Chemical Reactions & Equations

1. Chemical Reactions & Equations +

2. Evidence of Chemical Reactions • Release of a gas • Example: bubbles formed when magnesium and hydrochloric acid were mixed • Color change • Example: color went from clear to yellowish orange when potassium iodide was added to hydrogen peroxide • Formation of a precipitate • Example: cloudiness occurred when CO2 gas passed through limewater • Change in temperature, light, sound, smell • Example: temperature increased when hydrochloric acid and sodium carbonate were mixed

3. CaO(s) + H2O(l) Ca(OH)2(aq) + 82 kJ NH4NO3(s) + H2O(l) + energy NH4NO3(aq) Endothermic or Exothermic? Since energy is a product, this reaction is exothermic. This reaction will generate enough heat to fry an egg! Since energy is a reactant, this reaction is endothermic. This is the cooling reaction of a cold pack.

4. Products substances after the change Reactants substances before the change C3H8 + O2 CO2 + H2O + energy Chemical Equations Propane and oxygenreact to produce carbon dioxide, water, and energy. This reaction is not yet balanced.

5. + “plus” or “and” indicates multiple reactants or products “yields” or “produces” separates reactants & products, arrow points in the direction of the reaction (s) “solid” (l) indicate phase of a reactant or product “liquid” (g) “gas” (aq) “aqueous” dissolved in water (not the same as liquid (l)) indicates reaction is reversible D over the arrow, indicates heating is necessary N.R. “no reaction” Symbols in Equations

6. Balance the numbers of atoms of each element on both sides of the equation by determining the coefficients of the compounds. Balancing Equations • When balanced, an equation accounts for the conservation of: • mass • charge • energy DO NOT CHANGE SUBSCRIPTS!

7. Instead we double the coefficients of H2 and H2O 2H2 (g) + O2 (g) 2H2O (l) There are now 4 H’s and 2 O’s on each side. An Example: Forming Water H2 (g) + O2 (g) H2O (l) On left: 2 H and 2 O On right: 2 H and 1 OTherefore, not balanced We could use 1/2 O2 on the left but that is not done, we always want whole number coefficients.

8. 6H2O coefficient subscript Rules for Balancing Equations • Write the formulas for all reactants and products • Don’t change formulas (subscripts), only coefficients. • Balance elements that only occur once on each side of the equation first • To determine the number of atoms of an element, multiply the coefficient by the subscript for the element. Tips: for reactions involving oxygen, balance it last polyatomic ions can be balanced as a unit

9. C2H4 + O2 CO2 + H2O C2H4 + O22CO2 + 2H2O C2H4 + 3O2 2CO2 + 2H2O Balancing Example Write the equation for burning ethylene (C2H4) to produce carbon dioxide and water. C and H both appear once on each side of the equation whereas O appears in both compounds on the right hand side. Multiply CO2 by 2 to balance C Multiply H2O by 2 to balance H There are now 6 O’s on the right so multiply O2 by 3 to finish.

10. Balance F: K + F22KF Balance K: 2K + F22KF Balancing Practice #1 Unbalanced equation: K + F2 KF

11. Notice Ca is fine: Ca + H2O Ca(OH)2 + H2 Balance O: Ca + 2H2O Ca(OH)2 + H2 Balancing Practice #2 Unbalanced: Ca + H2O Ca(OH)2 + H2 Hydrogen is already taken care of so we’re done.

12. Cl: 2NH4Cl + Ca(OH)2 NH3 + H2O + CaCl2 N: 2NH4Cl + Ca(OH)22NH3 + H2O + CaCl2 O: 2NH4Cl + Ca(OH)2 2NH3 + 2H2O + CaCl2 H: 2NH4Cl + Ca(OH)22NH3 + 2H2O + CaCl2 10 H on each side, so we’re done! Balancing Practice #3 NH4Cl + Ca(OH)2 NH3 + H2O + CaCl2

13. 5 Types of Chemical Reactions • Combustion • Synthesis • Decomposition • Single Replacement • Double Replacement Some reactions fall into multiple categories.

14. Hydrocarbons (C, H, O) - heat homes, cook food, and power transportation. Always get CO2(g) and H2O(g). hydrocarbon+ O2 (g)CO2 (g) + H2O(g) Metals produce metal oxides (like rust and tarnishes). 2Mg(s)+ O2(g) 2MgO(s) Combustion Reactions A substance reacts with oxygen to create products containing oxygen. Heat may be required to initiate. Exothermic.

15. C7H16(g) + O2(g) CO2(g) + H2O(g) + heat C7H16(g) + 11 O2(g) 7 CO2(g) + 8 H2O(g) Practice Combustion Write the balanced equation for the combustion of heptane gas (C7H16) Write out the reactants and products: Balance the equation (ignore heat):

16. A + B AB general form Note: A & B can be elements or compounds Examples: sodium chloride 2Na+ Cl2 2NaCl magnesium oxide 2Mg+ O2 2MgO Synthesis Reactions • The combining of two or more substances to form a more complex substance.

17. AB A + B general form Examples: electricity 2H2O(l)2H2(g) + O2(g) NH4NO3(s)N2O(g) + 2H2O(g) Decomposition Reactions • Compound breaks down into two or more simpler substances (elements or compounds).

18. Example: Cl2(aq) + 2KBr(aq) 2KCl(aq) + Br2(aq) Single Replacement Reactions One element takes the place of another element. Metals replace metals. Non-metals replace non-metals. Not reversible. A + BC BA + C general forms A + BC AC + B

19. AB + XY AY + XB general form Example: Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq) Double Replacement Reactions When solutions of two soluble ionic compounds are mixed, an insoluble product (precipitate) forms. Remember “inny outy” rule

20. More on Double Replacement • When 2 ionic solutions are mixed there are several events that could happen: • Nothing, everything is soluble • A precipitate forms (possibly two) • A gas is formed (like FeS(s) + H2SO4(aq) --> FeSO4(aq) + H2S(g) • A covalent molecule forms (like H+ + OH- --> H2O) • Ions that remain in solution are called spectator ions because they don’t participate in the reaction.

21. Ca(s) + CuSO4(aq) CaSO4(aq) + Cu(s) 2AlCl3(l) 2Al(l) + 3Cl2(g) ZnSO4(aq) + SrCl2(aq) ZnCl2(aq) + SrSO4(s) 4Na(s) + O2(g) 2Na2O(s) Practice - Identifying Reaction Types single replacement decomposition double replacement synthesis, combustion