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Notes On Stoichiometry(Part 1)

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Published in: Chemistry
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IGCSE Chemistry Stoichiometry

Madeeha D / Dubai

5 years of teaching experience

Qualification: Masters of Science in Chemistry

Teaches: Chemistry, Biochemistry, IIT JEE, IGCSE/AS/AL

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  1. Stoichiometry
  2. N/A
  3. Element Symbols • Each element is represented by its own unique symbol as seen on the Periodic Table e.g. H is hydrogen Where a symbol contains two letters, the first one is always in capital letters and the other is small, eg. sodium is Na, not NA • Atoms combine together in fixed ratios that will give them full outer shells of electrons The chemical formula tells you the ratio of atoms Eg. H20 is a compound containing 2 hydrogen atoms which combine with 1 oxygen atom The chemical formula can be deduced from the relative number of atoms present
  4. H H If a molecule contains 3 Hydrogens and 1 Nitrogen, formula will be NH3
  5. Deduce the formula H—
  6. Chemical formulae The structural formula tells you the way in which the atoms in a particular molecule are bonded. This can be done by either a diagram (displayed formula) or written (simplified structural formula) The empirical formula tells you the simplest whole-number ratio of atoms in a compound The rnolecular formula tells you the actual number of atoms of each element in one molecule of the compound or element e.g. H2 has 2 hydrogen atoms, HCI has 1 hydrogen atom and 1 chlorine atom
  7. MOLECULAR FORMULA The molecular formula Of an Organic compound simply shows the number Of each type Of atom present. It tells you nothing about the bonding within the compound. EMPIRICAL FORMULA The empirical formula of an organic compound gives the simplest possible whole number ratio of the different types of atom within the compound. CONDENSED FORMULA The condensed formula is also text-based: here, each carbon atom is listed separately, With atoms attached to it following. An exception is cyclic parts Of molecules, e.g. benzene, where the carbons are grouped. DISPLAYED FORMULA A displayed formula shows all of the atoms and all of the bonds present in an organic compound. The bonds are represented as lines. STRUCTURAL FORMULA Similar to displayed formula • not all bonds are shcn,vn. although all atoms are still indicated using subscript numbers. Carbon hydrogen bonds are often simplified. C4H802 C2H40 CH3CH2CH2COOH CIO
  8. Deducing formulae by combining power The concept of valency is used to deduce the formulae of compounds Valency or combing power tells you how many bonds an atom can make with another atom Eg. carbon is in Group IV so a single carbon atom can make 4 single bonds or 2 double bonds The following valencies apply to elements in each group: GROUP IV V Ill VALENCY 2 4 2
  9. Deducing formula of Ionic Compounds Swap and Drop method Write down the oxidation states of each element . Swap . Drop 3
  10. ???“) ?????????) ??????? HYDROXIDE ?????? SULFATE ???????? HYDROGEN ???????? FORMULA AND CHARGE ??3+ ???;
  11. Names of compounds For compounds consisting of 2 atoms: • If one is a rnetal and the other a nonmetal, then the name of the metal atom comes first and the ending of the second atom is replaced by adding —ide Eg. NaCl which contains sodium and chlorine thus becomes sodium chloride If both atoms are nonmetals and one of those is hydrogen, then hydrogen comes first • Eg. hydrogen and chlorine combined is called hydrogen chloride For other combinations of nonmetals as a general rule, the element that has a lower Group number comes first in the name Eg. carbon and oxygen combine to form C02 which is carbon dioxide since carbon is in Group 4 and oxygen in Group 6
  12. For compounds that contain certain groups of atoms: • There are common groups of atoms which occur regularly in chemistry • Examples include the carbonate ion(C032-), sulfate ion (S042-), hydroxide ion (OH-) and the nitrate ion (N03-) • When these ions form a compound with a metal atom, the name of the metal comes first • Eg. KOH is potassium hydroxide, CaC03 is calcium carbonate
  13. Writing and balancing chemical equations These use the chemical symbols of each reactant and product When balancing equations, there needs to be the same number of atoms of each element on either side of the equation The following nonmetals must be written as molecules: H2, N2, 02, F2, C12, Br2 and 12 • Work across the equation from left to right, checking one element after another • If there is a group of atoms, for example a nitrate group (N03-) that has not changed from one side to the other, then count the whole group as one entity rather than counting the individual atoms. For example: NaOH HCI Nacl H20 There are equal numbers of each atom on either side of the reaction arrow so the equation is balanced
  14. State SOLID symbols LIQUID GAS AQUEOUS
  15. Write down the balanced equation of the following reaction Aluminium (s) + Copper (Il) Oxide (s) —Aluminium Oxide (s) + Copper (s)
  16. Write down the balanced equation of the following reaction Magnesium Oxide (s) + Nitric Acid (aq) —Magnesium Nitrate (aq) + Water (l)
  17. Balancing Ionic Equations • In aqueous solutions ionic compounds dissociate into their ions, meaning they separate into the component atoms or ions that formed them • Eg. hydrochloric acid and potassium hydroxide dissociate as follows HCI —+1+ + Cl- KOH -9 + OH-
  18. Write the ionic equation for the reaction of aqueous chlorine and aqueous potassium iodide Step 1: Write out the full balanced the equation: Kl(aq) + C12(aq) 12 Step 2: Identify the ionic substances and write down the ions separately: (aq) + 21-(aq) + C12(aq) Step 3: Rewrite the equation eliminating the ions which appear on both sides of the equation 21-(aq) + C12(aq)
  19. Relative atomic mass • The symbol for the relative atomic mass is Ar • This is calculated from the mass nurnber and relative abundances of all the isotopes of a particular element ATOMIC NUMBER 17 35 MASS NUMBER
  20. Equation for finding relative atomic mass X of isotope a x mass of isotope a) + (Z of isotope b x mass of isotope b) 400
  21. The table shows information about the Isotopes in a sample of rubidium ISOTOPE NUMBER OF PROTONS 37 NUMBER OF NEUTRONS 48 50 PERCENTAGE OF ISOTOPE IN SAMPLE 72 28 (72 x 85) + (28 x 87)+100 = 856 Relative Atomic Mass = 85.6
  22. Relative formula (molecular) mass The symbol for the relative molecular mass is Mr and it refers to the total mass of the molecule. To calculate the Mr of a substance, you have to add up the Relative Atomic Masses of all the atoms present in the formula sues T ANCE WATER porAssvuu CALC-NN HYDROXIDE SWEAT ATOMS PRESENT

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