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Chemistry

Chemistry. Tutorial #4. Chemical Compounds. Elements – pure substance made of one type of particle Compound – when two or more elements combine Elements are held together by chemical bonds Chemical bonds form when an element gains,loses or shares an electron (charged particle).

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Chemistry

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  1. Chemistry Tutorial #4

  2. Chemical Compounds • Elements – pure substance made of one type of particle • Compound – when two or more elements combine • Elements are held together by chemical bonds • Chemical bonds form when an element gains,loses or shares an electron (charged particle)

  3. Chemical Formulas • Use symbols and numbers (subscripts) to represent the composition of a pure substance • Symbols for each element are used • Subscript (#’s) represent the number of atoms of each element Eg. H2o – H=symbol of hydrogen o=symbol for oxygen 2= 2 atoms of hydrogen * note: if a symbol has no subscript it is said to represent having 1 atom.

  4. Molecular Compounds Two types of compounds: Molecular and Ionic • Ionic compounds form when two atoms transfer electrons to other atoms • Molecular compounds form when atoms share electrons • Molecule is smallest unit of a pure substance (cluster of atoms) • Diatomic molecule is a molecule made of two atoms of the same element. Eg. O, N, H are all diatomic and thus written as O2, N2, H2 • Most molecular compounds form sm.structures therefore, have strong bonds between atoms but very weak bonds between molecules. • As a result, they have low boiling points and low melting points

  5. Naming Molecular Compounds • Compounds are named by IUPAC (international union of pure and applied chemistry) recommendations. • Binary compounds are made from two elements. • Rules for naming: • Write the name of the first element • Change the ending of the second to “ide” • Use a prefix to indicate the # of each type of atom • Mono=1 (only used for the second element) • Di = 2 • Tri = 3 • Tetra = 4 • Eg. CO2 Carbon Dioxide • To change from name to chemical formula • Write the symbol for each element in the same order • Use subscripts to indicate the # of each atom. • When naming a compound, often it has sm.symbols in ( ) to indicate the state of matter (s), (l), (g) or (aq) which = aqueous (solid dissolved in water)

  6. Naming Molecular Compounds Try the following: CO _____________________________ SiO2_____________________________ Carbon tetrachloride ________________ Sulfur Dioxide _____________________

  7. Naming Molecular Compounds Try the following: CO carbon monoxide SiO2 Silicon dioxide Carbon tetrachloride CCl4 Sulfur Dioxide SO2

  8. Ionic Compounds • Form when atoms transfer electrons • An ion is formed when an atom gains or loses an electron and is no longer neutral • Ions always have a + or – charge • When dissolved in water ionic compounds separate into ions and thus carry an electric current.

  9. Naming Ionic Compounds • Rules are similar to molecular • The main difference is the name doesn’t indicate the #of ions in each element. • The Rules: • Name includes both elements with the metallic element first • Non metallic element is second with “ide” ending • Subscripts indicate the ratio of ions in a compound. Eg. CaCl2 calcium chloride (ratio = 1Ca to 2Cl

  10. Try the following: NaCl ______________ LiCl _______________ Sodium Fluoride ________ Zinc Sulfide ____________

  11. Try the following: • NaCl sodium chloride • LiCl Lithium chloride • Sodium Fluoride NaF • Zinc Sulfide ZnS

  12. Properties of Ionic & MolecularCompounds

  13. Chemical Reactions • Two or more substances combine to form a new substance. Cannot be reversed. • Reactants – substances that go into a reaction. • Products- what is formed in the reaction

  14. Chemical equations • How chemists describe reactions • Word equations and chemical equations • Chemical equations use symbols Eg. Magnesium + oxygen  magnesium oxide Or 2Mg(s) + O2  2 MgO (s) + light

  15. Balancing Chemical Equations • Remember the law of conservation of mass? • It says that atoms can’t be lost or created. The atoms essentially change partners. Therefore, you will have the same # of atoms of each element just combined differently. Eg. HC2H3O2 (aq) + NaHCO3 (s)  NaC2H3O2 (aq) + H2O (l) + CO2 (g) (vinegar) (baking soda)(sodium acetate) + (water) + (Carbon dioxide) • Take a minute to add up the number of atoms of each element on both sides of the equation

  16. It is important to balance a chemical equation so that each side of the equation has the same number of atoms of each element. • To balance an equation you need to count the number of atoms of each element on the each side of the equation and make both sides equal by using co-efficiants Eg. Magnesium + oxygen magnesium oxide Step 1: write the chemical equation Mg + O2 MgO *oxygen is a diatomic molecule so it is always written O2 Step 2: count the # of atoms of each element. 1Mg & 2O Step 3: balance the equation: there are 2 O’s on the left so we need 2 on the right. Mg + O2 2MgO Step 4: now, we have 2 Mg on the right and only 1 on the left. So, use a coefficiant. 2Mg + O2 2MgO Step 5: count the atoms and make sure eveyone is happy! You now have a balanced equation.

  17. Balancing Try the following: CH4 +_ O2 CO2 +_ H2O _H2 + O2  _ H2O _Na + Cl2 _NaCl _CaCO3 +_ H2SO4  _CaSO4 + _H2O +_ CO2

  18. Balancing Try the following: CH4 +2 O2 CO2 +2 H2O 2H2 + O2  2 H2O 2Na + Cl2 2NaCl _CaCO3 +_ H2SO4  _CaSO4 + _H2O +_ CO2

  19. Breaking Chemical Bonds • Chemical bonds are forces that cause a group of atoms to behave as a unit. • To break these bonds, energy must be added. • When bonds form energy is released • Energy is produced or absorbed during a reaction. • Exothermic reactions- eg. combustion • Endothermic reactions eg. Baking cookies; baking soda breaks down and release CO2 and puffs up the cookies • Eg. Corrosion – prevent by zinc / galvanization

  20. Reaction Rate • You can increase or decrease the rate of a reaction by: • Changing the temp. • Stirring • Changing size and conc. Of reactants • Catalysts – speed up a reaction without being changed Eg. Enzymes – these occur naturally in our bodies. Saliva (contains enzymes) helps breaks down the starches in our food • Inhibiter – slows down the reaction Eg. Preservatives in food.

  21. Quick Note: Memorize the chemical names of common chemicals. These are found on pg 442 -445 in your text. This will be on your midterm! • You should know the chemical formula’s for the following: • Vinegar • Table salt • Propane • Sugar (sucrose) • Glucose • Hydrogen peroxide

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