[CHEM] Chapter 3 - Mole Concept Chemical Equations

1 DARRELL ER (COPYRIGHTED) © DARRELL ER (COPYRIGHTED) © TOPIC 3: MOLE CONCEPT & CHEMICAL EQUATIONS

2 CHAPTER ANALYSIS THE ABOUT TIME EXAM WEIGHTAGE • Need to practice a lot • 5 key concepts • Heavily tested • Tested as add-on to other chapters  Acid & Bases, Electrolysis etc… • Heavy overall weightage • Constitute to 8% of marks for past 5 year papers DARRELL ER (COPYRIGHTED) ©

3 CHEMICAL EQUATION CHEMICAL FORMULA BALANCING CHEMICAL EQUATION IONIC EQUATION KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

4 CHEMICAL FORMULA IONIC COMPOUNDS Some common anions: Carbonate CO32- Nitrate NO3 - Phosphate PO4 3- Sulfate SO4 2- Chloride Cl- Forming of ionic compounds: For example, Cation: Ca2+ Anion: NO3- To balance out charges, 1 x Ca2+ & 2 x NO3 - Compound: Ca(NO3)2 DARRELL ER (COPYRIGHTED) © Cation: Anion:

5 CHEMICAL FORMULA COVALENT COMPOUNDS Prefixes are generally used to name compounds. Prefix: Mono – 1 Di – 2 Tri – 3 Tetra – 4 Pent – 5 For example, Nitrogen monoxide – NO Nitrogen dioxide – NO2 DARRELL ER (COPYRIGHTED) © Chlorine molecule Methane compound Water compound

6 CHEMICAL EQUATION STATE SYMBOLS Solid (s) Liquid (l) Gaseous (g) Aqueous (aq) – exist as ions in a solution, water was added. BALANCING EQUATIONS Check that the number of atoms for each element is equal on both sides of the equation (reactants & products). To balance the chemical equation, you will need to add a coefficient in front of the compounds that are not balanced. For example, MgCl2 (aq) + Na2CO3 (aq)  MgCO3 (s) + 2 NaCl (aq) MgCO3 (s) + 2 HCl (aq)  MgCl2 (aq) + CO2 (g) + H2O (l) Fe2O3 (s) + 3 CO (g)  2 Fe (s) + 3 CO2 (g) Practice makes perfect! DARRELL ER (COPYRIGHTED) ©

7 IONIC EQUATION Step 1 Write the balanced chemical equation for the reaction. CaCl2 (aq) + CuSO4 (aq)  CaSO4 (s) + CuCl2 (aq) Step 2 Ionic compounds that are in aqueous state should be written as ions. Ca2+ (aq) + 2Cl- (aq) + Cu2+ (aq) + SO4 2- (aq)  CaSO4 (s) + Cu2+ (aq) + 2Cl- (aq) Step 3 Remove all the spectator ions. Ca2+ (aq) + 2Cl- (aq) + Cu2+ (aq) + SO42- (aq)  CaSO4 (s) + Cu2+ (aq) + 2Cl- (aq) Step 4 Obtain the final ionic equation. Ca2+ (aq) + SO42- (aq)  CaSO4 (s) DARRELL ER (COPYRIGHTED) © An ionic equation is a chemical equation which only shows ions of the aqueous compounds that took part in the chemical reaction. Only ionic compounds that are in aqueous state should be written as ions.

8 MOLE CONCEPT Ar, Mr MOLE CONCENTRATION KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

9 RELATIVE MASS Relative atomic mass (Ar) Ar of an element is defined as the average mass of its atom compared to 1/12 of the mass of one carbon-12 atom. Relative molecular mass (Mr) Mr is defined as the average mass of a molecule of a substance compared to 1/12 of the mass of one carbon-12 atom. *Carbon–12 is used as a basis of comparison because it is the most commonly available element on Earth. Percentage by mass of an element present in a compound: DARRELL ER (COPYRIGHTED) © The term ‘relative mass’ is used when the mass of an atom is decided in ‘relative’ to the mass of a carbon-12 atom. In order words, an atom’s mass is defined by comparing it to the mass of a carbon-12 atom. 1 unit of mass is 1/12 of carbon-12 atom. ‘Average mass’ is also used as elements has isotopes, hence we need to use the element’s average mass! X 100% Ar x (no. of atoms) Mr of compound

10 MOLE WHAT IS MOLE? One mole of any substance would contain 6.02 × 1023 particles. The value 6.02 × 1023 is referred to as the Avogadro’s constant. MOLAR VOLUME OF GASES At room temperature and conditions, one mole of gas has a volume of 24 dm3 or 24 000 cm3. Any type of gas, regardless of their chemical formula & Mr, all have the same volume. DARRELL ER (COPYRIGHTED) © No. of particles = mole x 6.02 × 1023 No. of moles = Mass (in g) Mr 1 mole of gas = 24dm3