Unit 7: chemical reactions

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+.