICAL EXAMPLES Have you ever taken a bite out of a sour lemon? Lemon juice is a d _ c. Baking soda (NaHCO 3 ) taste? NaHCO 3 a is an e.g. of a b _ s _. Bl h and s p are also e.g. What happens if you re slicing lemons and some juice get into a cut on your finger? a d stings cuts Do you know any other common acids? What are some other common bases? What about water? Taste? 4

5 INQUIRY-BASED DEMO You will be given an activity sheet to investigate the effect of acidic, basic and neutral solutions on indicators. Instructions: 1. Dampen a piece of blue Litmus paper with tap water. 2. Insert one half of the blue Litmus paper into the unknown solution A. 3. Note your observations in a suitable table. 4. Use a new dampened blue Litmus paper. 5. Repeat steps 2 to 4, but apply it to solutions B, then use another one for C. 6. Decide which of the unknown solutions you believe is an acid, base, neutral. 5

6 POST-ACTIVITY QUESTIONS Identifying the Nature of Substances 1. What color does Litmus paper turn when it is a base? Chemical Indicator Unknown Solution A Unknown Solution B Unknown Solution C 2. What color does Litmus paper turn when it is an acid? Dampened blue Litmus 3. With a Litmus paper, how can you figure out what is an acid/base? Dampened red Litmus 4. What are neutrals? ACID GIVES RED, ALKALI BLUE, USING LITMUS, THIS IS TRUE! 6

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8 ACIDS, BASES AND SALTS For thousands of years, people have known that vinegar and lemon juice and many other juice taste sour. It was not until a few hundred years ago, it was discovered why these substances tastes sour - because they are all acids. The word acid comes from the Latin word acere, which means sour. Bases feel slippery. Acid reacts with base to produce salts. LETS EXPLORE EACH IN MORE DETAILS! 8

9 Mineral versus Organic Acids ACIDS Strong versus Weak Acids 9

10 An acid MAY be defined in three different ways: WHAT ARE ACIDS? (1) An acid is a substance which forms hydrogen ions (H + ) when dissolved in water. HCl (g) + water H + (aq) + H 2 O (l) Overall reaction: HCl (g) + H 2 O (l) H + (aq) + Cl - (aq) H 3 O + (aq) H 3 O + (aq) + Cl - (aq) OR An acid is a solution which has hydrogen ions (H + ) as its only positive ion. The more H + ions, the more acidic the solution. When an acid dissociates in water, it initially forms unstable H + ion, which then becomes attached rapidly to a water molecule to form hydroxonium ion (H 3 O + ). 10

11 WHAT ARE ACIDS? (2) An acid is a proton (H + ) donor. When a hydrogen atom loses an electron to form an H + ion, it becomes a hydrogen nucleus which contains a single proton. Hydrogen ions are therefore, protons and acids can donate these to other substances such as bases. (3)An acid is also a substance which contains hydrogen which can be displaced directly or indirectly by a metal to form a salt. Zn (s) + 2HCl (aq) ZnCl 2(aq) + H 2 (g) The zinc replaces the hydrogen to form the salt, zinc chloride. Who can write the ionic equation the fastest for the above reaction! HCl (g) + H 2 O (l) H 3 O + (aq) + Cl - (aq) Let us explore acid strengths - strong versus weak acids. 11

12 STRONG ACIDS AND WEAK ACIDS STRONG ACIDS A strong acid is one which is completely ionized in aqueous solution e.g. HCl (stomach acid) WEAK ACIDS A weak acid is one which is partially ionized in aqueous solution eg. Vinegar HCl H + and Cl - If you have 100 molecules of HCl, all 100 molecules will break down into H + and Cl -. There are six strong mineral acids: HCl, HBr, HI, H 2 SO 4, HNO 3, HCIO 4 CH 3 COOH CH 3 COO - + H + 85 mol 15 mol 15 mol If you had a 100 molecules of vinegar, only 15 molecules will break down. 12

13 STRONG ACIDS AND THEIR ANIONS 13

14 WEAK ACIDS AND THEIR ANIONS 14

15 NAMING OF MINERAL ACIDS 15

16 COMMON ACIDS & SIMPLE PREPARATIONS 16

17 PREPARATION OF ACIDS Acids can be prepared by dissolving a non-metal oxide in water. 17

18 ACIDS IN EVERYDAY LIFE Antacids contains NaHCO 3 and usually citric or tartaric acid. Baking powder is tartaric acid combined with NaHCO 3. NaHCO 3 neutralize stomach acid (gastric acid), but has an unpleasant taste. When water is added, some of the water reacts with the acid producing CO 2. The causes antacid to fizz and be less unpleasant tasting. When taken as a drink, the remaining neutralizes stomach acid. Heat causes the NaHCO 3 to decompose to liberate CO 2. NaHCO 3 Na 2 CO 3 + CO 2 + H 2 O The tartaric acid then reacts with the sodium carbonate in the presence of moisture forming one more carbon dioxide. The carbon dioxide forms bubbles in the cake causing it to rise. 18

19 DID YOU KNOW HOW TO PUT OUT A FIRE? In order to put out a fire, one of three things must be removed from it: heat, fuel or oxygen. Knowing this, firefighters don t always use water to put out a fire. Materials: empty soda bottle, 5 teaspoons of vinegar, ½ teaspoon of baking soda and tea light candle. Procedure: 1. Light the candle. 1. Pour the vinegar into the bottle and add the baking soda. (You may want to use a funnel.) The mixture should fizz. 2. Hold the bottle sideways over the lighted candle, making sure no liquid escapes. What happens to the flame? Explanation: The baking soda and vinegar react to make carbon dioxide, a gas that is heavier than oxygen. As it pours out the bottle, it pushes the lighter oxygen away from the candle. The fire, now deprived of oxygen, can no longer burn. 19

20 ACIDS IN YOUR BODY 20

21 USES OF ACIDS Sulphuric Manufacture of fertilizers Manufacture of detergents and soaps Manufacture of plant pigments Vulcanization of rubber Hydrochloric Manufacture of plastics Ascorbic acid in medicine. Nitric Manufacture of fertilizers Manufacture of explosives Manufacture of dyes Cleaning jewelry 21

22 ACID RAIN 22

23 BASICITIY (PROTICITY) OF ACIDS 23

24 PHYSICAL PROPERTIES OF ACIDS Acid generally have a sour taste. They have ph less than 7. They are colourless liquids. Corrosive, which means they break down certain substances. Many acids can corrode fabric, skin and paper. Conducts electricity, i.e. they are electrolytes. 24

25 CHEMICAL PROPERTIES OF ACIDS They react with alkalis/bases to form a salt and water. They react with reactive metals to form a salt and hydrogen gas. They react with carbonates/hydrogen carbonates to form a salt, water and carbon dioxide. They turn moist blue litmus red. **ACID GIVES RED, ALKALI BLUE, USING LITMUS, THIS IS TRUE! REFER TO WHITE BOARD FOR THE CHEMICAL REACTIONS 25

26 CHEMICAL REACTIONS WITH ACIDS How do acids react with metals? (Reactivity Series Must Know) How do acids react with a carbonate and hydrogen carbonate react with acid? How do acids react with bases? How do acids react with metallic oxides? 26

27 CHEMICAL REACTIONS WITH ACIDS 27

28 NEUTRALIZATION REACTIONS 28

29 NEUTRALIZATION REACTIONS 29

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31 NEUTRALIZATION REACTIONS 31

32 Insect Stings (Acidic) NEUTRALIZATION IN EVERYDAY LIFE Bee Stings (Alkaline) Rememember treatment: Bee Bicarb; Vinegar Vasp Ants Sting (Methanoic acid) Soil treatment (add quick lime) (CaO) + H 2 O Ca(OH) 2 (quick lime) (slake lime) 32

33 BASES A base is a solution that has an excess of OH - ions and will react with H + ions to form salt and water only. Bases are substances that can accept hydrogen ions. Another word for base is alkali (typically a soluble base). E.g. of alkalis - NaOH, KOH, Ca(OH) 2, ammonia solution. 33

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35 COMMON BASES (metal hydroxides) Sodium hydroxide, NaOH (lye for drain cleaner; soap) Potassium hydroxide, KOH (alkaline batteries) Magnesium hydroxide, Mg(OH) 2 (Milk of Magnesia) Calcium hydroxide, Ca(OH) 2 (lime; masonry) 35

36 PHYSICAL PROPERTIES OF BASES Feel Slippery or soapy to touch Taste Bitter Corrosive - concentrated bases can cause skin burn Can conduct electricity (Think alkaline batteries) Can be strong or weak electrolytes in aqueous solution. Do not react with metals ACID GIVES RED, ALKALI BLUE, USING LITMUS, THIS IS TRUE! 36

37 CHEMICAL PROPERTIES OF BASES 37

38 CHEMICAL PROPERTIES OF BASES 38

39 REACTIONS OF BASES How do bases react with metals? How do bases react with acids? How do bases react with metallic oxides? How do bases react with non-metallic oxides? 39

40 WEAK VERSUS STRONG BASES 40

41 NAMING BASES 41

42 FORMULA WRITING OF BASES 42

43 BASES STRENGTH 43

44 Bases give soaps, ammonia, and many other cleaning products (NH 3 solution) and drain or oven cleaners - NaOH); some of their useful properties. The OH - ions interact strongly with certain substances, such as dirt and grease. Chalk and oven cleaner are examples of familiar products that contain bases. Your blood is a basic solution. USES OF BASES 44

45 ACIDS AND BASES SIMILARITIES Can cause skin burn when concentrated. Neutralize each other to form a salt and water. 45

46 ACIDS AND BASE INDICATORS IN THE LABORATORY 46

47 You have probably been told not to taste or touch strong acids and bases. So how do we know how strong these chemicals are/ Acids and bases are terms used by chemists to categorize chemicals according to their ph. 47

48 ph (power of hydrogen) ph stands for power or potential hydrogen and is a measure of the number of H + in a solution. When a substance dissolves in water, it forms an aqueous solution that may be acidic, neutral or alkaline. ph is a measure of the relative acidity or alkalinity of a solution. Acids release H + into solutions and bases remove them. Water is a neutral liquid with a ph of 7 (green) Acidic solutions have ph less than 7 and the lower the number, the stronger the acid is. Neutral solutions have ph of 7. These are solutions of salt. Alkaline solutions have ph greater than 7, the stronger the alkali, the higher the ph. 48

49 ACID STRENGTHS (RECALL) The concentration of an acid is the number of moles (mol.) of the acid in solution. Dilute acids have a lower concentration. If you have 100 molecules of HCl, all 100 molecules will break down into H + and Cl -. The strength of acids is the extent to which the acids separates into ions. Some acids fully break down (ionize or dissociated) in aqueous solution and are termed strong acids. Eg. HCl Weak acids only break down partially in aqueous solutions. If you had a 100 molecules of vinegar, only 15 molecules will break down. CH 3 COOH (aq) CH 3 COO - + H + 85 mol 15 mol 15 mol 49

50 ph OF COMMON SUBSTANCES 50

51 DETERMINING ACIDS AND BASES There are four main ways to determine if a substance is an acid or a base. 1. Red litmus paper 2. Blue litmus paper 3. ph 4. Red cabbage indicator 51

52 RED AND BLUE LITMUS PAPERS Robert Boyle discovered the litmus paper. Certain plant extracts, such as litmus, can used to distinguish acids from bases. Blue and red litmus paper turn red when dipped in an acid. Red and blue litmus paper turn blue when exposed to a base. 52

53 UNIVERSAL INDICATOR/ ph CHART Universal indicator is a mixture of different dyes. It can change through a range of colours in both acid and alkali. We match these colours to numbers on a chart. The numbers tell us how strongly or weakly acidic or alkaline a solution is. This is essentially called the ph scale. 53

54 IMPORTANCE OF ph IN EVERYDAY LIFE 54

55 NEITHER ACID NOR ALKALI - If a solution is neither acidic nor alkaline. We say that is a neutral solution. NEUTRALS Neutral substances will turn blue and red litmus paper green. The main example of a neutral item is pure water. Alcohols are also neutral, as are solutions of table salt and sugar, milk and hand lotion. 55

56 SALTS To know whats a salt, we have to investigate acids and bases 56

57 MORE ABOUT SALTS A salt is any ionic compound composed of cations and anions so that the product is neutral without a net charge. E.g. NaCl. Salts contain an OH - ion are called basic salts. Salts which contain a H + ion are called acid salts. Normal salts are those that are neither acid or basic salt. When salts are d in water, they are called electrolytes, and are able to conduct electricity. Salts can be clear and transparent, opaque and even metallic and lustrous (iron disulphide). Different salts can elicit all five basic tastes (salt, sweet, sour, bitter and savoury). 57

58 TYPES OF SALTS NORMAL SALTS Normal salts are formed when all of the H + ions are replaced. ACID SALTS Acid salts are formed ONLY when SOME of the H + ions are replaced. Acid salts thus contain some H+ ions from the original acid. Only dibasic and tribasic acids can form acid salts. NaOH + H 2 SO 4 NaHSO 4 + H 2 O. acid salt Try to form an acid salt using NaOH with phosphoric acid. 58

59 1. Acid 2. Acidic oxide 3. Acid anyhydride 4. Salt 5. Base 6. Basic oxide 7. Alkali 8. Neutral oxide 9. Amphoteric oxide HANDOUT 59

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62 PLANNING AND DESIGNING DISCUSSION Problem statement A flour sample is known to be contaminated with one of the following substances: calcium chloride, sodium carbonate, zinc sulphate or lead nitrate. Plan and design an experiment in which would allow you to identify which of the four substances is the contaminant. 62

63 ACIDS, BASES & SALTS WORKSHEET 63

64 ACTIVITY SERIES HANDOUT & WORKSHEET 64

65 ACTIVITY SERIES HANDOUT & WORKSHEET 65