Topic 1 - Atoms (1).pptx PDF

Topic 1 Atoms, molecules and Stoichiometry PREPARED BY MRS. HANEEN KHEIR PAGES: 1-19 Atoms, Elements and Molecules An element is a substance that cannot be broken down using chemical reactions into simpler substances An element is a substance that contains ato...

Topic 1 Atoms, molecules and Stoichiometry PREPARED BY MRS. HANEEN KHEIR PAGES: 1-19 Atoms, Elements and Molecules An element is a substance that cannot be broken down using chemical reactions into simpler substances An element is a substance that contains atoms of only one type. Formula Element? Or not? H2 O O2 Ne Fe CO2 O3 Atoms, Elements and Molecules An atom is the smallest part of an element that has properties of that element. Atoms, Elements and Molecules A molecule is a neutral group of two or more atoms held together by covalent bonds. Atoms, Elements and Molecules A compound is a substance containing atoms of different elements combined together. Atoms, Elements and Molecules A mixture is formed when two or more substances mix with each other without participating in a chemical change,. Atoms, Elements and Molecules Properties of mixtures include: The components of a mixture each keep their original properties. The separation of components can be easily done. The proportion of the components is variable. Atoms, Elements and Molecules Atoms, Elements and Molecules An ion is a species consisting of one or more atoms joined together and having a positive or negative charge. Atoms, Elements and Molecules Other terms include: Atoms, Elements and Molecules This also applies to ions: Atoms, Elements and Molecules 1. 1 The table below illustrates the terms used in this topic: Atoms, Elements and Molecules Apply your knowledge: Worksheet 1 Writing Formulae for names Diatomic elements include: For e Don’ xamp t wri le, Hydr te H for ogen ! Writing formulae for polyatomic ions Common Polyatomic ions include: You need to know these! Writing chemical equations A chemical equation shows the overall change of reactants to products in a chemical reaction. Balancing chemical equations Rules for balancing an equation: 1. Only change the coefficients that appear in front of an element or a compound. 2. Never change any subscripts in a formula – If the subscript changes, the substance changes! Balancing chemical equations Rules for balancing an equation: 3. Coefficients should be written as the lowest possible ratios. 4. Begin by balancing elements that appear only once on each side of the equation. Using state symbols Many equations include state symbols: Physical state State symbol Solid (s) Liquid (l) Gas (g) Aqueous (dissolved in (aq) water) Using state symbols The physical states of the elements at 25°C are shown below: Writing chemical equations CONVERSION OF WORD EQUATION TO CHEMICAL EQUATION: STEP 1: IDENTIFY REACTANTS AND PRODUCTS AND PLACE THEM IN A WORD EQUATION, STEP 2: CONVERT THE CHEMICAL NAMES INTO CHEMICAL FORMULAS. PLACE THEM BASED ON THE CHEMICAL EQUATION AND WRITE THE STATE SYMBOLS. STEP 3: BALANCE THE CHEMICAL EQUATION. MULTIPLY EACH OF THE FORMULAS THE APPROPRIATE COEFFICIENT TO BALANCE THE NUMBERS OF ATOMS Balancing chemical equations Balancing chemical equations Writing and Balancing chemical equations Apply your knowledge: Worksheet 2 Arrows in Equations Most equations are shown with a conventional (left to right) arrow: However, some important reactions are reversible and the reaction can go in both the forward and the backward directions. Atomic Structure Representing an atom Isotopes ISOTOPES ARE ATOMS OF THE SAME ELEMENT WITH DIFFERENT NUMBER OF NEUTRONS! Relative Atomic Mass (Ar) One grain of sand contains millions of atoms, so atoms must be really small. How is the mass of an atom measured? Atoms are so small that their masses are not measured directly. Instead, all atoms are compared with the mass of carbon-12. The mass of an atom on this scale is called its relative Relative Atomic Mass (Ar)  Relative Atomic Mass (Ar) The relative atomic mass is defined as: The weighted mean (average) mass of an atom compared to 1/12 of the mass of an atom of 12C. Note: Relative atomic masses have no units Relative Atomic Mass (Ar) Relative Atomic Mass (Ar) The values of relative atomic mass (r.a.m.) are usually given in a data book or found in the periodic table. So you don’t have to work them out or remember them all! atomic number symbol relative atomic mass When looking up relative atomic mass in the periodic table, remember that it always the larger of the two numbers given. What is the other number? Relative Atomic Mass (Ar) It i c a s al s we l l e d o me ight an ed Relative Atomic Mass (Ar)  CALCULATE THE RELATIVE ATOMIC MASS OF COPPER. Relative Molecular Mass (Mr) Most substances are made of molecules, not individual atoms. Molecules are really small too, so can we work out their masses in the same kind of way? Of course! The mass of a molecule is called the relative formula mass. This is calculated by adding up the relative atomic masses of all the atoms in the molecule. Relative Molecular Mass (Mr) Relative molecular masses are used for molecules of both elements and compounds. Mr can be calculated by adding relative atomic masses. Make sure you use relative atomic masses shown on the Periodic table in the Data B Relative Formula Mass (Mr) Relative formula mass has the same symbol as relative molecular mass (Mr), but in includes both molecules and ions. The term Relative formula mass has to be used for compounds with giant structures- will be discussed in details in Topic 3. Relative Formula Mass (Mr) Relative Formula Mass (Mr) Relative Formula Mass (Mr) Relative Formula Mass and Relative molecular mass RELATIVE FORMULA MASS: THE SUM OF THE RELATIVE ATOMIC MASSES OF ALL THE ATOMS IN THE CHEMICAL FORMULA OF AN IONIC COMPOUND. IT SHOULD BE ONLY USED WITH GIANT STRUCTURES. RELATIVE MOLECULAR MASS: THE SUM OF THE RELATIVE ATOMIC MASSES OF ALL THE ATOMS IN THE CHEMICAL FORMULA OF A COVALENT COMPOUND THAT IS MADE OF MOLECULES. Definitions Relative Atomic Mass (Ar) The mass of an atom relative to the 12C isotope having a value of 12.000 Ar = average mass per atom of an element x 12 mass of one atom of carbon-12 Relative Isotopic Mass The mass of an of an isotope (example: 238U) relative to the 12C isotope having a value of 12.000 Relative Molecular Mass (Mr) The mass of an of a molecule (example: CO2, N2 ) relative to the 12C isotope having a value of 12.000 Relative Formula Mass The mass of any formula of a species or ion (example: NaCl; OH¯) relative to the 12C isotope having a value of 12.000 Molar Mass (M) The molar mass is just the numerical value of the relative atomic mass, with the unit g.mol-1. In other words, the molar mass of a substance tells you the mass of 1 mole of that substance!  Calculate the molar mass of each of the following compounds: Avogadro’s Constant (L) What is a moles (n)? What is a moles (n)? The mole is the amount of substance that contains the same number of particles as the number of carbon atoms in exactly 12 g of 12C. What is a moles (n)? Calculations of number of moles (n) The number of mole can be calculated using the formula: The number of particles, N, (atoms/ molecules/ ions) can be calculated using the formula: N=nxL Calculations of number of moles (n) Calculations of number of moles (n) Calculations of number of moles (n) Calculations of number of moles (n) Calculations of number of moles (n) Reacting Masses Many useful substances are made by chemical reactions. Scientists decide how much product they want to make and then work out the amount of reactants needed. The first step is to write a balanced symbol equation for the Reacting Masses The balanced equation for a chemical reaction shows the ratio of reactants and products involved. Reacting Masses The balanced equation for this chemical reaction shows that the ratio of Mg : O2 : MgO is 2 : 1 : 2. This ratio can be used to calculate the masses of reactants needed and the mass of product that will be made. These amounts are called the relative reacting masses. Reacting Masses To calculate the mass of a product given the mass of a reactant, use the following steps: 1. Calculate no. moles of 2. Determine mole ratio of reactant to reactant: product: no. moles = mass / Mr ensure the equation is balanced 3. Calculate no. moles of 4. Calculate mass of product: product: mass = moles × M use the mole ratio r Reacting Masses If you have 48 grams of magnesium, what mass of oxygen will react with this?  The relative atomic mass of Mg = 24  n(Mg) = m(Mg)/Mr (Mg) = 48/24 = 2 mol and the relative formula mass of O2 = 32.  The balanced equation shows the ratio  m (O2) = n (O2) x Mr (O2) = 1 x 32= 32 g. of Mg : O2 is 2 : 1. Therefore, n (O2) = n(Mg)/2 = 2/2= 1 mol Reacting Masses If you have 48 grams of magnesium, what mass of magnesium oxide will be produced?  The relative atomic mass of Mg = 24  n(Mg) = m(Mg)/Mr (Mg) = 48/24 = 2 mol and the relative formula mass of MgO = 24 + 16 = 40.  The balanced equation shows the ratio  m (MgO) = n (MgO) x Mr of Mg : MgO is 2:2. (MgO) = 2 x 40= 80 g. Therefore, n (MgO) = n(Mg) = 2 mol. Reacting Masses mass of first Moles of first reactant or reactant or product product Ratio from the balanced equation mass of second Moles of second m=nx reactant or reactant or M product product Topic 1: Atoms, molecules and stoichiometry  LIMITING REACTANT Application APPLYING THE INFORMATION: WORKSHEET 3 limiting reactant and reactant in excess.  LIMITING REACTANT Application APPLYING THE INFORMATION: WORKSHEET 4 Molar Volume Calculations If the temperature and pressure are fixed at convenient standard values, the molar volume of a gas can be determined. Molar Volume Calculations Molar Volume Calculations Example: what volume does 5 moles of CO2 occupy? volume occupied = no. moles × molar volume =5× 22.7 = 113.5 dm3 Volumes and Concentrations of solutions The molar concentration of a solution is the amount (in mol) of the solute divided by the volume of the solution. Volumes and Concentrations of solutions Volumes and Concentrations of solutions Example: 5 mole of sodium hydroxide is dissolved in water to make 1.5 dm3 of solution. What is the molar concentration of the solution? Concentration= no. moles ÷ volume of solution = 5 ÷ 1.5 = 3.33 mol. dm-3 Summary mass of solid A, or volume of gas A, or volume (or concentration) of solution containing A Moles of reactant or product A Ratio from balanced equation Moles of reactant or product B mass of solid B, or volume of gas B, or volume (or concentration) of solution containing B Empirical and Molecular Formulae  EMPIRICAL FORMULA THE EMPIRICAL FORMULA IS THE SIMPLEST WHOLE- NUMBER RATIO OF THE ATOMS IN A COMPOUND. EXAMPLES: BENZENE, CH  MOLECULAR FORMULA THE MOLECULAR FORMULA IS THE ACTUAL NUMBER OF THE ATOMS IN A SINGLE MOLECULE OF A COMPOUND. EXAMPLES: BENZENE, C6H6 How to find the Empirical Formula STEP 1: IF GIVEN THE PERCENTAGES OF EACH ELEMENT, ASSUME 100 GRAMS SAMPLE OF THE SUBSTANCE AND CONVERT % INTO GRAMS. STEP 2: CONVERT TO MOLES BY DIVIDING THE AMOUNT IN GRAMS BY THE ATOMIC MASS OF THAT ELEMENT. How to find the Empirical Formula STEP 3: SELECT THE SMALLEST VALUE AND DIVIDE ALL VALUES BY THIS SMALLEST ONE. IF ANY RESULT IS A DECIMAL MIXED NUMBER, YOU MUST MULTIPLY ALL VALUES BY SOME NUMBER TO MAKE IT A WHOLE NUMBER How to find the Empirical Formula STEP 4: USE THESE WHOLE NUMBER RESULTS AS SUBSCRIPTS AND WRITE THE EMPIRICAL FORMULA, LISTING THE ELEMENTS IN THE ORDER THEY ARE GIVEN IN THE PROBLEM. How to find the Empirical Formula APPLICATION FIND THE EMPIRICAL FORMULA OF A COMPOUND WHICH CONTAINS 54.93% POTASSIUM, 38.73% BORON AND 6.34% HYDROGEN. How to find the Molecular Formula STEP 1: CALCULATE THE MASS OF THE EMPIRICAL FORMULA (WHICH YOU HAVE ALREADY FOUND OR IT WILL BE GIVEN TO YOU ). STEP 2: DIVIDE THE KNOWN MOLAR MASS BY THE MASS OF THE EMPIRICAL FORMULA. How to find the Molecular Formula STEP 3: MULTIPLY THAT NUMBER BY THE SUBSCRIPTS OF THE EMPIRICAL FORMULA TO GET THE SUBSCRIPTS FOR THE MOLECULAR FORMULA. How to find the Molecular Formula Application: Example: What is the molecular formula of hydrogen peroxide given that its empirical formula is HO and the Mr is 34? 1. Determine relative mass of empirical formula: empirical formula mass = H + O = 1.0 + 16.0 = 17 2. Divide Mr by mass of empirical formula to get a multiple: relative molecular mass = 34 multiple = = 2 mass of empirical formula 17 3. Multiply empirical formula by multiple: HO × 2 = H2O2 How to find the Molecular Formula APPLICATION 1- THE MOLAR MASS OF A COMPOUND IS 181.50 G/MOL AND THE EMPIRICAL FORMULA IS C2HCL. WHAT IS THE MOLECULAR FORMULA? 2- FIND THE EMPIRICAL FORMULA FOR A COMPOUND CONTAINING ONLY CARBON AND HYDROGEN IF IT IS KNOWN TO CONTAIN 84.21% CARBON. IF THE MOLAR MASS IS 114 G/MOL, WHAT IS THE MOLECULAR FORMULA OF THIS COMPOUND? Application APPLYING THE INFORMATION: EMPIRICAL AND MOLECULAR FORMULAE WORKSHEET Percentage composition  PERCENTAGE COMPOSITION The percentage by mass of each element in a compound % mass = grams of element in 1 mol compound X 100% (composition) molar mass of compound Percentage composition  PERCENTAGE COMPOSITION Example: What is the percent composition of Potassium Permanganate (KMnO4)? Molar Mass of KMnO4 M(K) + M(Mn)+ 4M(O) = 1(39.1) + 1(54.9) + 4(16.0) = 158 g.mol-1 Percentage composition  PERCENTAGE COMPOSITION Percentage composition  TEST YOURSELF Determine the percentage composition of ethanol (C2H5OH)? % C = 52.13%, % H = 13.15%, % O = 34.72% Determine the percentage composition of sodium oxalate (Na2C2O4)? % Na = 34.31%, % C = 17.93%, % O = 47.76%