Chapter 11: Chemical Reactions

Chemical Reactions A rearrangement of atoms Chemical Reaction example: Methane in natural gas burns in oxygen; carbon dioxide and water are formed. Chemical Equation: CH4 + O2  H2O + CO2 Reactants: present before the reaction (methane and oxygen) Products: chemicals formed in a reaction (water and carbon dioxide)

Evidence of a Chemical Reaction Color Change Bubbles form Precipitate forms (solid from the combination of two liquids) Heat is absorbed or released A flame occurs (combustion)

Law of Conservation of Mass In a chemical reaction, atoms are neither created nor destroyed. All atoms in the reactants must be accounted for in the products Chemical rxns list state of compounds (s) = solid (l) = liquid (g) = gas (aq) = in aqueous solution (dissolved in water)

Example Potassium reacts with water to produce hydrogen gas and potassium hydroxide. Write the balanced chemical equation with physical states listed. K(s) + H2O(l)  H2(g) + KOH(aq)

Rules for Balancing Never change chemical formulas Only balance using coefficients The coefficient is distributed to all of the atoms in a compound Example: 2 NaOH = 2 Na, 2 O, 2 H Reduce coefficients if possible

Practice Problems Solid Zinc reacts with hydrochloric acid and produces hydrogen gas and solid zinc chloride. Zn(s) + HCl(aq)  H2(g) + ZnCl2(s) Sulfuric acid plus solid sodium hydroxide yields water and solid sodium sulfate. H2SO4(aq) + NaOH(s)  H2O(l) + Na2SO4(s)

Practice Problems Solid Potassium chlorate is broken down into solid potassium chloride and oxygen gas. KClO3(s)  KCl(s) + O2(g) Lead(II)nitrate reacts with sodium iodide to produce Lead(II)iodide and sodium nitrate. Pb(NO3)2 + NaI  PbI2 + NaNO3

Practice Problems Solid Copper reacts with aqueous silver hypoiodite to make aqueous copper(I)hypoiodite and solid silver. Cu(s) + AgIO(aq)  CuIO(aq) + Ag(s) Aluminum carbonate reacts with barium chloride to produce aluminum chloride and barium carbonate. Al2(CO3)3 + BaCl2  AlCl3 + BaCO3

Combination (synthesis) Reactions Two or more substances combine to form a new compound A + X  AX 2 Mg + O2  2 MgO 8 Ba + S8  8 BaS

Decomposition Reactions A single compound undergoes a reaction that produces two or more similar substances AX  A + X 2 HgO  2 Hg + O2 CaCO3  CaO + CO2

Single-Replacement Reaction One element replaces a similar element in a compound A + BX  AX + B Mg + 2 HCl  H2 + MgCl2 2 Al + 3 Pb(NO3)2  3 Pb + 2 Al(NO3)3

Metal Reactivity When finely divided copper metal is heated, it gradually reacts with oxygen in the air to produce a black copper(II) oxide: 2 Cu(s) + O2(g)  2 CuO(s) Magnesium metal burns in oxygen with a blinding light: 2 Mg(s) + O2(g)  2 MgO(s) Gold does not react with oxygen What is the relative reactivity of Cu, Mg, Au?

Metal Activity Series Why did early Man discover gold, but not sodium? Element Metal ion(s) found in Minerals Na Na+ Mg Mg2+ Zn Zn2+ Fe Fe3+, Fe2+ Cu Cu2+ Ag Ag+ Au Au3+ most active metals (not found as free element) least active metals (can be found as free element)

Trends in Metal Activity A more reactive metal (higher in the activity series) will cause ions of a less reactive metal (lower in the activity series) to change to their corresponding metal Zn + CuCl2  ZnCl2 + Cu this reaction will take place; Zn is high on the Activity Series and tends to form it’s cation Cu + ZnCl2  N. R. this reaction does not occur; Cu is low on the Activity Series and does not easily form it’s cation

Metal Reactivity Activity Series – Elements ranked by relative order of chemical reactivities Gold does not react with any components of air, including oxygen Long-lasting decorative objects (jewelry) Gold-plated electrical connectors for automobile air bags and audio cable connectors

Activity Series How would you rank copper, gold, and magnesium based on what we observed? Magnesium – reacts readily Copper – reacts with much added heat Gold – does not react

Double-Replacement Reactions The ions of two compounds exchange places in an aqueous solution to form two new compounds AX + BY  AY + BX 2 KI + Pb(NO3)2  PbI2 + 2 KNO3 HCl + NaOH  NaCl + HOH (H2O)

Combustion Reactions A substance combines with oxygen, releasing a large amount of energy in the form of light and heat Hydrocarbon + oxygen  carbon dioxide + water CxHy + O2  CO2 + H2O C3H8 + 5 O2  3 CO2 + 4 H2O

SR Types of Reactions Synthesis Decomposition (direct combination-DC) Large molecule breaks down to smaller pieces opposites Small pieces are put together to form large molecules AB  A + B A + B  AB Only one reactant Only one product Ex:  + Ex: +  Types of Reactions SR Single Replacement C DR Double Replacement Combustion One element replaces an element from a compound. Combustible organic (Carbon-containing) compound that reacts with O2 to form CO2 + H2O Two compounds exchange ions Active metal can replace another less active metal (cationic SR) AX + BY  AY + BX A + BX  AX + B cpd. + cpd.  cpd. + cpd. +  + metal Less active metal Active non-metal can push out less active non-metal (anionic SR) +  + The cpds. switch cations A + BX  BA + X CH4 + 2O2  CO2 + 2H2O +  + Look for 2 cpds. for reactants, 2 cpds. for products Non-metal Weaker non-metal in compound Non-metal (always the products)