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Back Chapter 8: Chemical Reactions Chemical Reactions work like mathematical formulas: 1 2 3 Na Cl NaCl A reaction is balanced when the left and right side have the same number of each element Recall Dalton’s Law of Conservation of Mass Bires, 2009 Slide 1 Chemical Reactions Products and Reactants Back • Compounds that react on the left and the Compounds produced on the right. compounds to react compounds produced reactants products Read as “yields” • Remember, Bires, 2009 reactants react to produce products Slide 2 Back Symbols and Notation PbI 2 ( s ) – compound is a solid (l) – cmpd is a liquid (g) – gas (aq) – aqueous (dissolved in water, exists as ions) ↓ - a precipitate has formed • (s) • • • • – (solid falling out of the reaction) • ↑ - a gas is evolved – (bubbling out of the reaction) • Various things can be put over the “yields” sign (→) to indicate special reaction conditions. Bires, 2009 H 2O(l ) CH 4 ( g ) KI (aq ) PbI 2 H2 ca ta lyst 00 C Slide 3 Back Reaction Types • There are five basic reaction types: • Synthesis (Composition) – two or more reactants produce a single product. • Decomposition A B AB A B C – a single reactant breaks down into its parts. • Combustion AB A B – reaction between fuel(A) and oxygen C A B to produce H2O and CO2. Bires, 2009 A O2 CO2 H 2O Slide 4 Back Reaction Types • Single Replacement (Displacement) – One ion kicks one of the ions in the pair out. AB D AD B AB C CB A • Double Replacement (Displacement) – Ion pairs switch places (partners). AB CD AD CB • Can a negative-negative compound exist? Bires, 2009 Slide 5 Back Chemical Equations • We express a chemical reaction with a chemical equation. • Shows relative number of products and reactants required to satisfy the Law of Conservation of Mass. • Our lab reaction: Pb( NO3 ) 2 ( aq) KI ( aq) PbI 2 ( s ) KNO3( aq) • Must be written: Pb( NO3 ) 2( aq) 2KI( aq) PbI 2( s ) 2KNO3( aq) Bires, 2009 Slide 6 Back Recall: Diatomic Molecules • Some elements must form a pair with itself. • In chemical reaction, these elements must be written as a X2. H 2O 2H O ? 2H 2O 2H 2 O2 Bires, 2009 Diatomic Molecules Hydrogen = H2 Nitrogen = N2 Oxygen = O2 Fluorine = F2 Chlorine = Cl2 Bromine = Br2 Iodine = I2 Slide 7 Back Rules for Balancing Chemical Rxns • Never Change Subscripts • Balance Groups First • Balance H2O, O2, and H2 last. H 2 SO4 • We balance by adding H O H O 2 2 2 coefficients. • Two reactant 2 H 2 O2 2 H 2O oxygens • Four product hydrogens • Properly balanced 2H 2 O2 2H 2O Bires, 2009 Slide 8 Back Balancing Practice • Balance the following reactions on your own: 4 Fe 3O2 2Fe2O3 Rust 2 Al Fe2O3 Al2O3 2 Fe Thermite + Energy 2 C2 H 2 5 O2 4 CO2 2 H 2O Acetylene torch + Energy C6 H12O6 6 O2 6 CO2 6 H 2O Respiration Bires, 2009 + Energy Slide 9 Back Mole Ratios • Mole ratios: – how many moles of products are produced with given a number of moles of reactants. • Here, the mole ratio is 1:1:2 (1H2 : 1Cl2 : 2 HCl) • This means two moles of HCl will be produced when one mole of H2 and one mole of Cl2 react. Bires, 2009 Slide 10 Back Energy In and Out • Recall that a reaction can be Exothermic (releasing energy) or • Endothermic (absorbing energy) • Energy Barrier: – Even exothermic reactions need a little energy to get started Energy Barrier • Activation Energy – energy needed to get the reaction going. Exothermic Bires, 2009 Slide 11 Back Energetic Reactions • Exothermic Reaction – energy acts like it is a product. CH4 2O2 2H 2O CO2 888 kJ / mol • Endothermic Reaction – energy acts like a reactant. 2H 2O(l ) 571 kJ / mol 2H 2( g ) O2( g ) Bires, 2009 Slide 12 Back Activity Series • Will an element replace another element in a single displacement reaction? • More reactive elements – placed high on the activity series. • Less reactive elements – placed low on the series. Activity Series (reaction with acids and water) Most Active Li Rb K React with cold H2O and acids, replacing hydrogen Ba Ca Na Mg Al React with acids and steam, replacing Mn hydrogen Zn Cr Fe Ni React with acid, Sn but not water Pb Halogen H2 React with Activity Cu oxygen F2 Hg Cl2 Ag Mostly Br2 Pt Nonreactive I2 Au Least Active When comparing reactions, the elements above will replace (kick out) elements below in solutions. Bires, 2009 Slide 13 Back Predict the Products • Use the reactants and the reaction type to determine the products… Synthesis: ___ Al( s ) ___ O2( g ) __________ ( s ) Decomposition: Combustion: ___ CuCl3 ______ ( g ) ______ ( s ) ___ C2 H 6( g ) ___ O2( g ) ______ ( g ) ______ ( g ) Single Replacement: Use Cu2+ ___ Ag3 PO4( aq) ___ Cu( s ) ______ ( s ) ______ ( aq) Double Replacement: ___ MnBr7 ( s ) ___ Na2O( s ) ______ ( s ) ______ ( s ) Bires, 2009 Slide 14 Ionic Cell Potentials (Later…) Back • Ionic solutions conduct electricity as charged ions flow through a solution. (Electrolytes) • We can measure the current that flows when certain ions react in solution. • Similar to the activity series, we can measure cell potentials and create a table of electricity produced when metals react with ions. Table: Page 615 This is due to oxidation reduction: when the oxidation number of a metal changes and electrons flow. Bires, 2009 Slide 15