Unit 8: Chemical Reactions and Equations

Transcription

1 1 Chemical Reactions Unit 8: Chemical Reactions and Equations What are chemical reactions and how do they occur? How are chemical reactions classified? How are products of chemical reactions predicted? How are chemical reactions symbolically represented? Is the Law of Conservation of Matter obeyed when a chemical reaction takes place? Chemical Reaction (Rxn) Change in identity of a substance(s) Reactants (Reagents) Substance(s) you start with Left Side Products Substance(s) you end up with Right Side Reactants turn into/create the products Chemical Equations Chemical Equation Formula for a reaction Arrow separates the reactants from products Reactants Products Substances separated with a + sign Read: Arrow as: reacts to form or yields + as and Symbols after formula indicate substance s state of matter: (s) Solid (l) Liquid (g) Gas (aq) Aqueous solution Dissolved in water Solid is formed (precipitate) Gas is produced as a product Gas is evolved Reversible reaction Heat is supplied to the reaction Pt Catalyst (Can be an element or compound)

2 2 Writing Equations Word Equation Uses words to illustrate substances Ex: Copper + chlorine copper (II) chloride Ex: Copper reacts with chlorine to form copper (II) chloride. Skeleton Equation Uses formulas Not necessarily balanced Ex: Cu + Cl 2 CuCl 2 Exothermic Give off heat Ex: Explosions Endothermic Absorb heat Feel cold afterwards Ex: Ice packs from the nurse Reaction Energy Catalyst Substance that speeds up a reaction Not changed or used up by the reaction Lowers Activation Energy Energy needed to start the reaction Enzymes Biological catalysts Catalysts Balancing Equations Law of Conservation of Mass Atoms can t be created or destroyed In rxn, all atoms we start with we must end up with Balanced Equation Same number of each element on both sides of the equation Balancing Equations 1. Write the equation with the correct formulas for all reactants and products 2. Count the number of atoms of each element on both sides 3. Balance the elements one at a time by adding Coefficients Number in front of substance 4. Save H and O until LAST 5. Recount to make sure it is balanced Balancing Don ts Never change a subscript to balance an equation If you change the formula, you change the substance H 2 O is a different compound than H 2 O 2 Never put a coefficient in the middle of a formula 2 NaCl is okay, Na2Cl is not

3 3 Synthesis Synthesis Reaction 2+ substances combine to make one compound Examples: Ca + O 2 CaO SO 3 + H 2 O H 2 SO 4 Predict the products if reactants are two elements Cross charges and write ionic formula Ex: Mg + N 2 Mg 3 N 2 One reactant breaks up into 2+ substances Energy is usually required Examples: electricity NaCl Na + Cl 2 CaCO 3 CaO + CO 2 For Binary Compounds: Decompose into elements electricity 1. 2 H 2 O 2 H 2 + O HgO 2 Hg + O 2 If the compound has more than two elements you will only be asked to identify and balance them NiCO 3 NiO + CO 2 KClO 2 3 KCl O 2 Activity Series Metals Lithium Potassium Decreasing Activity One element replaces another Reactants are an element and a compound Products will be a different element and a different compound K + NaCl Na + KCl F 2 + NaCl NaF + Cl 2 Some metals are more reactive than others Activity Series of Metals Lists metals in order of reactivity Higher on the list is more active Used to determine if rxn happens Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Tin Lead Hydrogen Copper Silver Mercury Platinum Gold

4 4 Activity Series If free metal is higher than cation in compound: Reaction occurs Free metal replaces cation Cation is neutral, free element as product If free metal is lower than cation: No reaction Write NR or No Reaction after arrow Metal/Water Reactions Think of water as HOH Metals replace one of the H, combine with hydroxide (OH - ) H 2 released Example: 2 Li + 2 H 2 O 2 LiOH + H 2 Activity Series of Halogens Lower mass halogen is more active F replaces all, I does not replace You can use list on Activity Series or periodic table Halogen Reactions Halogen that is kicked out is diatomic element on other side 1. Br NaI 2 NaBr + I 2 2. Cl 2 + CaF 2 No Rxn Double Replacement Double Replacement Two compounds switch partners Reactants must be two ionic compounds or acids Usually in aqueous solution Ex: NaOH + AlCl 3 NaOH + AlCl 3 Al +3 OH - + Na +1 Cl -1 3 NaOH + AlCl 3 Al(OH) NaCl Double Replacement Double Replacement Will only happen if one of the products is: Insoluble in water and forms a precipitate A gas that bubbles out A covalent compound (usually water)

5 5 Combustion Combustion Oxygen gas reacts with Hydrocarbon Compound composed of only C, H, and maybe O Products will be CO 2 and H 2 O Known as complete combustion Put a 2 here Combustion C x H y + O 2 CO 2 + H 2 O Balance last If this is odd Balance first Balancing Tips: Balance H 2 O First If coefficient is odd Put a 2 in front of the hydrocarbon Balance O 2 last Dissociation Separation into cations and anions Generally occurs in aqueous solution Complete Ionic Equation If aqueous, write compound as its ions If a solid, liquid, or gas, leave the formula as is Complete Equation: HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) Complete Ionic Equation: H + (aq) + Cl - (aq) + Na + (aq) + OH - (aq) Na + (aq) + Cl - (aq) + H 2 O (l) Spectator Ions Appear on both sides of the equation Same state/charge on both sides Are present in solution, but do not affect reaction Net Ionic Equation Shows what ions actually react Does not include spectator ions