[CHEM] Chapter 7.1 - Acid Bases

1 DARRELL ER (COPYRIGHTED) TOPIC 7.1: ACID & BASES DARRELL ER (COPYRIGHTED) ©

2 CHAPTER ANALYSIS THE ABOUT TIME EXAM WEIGHTAGE • Very important chapter that is needed for Salts & QA • 4 key concepts: Acids, Bases, pH & indicators • 2 advanced concepts: Strength/Concentration/Basicity & Base vs Alkaline • Commonly tested every year Need to know relevant chemical and ionic equations (not going through in detail here, will focus on equations for “Chemical Equations” chapter instead) • Medium overall weightage • Constitute to 6% of marks for past 5 year papers DARRELL ER (COPYRIGHTED) ©

3 ACIDIC PROPERTIES ACID’S CHEMICAL REACTIONS STRENGTH, CONCENTRATION, BASICITY KEY CONCEPTS DARRELL ER (COPYRIGHTED) ©

What makes a compound/substance an ‘acid’? A compound is considered as an ‘acid’ due to the ability to dissociate H+ ions when they are dissolved in water. For example, HCl(l) in liquid state is called hydrogen chloride. (not acid yet) HCl(aq) in aqueous state is called hydrochloric acid. Hydrogen Chloride Hydrochloric Acid This is because when HCl dissolves in water, it dissociates to produce H+ ions which classifies it as an acid. HCl (aq)  H+ (aq) + Cl- (aq) In other words, an acid is only considered as an acid after it has been dissolved in water! 4 ACIDS Physical properties of acids 1) Acids tastes sour. 2) Dilute acids are irritants and will result in rashes and blisters. 3) Acids has the ability to change the colour of indicators, turning blue litmus paper to red. Examples of acids: Hydrochloric acid, HCl Sulfuric acid, H2SO4 Nitric acid, HNO3 Phosphoric acid, H3PO4 Hydrofluoric acid, HF Hydrobromic acid, HBr *Aqueous refers to a compound being in a solution. In other words, water is added. DARRELL ER (COPYRIGHTED) © Cl Cl Cl- Cl- H H H + H +

5 3 CHEMICAL REACTIONS ACID + METAL ACID + BASE ACID + CARBONATE 1) ACID + METAL  SALT + HYDROGEN GAS For example, H2SO4 + Mg  MgSO4 + H2 In order to test for hydrogen gas, use a lighted splint, it should extinguish with ‘pop’ sound. 2) ACID + BASE  SALT + WATER For example, H2SO4 + MgO  MgSO4 + H2O This is also known as a neutralisation reaction. 3) ACID + CARBONATE  SALT + WATER + CARBON DIOXIDE For example, H2SO4 + MgCO3  MgSO4 + H2O + CO2 In order to test for carbon dioxide gas, bubble the carbon dioxide gas into limewater, Ca(OH)2, a white precipitate will be formed. 1) ACID + METAL  SALT + HYDROGEN GAS 2) ACID + BASE  SALT + WATER 3) ACID + CARBONATE  SALT + WATER + CARBON DIOXIDE GAS DARRELL ER (COPYRIGHTED) ©

6 What is the relationship between the 3? STRENGTH CONCENTRATION BASICITY Let’s understand how all 3 are distinct yet related. Let’s run through each of them individually and keep it simple. ADVANCED CONCEPT DARRELL ER (COPYRIGHTED) ©

7 Strength of acid STRENGTH Strength of acid depends on the nature of the acid. Strong acids have the ability to dissociate fully in water to produce a high concentration of H+ ions. Examples: nitric acid, hydrochloric acid, sulfuric acid Weak acids only have the ability to dissociate partially in water to produce a low concentration of H+ ions. Examples: citric acid, hydrofluoric acid, organic acid such as ethanoic acid Hydrochloric acid fully dissociate in water to produce a high concentration of H+ ions. While for hydrofluoric acid, not all of them have dissociated, so there are much lesser H+ ions. So if you were to compare the strength of the acid, hydrochloric acid is the stronger acid, since it produces more H+ ions. DARRELL ER (COPYRIGHTED) ©

8 Concentration of acid Concentration of an acid is dependent on the dilution factor of the solution, or put in simple terms, the amount of water added. Concentration refers to the number of acid molecules present per water molecule. Concentration is usually expressed in moldm-3. (Learn in detail in chapter “Mole Concept”.) Visualise this: A strong acid like nitric acid can be very corrosive and dangerous at high concentrations. However, students use nitric acid on a regular basis in the school laboratories . To make it safer, the lab staff adds a large amount of water to dilute the nitric acid. Dilute nitric acid will then be able to be used in the chemical experiments, also being safe to use for the students as it is diluted. Another example: We buy ribena syrup from NTUC. Do we drink it straight? No, that will be too sweet because it is so concentrated. So, what we do is to dilute the syrup by adding water. This is exactly the same for acid! DARRELL ER (COPYRIGHTED) ©

9 Basicity of acid The basicity of an acid depends the acid’s chemical formula. Basicity refers to the number of H+ ions produced per molecule. 1 H+ ion produced: monobasic 2 H+ ions produced: dibasic 3 H+ ions produced: tribasic BASICITY DARRELL ER (COPYRIGHTED) © Acid Chemical Formula Basicity Hydrochloric acid HCl monobasic Nitric acid HNO3 monobasic Sulfuric acid H2SO4 dibasic Phosphoric acid H3PO4 tribasic

10 What is the relationship between the 3? STRENGTH CONCENTRATION BASICITY Strength of acid depends on the nature of the acid. Concentration of an acid depends on the dilution factor. Basicity of an acid depends the acid’s chemical formula. Can a strong acid have a low concentration of H+ ions? Yes, use a strong acid like nitric acid and add a large amounts of water to it. If we were to compare nitric acid & sulfuric acid, both strong acids, which one will produce a higher concentration of H+ ions? Even though both strong acids will dissociate fully in water, sulfuric acid would produce a higher concentration of H+ ions . Sulfuric acid is dibasic and has a formula of H2SO4, producing 2 H+ ions per molecule. Nitric acid is monobasic and has a formula of HNO3, producing 1 H+ ions per molecule. DARRELL ER (COPYRIGHTED) ©