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L EARNING O UTCOMES

Chapter 4. Elements and Compounds. L EARNING O UTCOMES. Write formulae to represent ions and molecules Write balanced equations including state symbols to represent chemical reactions referred to in the syllabus. Chapter 4. An element. Elements and Compounds.

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L EARNING O UTCOMES

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  1. Chapter 4 Elements and Compounds LEARNINGOUTCOMES • Write formulae to represent ions and molecules • Write balanced equations including state symbols to represent chemical reactions referred to in the syllabus

  2. Chapter 4 An element Elements and Compounds • When mercuric oxide is heated, it decomposes into oxygen and mercury. • However, both oxygen and mercury cannot be broken down into anything simpler. • Oxygen and mercury are examples of elements. An element is a substance which cannot be broken down into any simpler substances by chemical means.

  3. Chapter 4 Elements Elements and Compounds • Elements are the fundamental building blocks of matter in our universe. • There are about 92 natural elements and more than 10 man-made elements. • Each element has a name and a chemical symbol. • A list of elements with their symbols is given in the Periodic Table. The Periodic Table of the Elements http://www.chemicool.com/

  4. Chapter 4 Atoms of elements Elements and Compounds • An element is made up of onlyone kind of atom. • For example, gold is made up of only gold atoms. • Oxygen is made up of molecules each consisting of two oxygen atoms, while ozone is made up of molecules each containing three oxygen atoms.

  5. Chapter 4 Metals and non-metals Elements and Compounds • Elements can be classified into metals and non-metals. • Metals are usually hard and shiny. They are malleableandductileand aregood conductors of heat and electricity. • Non-metals are usually soft and brittle, and are poor conductors of heat and electricity. • There are more metals than non-metals. Copper: a metal Sulphur: a non- metal

  6. Chapter 4 Quick check 1 Elements and Compounds • “Magnesium is an element”. Explain what this statement means. • “A piece of copper can be broken down into very tiny pieces, hence copper is not an element.” Explain what is wrong with this statement. • Give the symbol for each of the following elements. State whether it is a metal or non-metal.(a) Mercury, (b) Lead, (c) Silver, (d) Chlorine, (e) Strontium, (f) Tungsten. Solution

  7. Chapter 4 Solution to Quick Check 1 Elements and Compounds • Magnesium is an element because it cannot be broken down into simpler substances. Magnesium is made up of magnesium atoms and nothing else. • A piece of copper can be broken down into very tiny pieces but each tiny piece of copper is still made up of only copper atoms, hence copper is an element. • (a) Mercury: Hg (metal), (b) Lead: Pb (metal), (c) Silver: Ag (metal), (d) Chlorine: Cl (non-metal), (e) Strontium: Sr (metal), (f) tungsten: W (metal). Return

  8. Chapter 4 A Compound Elements and Compounds A compound is a substance made up of two ormore elementschemically combined together. • Mercuric oxide is not an element because it is made up of mercury and oxygen. • It is called a compound. Element + Element Compound

  9. Chapter 4 Atoms of compounds Elements and Compounds • A compound is made up of molecules. • A molecule of a compound is made up of two or more different types of atoms chemically joined together. carbon dioxide molecules water molecules methane molecules

  10. Chapter 4 Formulae of compounds Elements and Compounds • As a compound has a fixed composition, it can be represented by a formula. • The formula of a compound shows: • the symbols of the elements present • the ratio of the atoms present For example, water has the formula H2O. H2O shows 2 hydrogen atoms and 1 oxygen atom

  11. Chapter 4 Formulae of some compounds Elements and Compounds

  12. Chapter 4 Valency of an element Elements and Compounds • To help us write chemical formulae more easily, we can use the valency of an element. • The valency of an element can be treated as the “combining power” of an element. • The valency of an element is related to the electronic structure of the atom and the Group number of the element in the Periodic Table.

  13. Chapter 4 Valencies of some common elements (metals) Elements and Compounds

  14. Chapter 4 Valencies of some common elements (non-metals) Elements and Compounds

  15. Chapter 4 Valencies of some group atoms Elements and Compounds

  16. Chapter 4 Writing formulae Elements and Compounds • Metals (or cations) can react with non-metals (or anions). • When a formula is formed, the positive charges of the cation have to be balanced by the negative charges of the anion. • The total charge of a neutral compound must be zero. • Examples:1. Sodium chloride: Na+ + Cl− NaCl 2. Copper(II) oxide: Cu2+ + O2− MgO

  17. Chapter 4 Writing formula Elements and Compounds • Example 3: magnesium chloride Mg2+ + (Cl−)x 2 MgCl2 • Example 4: aluminium oxide (Al3+)x2 + (O2−)x3 Al2O3 • Example 5: ammonium sulphate (NH4+)x2 + (SO42−) (NH4)2SO4

  18. Chapter 4 Quick check 3 Elements and Compounds Write the formula for each of the following compounds: Solution

  19. Chapter 4 Solution to Quick check 3 Elements and Compounds Return

  20. Chapter 4 Writing chemical equations Elements and Compounds • A chemical equation tells us what chemical changes take place during a reaction. • It tells us what the reactants (things that react) and what the products(things that are formed) are. • A chemical equation must be balanced. • This means that the total number and types of atoms on the right side of the equation must be equal to those on the left side of the equation. This is because atoms cannot be created or destroyed. Word equation Mercury + oxygen  mercuric oxide [ Reactants ] [ product ]

  21. Chapter 4 Writing chemical equations Elements and Compounds Mercury + oxygen  mercuric oxide Step 1: Write down the chemical formula for each reactant and product: Hg + O2 HgO Step 2: Count the number of atoms on each side of the equation: Left side:1 Hg atom + 2 O atoms Right side: 1 Hg atom + 1 O atom

  22. Chapter 4 Balancing chemical equations Elements and Compounds The equation is not balanced because the right side has 1 less oxygen atom. Step 3: To balance the equation, add 2 in front of HgO, and again count the number of atoms on both sides of the equation: Hg + O2 2 HgO Left side: 1 Hg atom + 2 O atoms Right side: 2 Hg atoms + 2 O atoms The equation is still not balanced because the left side has 1 less mercury atom.

  23. Chapter 4 Writing chemical equations Elements and Compounds Step 4: To balance the equation, add 2 in front of Hg, and again count the number of atoms on both sides of the equation: 2 Hg + O2 2 HgO Left side: 2 Hg atom + 2 O atoms Right side: 2 Hg atoms + 2 O atoms The equation is now correctly balanced. 2 Hg + O2 2 HgO

  24. Chapter 4 State symbols in chemical equations Elements and Compounds • The state symbols tell us the physical states of the reactants and products in a chemical reaction. • (s)  solid state • (l)  liquid state • (g)  gaseous state • (aq)  aqueous state (solution in water) E.g. Write a balanced chemical equation, with state symbols, for the reaction between dilute hydrochloric acid and limestone chips (calcium carbonate). 2 HCl(aq) + CaCO3(s)  CaCl2(aq) + H2O(l) + CO2(g)

  25. Chapter 4 Quick check 4 Elements and Compounds 1. Balance the following chemical equations by writing numbers in the blank spaces provided: • __ Ca + __ O2 __ CaO • __ N2 + __ H2  __ NH3 • __ C2H4 + __ O2  __ CO2 + __ H2O • __ Mg + __ Fe2O3  __ MgO + __ Fe • __ NH3 + __ O2  __ N2 + __ H2O • __ Pb(NO3)2  __ PbO + __ NO2 + __ O2 • __ KClO3  __ KCl + __ O2 • __ CaCl2 + __ Na2CO3  __ CaCO3 + __ NaCl Solution

  26. Chapter 4 Quick check 4 Elements and Compounds 2. Write balanced chemical equations with state symbols for the following word equations: • Magnesium + Oxygen Magnesium oxide • Hydrogen + Oxygen  Water • Mercuric(II) oxide  Mercury + Oxygen • Sodium + Oxygen  Sodium oxide • Ammonia + Sulphuric acid  Ammonium sulphate • Ammonium chloride + Sodium hydroxide  Sodium chloride + water + ammonia • Zinc + Hydrochloric acid  Zinc chloride + hydrogen Solution

  27. Chapter 4 Solution to Quick check 4 Elements and Compounds Q1. • 2_Ca + __ O2 2_ CaO • __ N2 + 3_H2  2_ NH3 • __ C2H4 + 3_O2  2_CO2 + 2_H2O • 3 Mg + __ Fe2O3  3_ MgO + 2_ Fe • 4_NH3 + 3_O2  2_N2 + 6 H2O • 2_ Pb(NO3)2  2_PbO + 4_ NO2 + __ O2 • 2_ KClO3  2_KCl + 3_O2 • __ CaCl2 + __ Na2CO3  __ CaCO3 + 2_NaCl Return

  28. Chapter 4 Solution to Quick check 4 Elements and Compounds Q2. • 2Mg(s) + O2(g) 2MgO(s) • 2H2(g) + O2(g)  2H2O(l) • 2HgO(s)  2Hg(l) + O2(g) • 4Na(s) + O2(g)  2Na2O(s) • 2NH3(g) + H2SO4(aq)  (NH4)2SO4(aq) • NH4Cl(s) + NaOH(aq)  NaCl(aq) + H2O(l) + NH3(g) • Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g) Return

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