Chemical Reactions & Equations

Transcription

1 Chemical Reactions & Equations + 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 CO 2 gas passed through limewater Change in temperature, light, sound, smell Example: temperature increased when hydrochloric acid and sodium carbonate were mixed 1

2 Endothermic or Exothermic? CaO (s) + H 2 O(l) Ca(OH) 2 (aq)) + 82 kj Since energy is a product, this reaction is exothermic. This reaction will generate enough heat to fry an egg! NH 4 NO 3 (s) + H 2 O(l) + energy NH 4 NO 3 (aq) Since energy is a reactant, this reaction is endothermic. This is the cooling reaction of a cold pack. Chemical Equations Reactants substances before the change C 3 H 8 + O 2 Products substances after the change CO 2 + H 2 O + energy Propane and oxygen react to produce carbon dioxide, water, and energy. This reaction is not yet balanced. 2

3 Symbols in Equations + plus or and indicates multiple reactants or products (s) (l) (g) (aq) Δ N.R. yields or produces separates reactants & products, arrow points in the direction of the reaction solid liquid gas indicate phase of a reactant or product aqueous dissolved in water (not the same as liquid (l)) indicates reaction is reversible over the arrow, indicates heating is necessary N.R. no reaction Balancing Equations When balanced, an equation accounts for the conservation of: mass charge energy Balance the numbers of atoms of each element on both sides of the equation by determining the coefficients of the compounds. DO NOT CHANGE SUBSCRIPTS! 3

4 An Example: Forming Water H 2 (g) + O 2 (g) H 2 O (l) On left: 2 H and 2 O On right: 2 H and 1 O Therefore, not balanced We could use 1/2 O 2 on the left but that is not done, we always want whole number coefficients. Instead we double the coefficients of H 2 and H 2 O 2H 2 (g) + O 2 (g) 2H 2 O (l) There are now 4 H s H s and 2 O s O s on each side. Rules for Balancing Equations coefficient 6H 2 O subscript 1. Write the formulas for all reactants and products 2. Don t t change formulas (subscripts), only coefficients. 3. Balance elements that only occur once on each side of the equation first 4. 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 4

5 Balancing Example Write the equation for burning ethylene (C 2 H 4 ) to produce carbon dioxide and water. C 2 H 4 + O 2 CO 2 + H 2 O C and H both appear once on each side of the equation whereas O appears in both compounds on the right hand side. Multiply CO 2 by 2 to balance C Multiply H 2 O by 2 to balance H C 2 H 4 + O 2 2CO 2 + 2H 2 O There are now 6 O s O s on the right so multiply O 2 by 3 to finish. C 2 H 4 + 3O 2 2CO 2 + 2H 2 O Balancing Practice #1 Unbalanced equation: K + F 2 KF Balance F: K + F 2 2KF Balance K: 2K + F 2 2KF 5

6 Balancing Practice #2 Unbalanced: Ca + H 2 O Ca(OH) 2 + H 2 Notice Ca is fine: Ca + H 2 O Ca(OH) 2 + H 2 Balance O: Ca + 2H 2 O Ca(OH) 2 + H 2 Hydrogen is already taken care of so we re done. Balancing Practice #3 NH 4 Cl + Ca(OH) 2 NH 3 + H 2 O + CaCl 2 Cl: N: O: 2NH 4 Cl + Ca(OH) 2 NH 3 + H 2 O + CaCl Cl 2 2NH 4 Cl + Ca(OH) 2 2NH 3 + H 2 O + CaCl 2 2NH 4 Cl + Ca(OH) 2 2NH 3 + 2H 2 O + CaCl 2 H: 2NH 4 Cl + Ca(OH) 2 2NH 3 + 2H 2 O + CaCl 2 10 H on each side, so we re done! 6

7 5 Types of Chemical Reactions Combustion Synthesis Decomposition Single Replacement Double Replacement Some reactions fall into multiple categories. Combustion Reactions A substance reacts with oxygen to create products containing oxygen. Heat may be required to initiate. Exothermic. Hydrocarbons (C, H, O) - heat homes, cook food, and power transportation. Always get CO2(g) and H2O(g). O(g). hydrocarbon + O2 (g) CO2 (g) + H2O(g) Metals produce metal oxides (like rust and tarnishes). 2Mg(s) + O2(g) 2MgO(s) 7

8 Practice Combustion Write the balanced equation for the combustion of heptane gas (C 7 H 16 ) Write out the reactants and products: C 7 H 16 (g) + O 2 (g) CO 2 (g) + H 2 O(g) + heat Balance the equation (ignore heat): C 7 H 16 (g) + 11 O 2 (g) 7 CO 2 (g) + 8 H 2 O(g) Synthesis Reactions The combining of two or more substances to form a more complex substance. A + B AB general form Note: A & B can be elements or compounds Examples: 2Na + Cl 2 2NaCl 2Mg + O 2 2MgO sodium chloride magnesium oxide 8

9 Decomposition Reactions Compound breaks down into two or more simpler substances (elements or compounds). AB A + B general form Examples: 2H 2 O(l) NH 4 NO 3 (s) electricity 2H 2 (g) + O 2 (g) N 2 O(g) + 2H 2 O(g) Single Replacement Reactions One element takes the place of another element. Metals replace metals. Non-metals replace non- metals. Not reversible. A + BC A + BC BA + C AC + B general forms Example: Cl 2 (aq)) + 2KBr(aq aq) ) 2KCl(aq aq) ) + Br 2 (aq) 9

10 Double Replacement Reactions When solutions of two soluble ionic compounds are mixed, an insoluble product (precipitate) forms. AB + XY AY + XB general form Remember inny outy rule Example: Pb(NO 3 ) 2 (aq)) + 2KI(aq aq) ) PbI 2 (s) + 2KNO 3 (aq) 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) + H 2 SO 4 (aq)) --> FeSO 4 (aq)) + H 2 S(g) A covalent molecule forms (like H + + OH - --> H 2 O) Ions that remain in solution are called spectator ions because they don t t participate in the reaction. 10

11 Practice - Identifying Reaction Types Ca(s) + CuSO4(aq aq) ) CaSO4(aq aq) ) + Cu(s) single replacement 2AlCl 3 (l) 2Al(l) + 3Cl 2 (g) decomposition ZnSO 4 (aq)) + SrCl 2 (aq) 4Na(s) + O 2 (g) ZnCl 2 (aq)) + SrSO 4 (s) double replacement 2Na 2 O (s) synthesis, combustion 11