UGEB2380 The Chemistry of Life 2024-25 Lecture Notes PDF

UGEB2380 The Chemistry of Life Academic Year 2024-25 Dr. Sam CK HAU (Department of Chemistry) Acids and Bases Water Chemistry The oceans may seem remote from our living environment But this watery world is greatly affected by our activities and the pollutants we generate and release i...

UGEB2380 The Chemistry of Life Academic Year 2024-25 Dr. Sam CK HAU (Department of Chemistry) Acids and Bases Water Chemistry The oceans may seem remote from our living environment But this watery world is greatly affected by our activities and the pollutants we generate and release into it 3 Acids, Bases and Neutralisation Acid Latin “Acidus” - sour, or having a sharp taste Turn litmus paper red React with certain metals, like magnesium or zinc to liberate hydrogen gas bubbles Many common foods like lemon or vinegar contain acid and taste distinctly acid Litmus paper - an absorbent paper strip containing a natural 4 dye that turns red in acids and blue in bases Acids, Bases and Neutralisation Bases Turn litmus paper blue Feel slippery React with acids to form salts 5  Acids, Bases and Neutralisation Toilet bowl cleaner contain hydrochloric Vitamin C is also acid known as ascorbic HC acid, C6H8O6 Vinegar is a dilute solution of acetic acid, CH3COOH Lemons are a good source of citric acid, C6H8O7 6  Acids, Bases and Neutralisation Drain and oven Baking soda is also cleaners can contain known as NaHCO3 sodium hydroxide Calcium carbonate, CaCO3, use to combat excess stomach acid and relieve heartburn · 7 Definition of Acids and Bases Arrhenius definition (1887) Acid as any substances that produce hydrogen ions, H+, in water; Bases as any substances that produce hydroxide ions, OH ˉ in water E.g. when hydrochloric acid (HCl) dissolves in water, it dissociation into hydrogen ion (H+) and chloride (Cl ˉ ) Restricted to aqueous, or water-based solutions Definition of Acids and Bases Brønsted-Lowry definition (1923) Acid as any substances that donate hydrogen ions, H+, in water; Bases as any substances that accept hydrogen ions, H+ in water E.g. when hydrochloric acid (HCl) donates a proton (H+), to the base, (NH3), to form ammonium ion (NH4+) and chloride (Cl ˉ ) Apply to solutions of any kinds, both aqueous and non-aqueous Neutralisation Reactions bE-Ek - A reaction between an acid and a base to produce a salt and water HCl + NaOH → NaCl + H2O The H of the HCl and the OH of the NaOH combine to form water H2O; a combination of Cl and Na to produce NaCl Neutralisation can be found in a variety of practical everyday uses Use vinegar to clean a shower-head Epsom salt as the bath salts 2CH3CO2H + CaCO3 H2SO4 + Mg(OH)2 → MgSO4 + 2H2O → Ca(CH3CO2)2 + H2O + CO2 10 Conjugate Acid-Base Pairs A pair of chemical species differ by the presence of a proton (H+) is an acid-base conjugate pair Base Carbonic Acid X H2CO3 + H2O H3O+ + HCO3 Conjugate Conjugate Base Base Bicarbonate ion Carbonate ion Acetic Acid CH3COOH + H2O H3O+ + CH3CO2 Conjugate Base Acetate ion Ammonia NH3 + H2O OH + NH4+ Conjugate Acid Ammonium ion 11 The pH Scale Amphoteric Water Amphoteric - able to act as either an acid or a base Water ionises and then re-form itself, the amount of ionised products is exceedingly small Brønsted-Lowry Arrhenius Definition Definition H O H+ + HO O + O + OH H H H H H H O proton hydroxide ion H H Hydronium H 2O ion H O H H Hydronium ion 12 The pH Scale Amphoteric Water The simultaneous ionisation and recombination of water is a dynamic equilibrium Dynamic equilibrium - a state in which the rate of forward reaction equals the rate of the reverse reaction, so that the concentrations of all chemical species remain constant Compound A Compound B 3A→3B 3B→3A 13 The pH Scale Amphoteric Water At any instant, countless covalent water molecules to hydronium and hydroxide ions; at the same instant, equal amount of hydronium and hydroxide ions recombine to generate water The concentrations of all the transient hydronium and hydroxide ions at equilibrium are fixed at any given temperature and equal to each other H 2O H3O+ + HO- Concentration of hydronium ion H3O+ [H3O+] = 1 x 10-7 M, Equally, [OH ˉ ] = 1 x 10-7 M 14 The pH Scale In 1909, Danish biochemist Søren Sørensen proposed the concentration of [H+] be treated as exponential values pH value = “power of the hydrogen (hydronium) ions”, as a symbol of acidity pH - a measure of acidity, and defined as the negative logarithm of the hydronium ion concentration pH = –log[H3O+] H 2O H3 O+ Eve + HO- Concentration of hydronium ion H3O+ at 25℃ [H3O+] = 1 x 10-7 M, Equally, [OH ˉ ] = 1 x 10-7 M pH = –log[H3O+] = –log(1 x 10-7) =7 15 The pH Scale * In water solution at 25℃, [H3O+] x [OH ˉ ] = 1 x 10-14 i.e. pH + pOH = 14 Increase the [H3O+] of a solution by adding acid Adding base to water increase [OH ˉ ] but lowers [H3O+] 16 The pH Scale Measuring pH and acid-base indicators A pH meter very accurately measures Universal test strips are saturated with acid- hydronium ion concentration and can be used base indicators that turn various colours as the to record the pH of a sample of yogurt pH changes, but are less accurate than pH meters EE -JE 17  Strong vs. Weak Acids Water generates a very small concentration of hydronium ions from very slight ionisation In contrast, the ionisation of hydrochloric acid (HCl) in water produces a relatively large number of hydronium and chloride ions This is a characteristic of a strong acid Hydronium ion H 2O H Cl H3O+ + Cl Hydrochloric acid Chloride ion Before dissolving in water, HCl has not After dissolving in ionised, so consist of water, HCl is only acid molecules completely ionised and no ions HCl H3O+ Cl 18 HCl H3O+ Cl  Strong vs. Weak Acids In contrast, acetic acid ionises reversibly and partially in water O - O C + H 2O C + H3O+ HO CH3 O CH3 Acetic Acid Acetate Before dissolving in After dissolving in water, acetic acid has water, at equilibrium, not ionised, so consist only a fraction of the of only acid molecules has ionised. and no ions Acetic Acetate H3O+ Acetic Acetate H3O+ acid acid Incomplete ionisation is characteristic of a weak acid 19 Strong vs. Weak Acids In summary, Strong acid - an acid ionises completely in water to form an equivalent amount of hydronium ions Weak acid - an acid ionises partially in water 20 Strong vs. Weak Bases Weak bases are similar to weak acids - ionises reversibly and partially in water Bases accept protons from water as opposed to donating them H H N H + H 2O H N H + OH H H Ammonia Ammonium ion Before dissolving in After dissolving in water, ammonia has water, at equilibrium, not ionised, so consist only a fraction of the of only base molecules base has ionised. and no ions NH3 OH NH4+ NH3 OH NH4+ Strong bases like NaOH are similar to strong acids that dissociate completely in water 21 Acids and Bases in Everyday Life Strongest acid that we encountered - gastric juices secreted by our stomach containing HCl The digestion of proteins requires an acidic environment Stomach enzyme pepsin - cut large proteins into smaller ones but works best at a pH of about 1.5 - 2.5, and not effective when pH at 4 to 5 24 Acids and Bases in Everyday Life To maintain our acidic digestive environment, our foods are generally acidic to neutral, with a few in the weak basic category. Citrus fruits, vinegar, pickles, soft drinks, wines, and tomatoes are generally run in a pH range of 2.0 to 4.5 Cow’s milk is slightly acidic (6.4~6.8); Fresh eggs are slightly basic (7.5~8.0) 25 Acids and Bases in Everyday Life Many of the acids present in our foods are carboxylic acids O O C C OH OH HO C E L Carboxyl group - O Acetic acid (Vinegar) is the simplest carboxylic acid IOxalic Acid Ent Ca-ex Th 3 e. g Milk. Oxalic acid - dicarboxylic acid, occurs in green leafy H 2C COOH veggies like spinach HO C COOH Citric acid - tricarboxylic H 2C COOH acid, the principal acid of the Citrus Acid citrus fruits 26 Acids and Bases in Everyday Life Citrus acid - an example of alpha-hydroxy acids (AHAs) Carboxyl group COOH 𝜶 𝛃 OH O H C C OH I ↳ OH Salicylic acid Alpha (𝜶) carbon Hydroxy group Carboxyl group ge-h C by < 2nd eB..... 4theD Widely used in a variety of skin and facial creams to improve skin texture and reduce wrinkles The greater the AHA content, the stronger the exfoliating (去⾓質) effect 27 Acids and Bases in Everyday Life Antiacid Antacids An acidic stomach is essential to good health, but excess stomach acidity can cause uncomfortable and dangerous strong : too vigorous Contains weak bases to neutralise Cont ~ - excess stomach acid > with acid > - - Sodium bicarbonate (NaHCO3, pH ~8.4), Calcium carbonate (CaCO3, pH ~9.4), Magnesium hydroxide (Mg(OH)2, pH ~10.5) 28 Acids and Bases in Everyday Life Buffers -weak acid + weak alkali Living organism are generally sensitive to changes in pH and have buffers to help maintain a stable pH A combination of a weak acid and its conjugate base that resist changes in pH Eg. Blood is buffered to maintain a pH of about 7.4, which is slightly basic by the presence of carbonic acid and bicarbonate ion Added OHˉ Carbonic acid neutralise added base, OHˉ weak was H2CO3 base HCO3 + H2O Carbonic Acid Conjugate Base Bicarbonate ion Bicarbonate ions neutralise added acid, Added H+ H+ 29 Acids in our Environment Acid Rain All rainfall is naturally acidic as the water absorbs atmospheric carbon dioxide (CO2) to form carbonic acid (H2CO3) With a high enough concentration of CO2, the pH of rainwater can fall as low as 5.6 When pH < 5.6 due to other acidic components → acid rain - 30 Acids in our Environment Acid Rain Generate by the absorption of a variety of atmospheric pollutants, by both natural and human activities Volcanic eruption and burning fossil fuels - Sulphur dioxide (SO2) → Sulphuric acid (H2SO4), a strong acid Lighting and bacterial action, also combustion of fossil fuels in cars and trucks - Nitrogen oxides (NOx) → Nitric acid (HNO3), a strong acid Produce a rainfall with a pH as low as vinegar (pH ~2) 31 Acids in our Environment Ocean Acidification The oceans absorb a significant amount (more than a quarter) of the CO2 generated by burning the fossil fuels The CO2 then dissolves and converts into carbonic acid (H2CO3) The ocean pH is predicted to drop from 8.1 to 7.8 in next century Can harm the ocean life in a variety of ways, including the slowing of growth or decay the shells t broke of marine organisms 32