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Unit 7: chemical reactions

Unit 7: chemical reactions. Evidence of Chemical Change. Changes in Energy Release of energy as heat Release of energy as light Production of sound Reduction or increase in temperature Absorption or release of electrical energy. Formation of new substances Formation of a gas

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Unit 7: chemical reactions

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  1. Unit 7: chemical reactions

  2. Evidence of Chemical Change • Changes in Energy • Release of energy as heat • Release of energy as light • Production of sound • Reduction or increase in temperature • Absorption or release of electrical energy • Formation of new substances • Formation of a gas • Formation of a precipitate (an insoluble solid) • Change in color • Change in odor A NEW SUBSTANCE MUST BE FORMED!

  3. Writing Chemical Equations • Shows chemical formulas and relative amounts of all reactants and products – like a recipe • Start with a word equation • Reactantproduces/yields/formsproduct • R + R P + P • Separate the chemical names with plus signs • Next, write the formulas for each chemical – this does not show amounts • The equation must be balanced to complete the recipe

  4. The arrow separates the reactants from products • The s, l, g, aq are written after the formulas to indicate state

  5. Balancing Equations • The Law of Conservation of Mass must be observed!! Matter, mass, atoms, electrons, etc. are not created or destroyed only REARRANGED! • Once the formulas are established for the chemicals, they cannot be changed (this changes the substance) – DO NOT Change subscripts! • COEFFICIENTS are used to balance the elements in the equation

  6. Balancing Chemical Equations • Hydrogen and oxygen react to form water • H2(g) + O2(g) H2O(g) • H: 2 2 • O: 2 1 • 2H2(g) + O2(g) 2H2O(g) • H: 44 • O: 2 2 • BALANCED! • Identify reactants and products • Count atoms • Insert Coefficients • Verify Results 2 x 2 = 4

  7. Helpful Hints • Polyatomic ions can be balanced as a unit, provided it stays in tact on both sides • Balance polyatomic ions and metals first • Balance hydrogen, oxygen, and water last! • If you run into a problem, for instance a whole number coefficient will not work, try doubling ALL coefficients • Don’t worry if your numbers are too big, just reduce all numbers by the same factor if possible

  8. There are 5 types of reactions • Synthesis • Decomposition • Single Displacement (or Replacement) • Have to use the Activity Series of Metal • Double Displacement (or Replacement) • Have to use the solubility rules • Combustion

  9. Synthesis (Combination) – “building”, comes from Greek word that means “to put together”

  10. Decomposition – opposite of synthesis, rotting, breaking apart

  11. Decomposition continued

  12. Single replacement – an element replaces another in a compound Activity Series

  13. Double Replacement: 2 elements switch placesCombustion: burning in oxygen Solubility rules

  14. Using the reference tables • Just because an equation can be written does NOT mean that the reaction will actually happen! • If it is single replacement – Refer to the activity series • The reacting metal must be more active (higher up) than the metal that is to be replaced, otherwise NO RXN occurs • If it is double replacement – refer to solubility rules • The rxn only occurs if water, a gas, or a precipitate is formed • Precipitate – an insoluble compound, determine by using solubility rules

  15. Solubility Rules • 1 All compounds containing alkali metal cations and the ammonium ion are soluble. • 2 All compounds containing NO3-, ClO4-, ClO3-, and C2H3O2- anions are soluble. • 3 All chlorides, bromides, and iodides are soluble except those containing Ag+, Pb2+, or Hg22+. • 4 All sulfates are soluble except those containing Hg22+, Pb2+, Sr2+, Ca2+, or Ba2+. • 5 All hydroxides are insoluble except compounds of the alkali metals, Ca2+, Sr2+, and Ba2+. • 6 All compounds containing PO43-, S2-, CO32-, and SO32- ions are insoluble except those that also contain alkali metals or NH4+.

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