ASRJC H2 Chem 2. Atoms, Molecules, and Stiochiometry

2024 JC1 H2 The Mole Concept, Stoichiometry & Redox Reactions 2024/ASRJC/Chemistry 1 Anderson Serangoon Junior College JC1 H2 Chemistry THE MOLE CONCEPT, STOICHIOMETRY & REDOX REACTIONS Learning Outcomes [the term relative formula mass or Mr will be used for ionic compounds] Candidates should be able to: (a) define the terms relative atomic, isotopic, molecular and formula mass (b) define the term mole in terms of the Avogadro constant (c) calculate the relative atomic mass of an element given the relative abundances of its isotopes (d) define the terms empirical and molecular formula (e) calculate empirical and molecular formulae using combustion data or composition by mass (f) write and/or construct balanced equations (g) perform calculations, including use of the mole concept, involving: (i) reacting masses (from formulae and equations) (ii) volumes of gases (e.g. in the burning of hydrocarbons) (iii) volumes and concentrations of solutions [when performing calculations, candidates’ answers should reflect the number of significant figures giv en or asked for in the question] (h) deduce stoichiometric relationships from calculations such as those in (g) (i) describe and explain redox processes in terms of electron transfer and/or of changes in oxid ation number (oxidation state) (j) construct redox equations using the relevant half−equations (k) Describe and explain the use of Fe 3+/Fe2+, MnO4−/Mn2+, Cr2O72−/Cr3+ and iodometric titration as examples of redox systems References 1. Chemistry for Advanced Level, Peter Cann & Peter Hughes 2. Chemistry, JGR Briggs 3. Chemistry in Context, 5th edition, Graham Hill & John Holman 4. A−Level Chemistry, Fourth Edition, E.N. Ramsden 5. Chemistry, Sixth Edition, Steven S. Zumdahl & Susan A. Zumdahl 6. Chemistry: The Molecular Nature of Matter and Change, 4th Edition, Martin S. Silberberg

2024 JC1 H2 The Mole Concept, Stoichiometry & Redox Reactions 2024/ASRJC/Chemistry 2 TABLE OF CONTENTS Pg 1. Relative Masses of Atoms and Molecules ……...……………………………………. 3 1.1 Relative Masses of Atoms …………………………………………………………. 3 1.2 Relative Molecular Mass and Relative Formula Mass …………………………... 4 2. The Mole, The Avogadro Constant and Molar Mass ……………………………..… 6 2.1 The Mole, The Avogadro Constant ………………………………………………. 6 2.2 Molar Mass ………………………………………………………………………….. 7 3. Stoichiometry ………………………………………………………………………….….. 8 3.1 Balancing Chemical Equations …………………………………………………… 8 3.2 Stoichiometry involving Gases ……………………………………………………. 9 3.2.1 Relationship between the Mole and Volume of Gases ……………….. 9 3.2.2 Molar Volumes of Gases ………………………………………………… 9 3.3 Calculations involving Reacting Masses ………………………………………… 11 3.4 Calculations involving Limiting Reagents ………………………………………... 13 3.5 Theoretical, Experimental and Percentage Yield ……………………………….. 14 4. Empirical and Molecular Formulae ………………………………………………… ..... 17 4.1 Determination of Empirical and Molecular Formulae using Composition by Mass …………………………………………………………………………………. 17 4.2 Determination of Molecular Formula using Combustion Data ……..………….. 19 5. Volumetric Analysis ……………………………………………………………………… 22 5.1 Theory of Volumetric Analysis ……………………………………………………. 22 5.2 Solutions and Standard Solutions ………………………………………………... 23 5.3 Dilution and Sampling ……………………………………………………………… 25 5.4 Back Titration ……………………………………………………………………….. 30 6. Redox Processes …………………………………………………………………………. 34 6.1 Definition of Oxidation and Reduction …………………………………………… 34 6.2 Oxidation Number ………………………………………………………………….. 34 6.2.1 Definition of Oxidation Number …………………………………………. 34 6.2.2 Rules for Assigning Oxidation Number ………………………………… 34 6.3 Oxidising and Reducing Agents ………………………………………………….. 37 6.3.1 Oxidising Agents ………………………………………………………….. 37 6.3.2 Reducing Agents …………………………………………………………. 38 6.3.3 Application of Oxidising and Reducing Agents ……………………… 38 6.4 Redox Reactions …………………………………………………………………… 39 6.5 Balancing Redox Equations ………………………………………………………. 40 6.5.1 The Oxidation Number Method …………………………………………. 40 6.5.2 The Half−Equation Method ……………………………………………… 41 6.5.3 Writing Overall Redox Equation ……………………………………....... 42 7. Redox Titrations ……………………………………………………………………….….. 44 7.1 Manganate(VII) (MnO4−) Titration ……………………………………………….. 44 7.2 Dichromate(VI) (Cr2O72−) Titration ……………………………………………... 46 7.3 Iodometric Titration using Thiosulfate (S2O82−) …………………………………. 47 7.4 Determination of Unknown Oxidation Number ………………………………….. 49

2024 JC1 H2 The Mole Concept, Stoichiometry & Redox Reactions 2024/ASRJC/Chemistry 3 Introduction Chemistry is observed all the time. The chemical reactions occurring in your brain allows you to read and understand this statement. The food you ate is now providing you with energy through chemical reactions. In the study of Chemistry, we learn about the structure and properties of matter, its interactions and transformation. During transformation of matter, a chemical and/or physical change takes place. Matter and energy are conserved in all of such transformations. In this topic, we will focus on the stoichiometry − study of the quantitative relationship between the amounts of reactants and products − in chemical reactions. This key concept is an important tool for chemists in the area of food, health, medicine and engineering. 1. Relative Masses of Atoms And Molecules • What do the terms relative isotopic, atomic, molecular and formula mass mean? • Can you calculate the relative atomic mass of an element given the relative abundances of its isotopes? 1.1 Relative Masses of Atoms Atoms have very small masses. For example, a hydrogen atom weighs 1.66 x 10−24 g. Instead of using their actual masses, we measure how heavy one atom is compared with another atom. The carbon−12 isotope, 12C is used as the standard of reference. Relative isotopic mass of an isotope = isotope Cof atom one of mass isotope the of atom one of mass 12 12 1  Example: Relative isotopic mass of 13C = 13 Relative isotopic mass of 37Cl = 37 Relative atomic mass of an element, Ar = 121 12 average mass of one atom of the element mass of one atom of C isotope Most elements consist of a mixture of isotopes. The average of the relative isotopic masses of the element is equal to the relative atomic mass of the element. Recall: Isotopes are atoms of the same element with different number of neutrons. They have identical atomic (proton) numbers but different mass numbers.

2024 JC1 H2 The Mole Concept, Stoichiometry & Redox Reactions 2024/ASRJC/Chemistry 4 1.2 Relative Molecular Mass and Relative Formula Mass Relative molecular mass of a substance, Mr = The relative molecular mass of a compound is also the sum of the relative atomic masses of all the atoms in a molecule of a compound. Example: Mr of HCl = 1 x 1.0 + 1 x 35.5 = 36.5 (1 d.p.) Mr of O2 = 2 x 16.0 = 32.0 (1 d.p.) For giant ionic and giant covalent compounds, they do not consist of individual molecules. Relative formula mass, with the same symbol Mr is used for such substances. Relative formula mass of a substance, Mr = Similarly, the relative formula mass of a compound is the sum of the relative atomic masses of all the atoms in one formula unit of a