Chemical Equations and Reactions

Describing Chemical Changes or Reactions Chemical Reaction: process by which one or more substances are changed into one or more different substances. Original Substances  Resulting Substances Reactants  Products

Evidence of a Chemical Reaction: Easily observed changes that indicate a chemical reaction has occurred Evolution of heat and light Production of a gas Formation of a precipitate Colour Change

Word Equations Write a word equation to describe the reaction of methane (natural gas) as it burns in the air to produce carbon dioxide and water vapor. Methane + oxygen  carbon dioxide + water

Representing a Chemical Change Chemical Equation: represents with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction.

Chemical Equation Law of Conservation of Mass must be taken into account. Adjust equation so that the same type of atoms and numbers of atoms are the same on both sides of the equation. CH4(g) + O2(g)  CO2(g) + H2O(g) CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) Symbols used in chemical equations, see p. 77.

Write a word & chemical equation Solid sodium oxide is added to water (at room temp.) and forms sodium hydroxide which is dissolved in water. Sodium oxide(s) + water(l)  sodium hydroxide (aq) Na2O(s) + H2O(l)  NaOH(aq) Na2O(s) + H2O(l)  2NaOH(aq)

Write a word & chemical equation Hydrogen peroxide in an aqueous solution decomposes to produce oxygen and water. Hydrogen peroxide  oxygen + water H2O2(aq)  O2(g) + H2O(l) 2H2O2(aq)  O2(g) + 2H2O(l)

Write a word & chemical equation Solid copper metal reacts with aqueous silver nitrate to produce solid silver metal and aqueous copper(II) nitrate. Copper(s) + silver nitrate(aq)  silver(s) + copper(II)nitrate(aq) Cu(s) + AgNO3 (aq)  Ag (s) + Cu(NO3)2 (aq) Cu(s) + 2AgNO3(aq)  2Ag (s) + Cu(NO3)2 (aq)

Write a word & balanced chemical equation Solid zinc metal reacts with aqueous copper(II) sulfate to produce solid copper metal and aqueous zinc sulfate. Zinc(s) + copper(II)sulfate(aq)copper (s) + zinc sulfate (aq) Zn(s) + CuSO4 (aq) Cu(s) + ZnSO4(aq)

Guidelines for Balancing Equations Write the chemical equation with the correct formulas for the reactants and products. Count the number and types of atoms on both sides of the equation. Balance the atoms of the elements that appear only once on each side of the equation.

Balancing Equations Balance polyatomic ions that appear on each side of the equation. Balance hydrogen and oxygen after all other atoms have been balanced. Check both sides – count each element. Use lowest coefficients.

HOFBrINCl What’s it spell? Bromine Iodine Nitrogen Chlorine Hydrogen Oxygen Fluorine HOFBrINCl

BrINClHOF Elements that exist as diatomic molecules. Cl2 is electrically neutral, whereas in a compound such as NaCl, chlorine is the chloride ion with a negative charge.

Write a Balanced Chemical Equation: Zinc(s) + hydrochloric acid(aq) zinc chloride (aq) and hydrogen gas. Zn(s) + HCl (aq)  ZnCl2(aq) + H2(g) Zn(s) + 2 HCl(aq)  ZnCl2(aq) + H2(g)

Write a Balanced Chemical Equation: Aluminum carbide, Al4C3(s) + watermethane gas, CH4 + aluminum hydroxide(s) Al4C3 (s) + H2O(l)  CH4(g) + Al(OH)3(s) Al4C3(s) + 12 H2O(l) 3CH4(g) + 4Al(OH)3(s)

Write a Balanced Chemical Equation: Aluminum sulfate + calcium hydroxide aluminum hydroxide + calcium sulfate Al2(SO4)3 + Ca(OH)2 Al(OH)3 + CaSO4 Al2(SO4)3 +3Ca(OH)2 2Al(OH)3 + 3CaSO4

CHO method: Combustion Reactions Combustion Reaction contains a hydrocarbon + oxygen to yield carbon dioxide and water. Balance carbon first, then hydrogen and finally oxygen. C2H6 + O2  CO2 + H2O 2C2H6 +7 O2 4 CO2 + 6H2O

Types of Chemical Reactions Five Basic Types of Chemical Reactions: Synthesis Decomposition Single-replacement Double-replacement Combustion

Synthesis Two or more substances combine to form a new compound. A + X  AX (A & X may be compounds or elements)

Synthesis Reactions Here is an example

Synthesis Examples Mg(s) + O2(g)  MgO(s) 2Mg(s) + O2(g) 2MgO(s) Ba (s) + S8 (s)  BaS(s) 8Ba (s) + S8 (s)  8 BaS(s) Na(s) + Cl2(g)  NaCl 2 Na(s) + Cl2 (g)  2NaCl

Basic Oxides Metal oxides react with water to produce Bases e.g. Na2O + H2O → 2NaOH K2O +H2O  2KOH CaO +H2O  Ca(OH)2

Basic Oxides Metal oxides react with water to produce Bases e.g. Na2O + H2O → 2NaOH K2O +H2O  2KOH CaO +H2O  Ca(OH)2

Acidic oxides Non-metal oxides react with water in a synthesis reaction to produce oxy-acids e.g. SO2 +H2O  H2SO3 sulphurous acid SO3 +H2O  H2SO4 sulphuric acid NO2 +H2O  HNO3 nitric acid CO2 +H2O  H2CO3 carbonic acid P4O10 +H2O  H3PO4 phosphoric acid B2O3 +H2O  H3BO3 boric acid

Decompositon A single compound undergoes a reaction that produces 2 or more simpler substances. AX  A + X (A & X can be elements or compounds)

Decomposition Reactions Another view of a decomposition reaction:

Decomposition Examples H20 (l)  H2 (g) + O2 (g) 2H20 (l)  2H2 (g) + O2 (g) HgO (s)  Hg (g) + O2 (g) 2HgO (s)  2Hg (g) + O2 (g) CaCO3 (s)  CaO (s) + CO2 (g)

Decomposition is the reverse of synthesis Ca(OH)2  CaO +H2O HNO3  NO2 +H2O

Decomposition Exceptions Carbonates and chlorates are special case decomposition reactions that do not go to the elements. Carbonates (CO32-) decompose to carbon dioxide and a metal oxide Example: CaCO3  CO2 + CaO Hydrogen Carbonates (HCO3-) decompose to carbon dioxide, water and a metal oxide Example: Ca(HCO3)2  2CO2 + H2O + CaO

Single Replacement Reactions One element replaces a similar element in a compound. A + BX  AX + B or A + BX  BA + X

Single Replacement Reactions Another view:

Single Replacement Examples 2Al(s) + 3Pb(NO3)2 (aq)  3Pb(s) + 2Al(NO3)3 (aq) Al replaces Pb Cl2(g) + 2 KBr(aq)  2KCl(aq) + Br2(g) Cl replaces Br

Activity Series The ability of an element to react is called the elements “activity.” The more readily an element reacts – the greater “activity” it has.

Activity series of metals potassium sodium React vigorously with cold water  calcium React slowly with cold water magnesium aluminum zinc chromium React very slowly with steam but are quite reactive in acid increasing reactivity iron nickel Tin Lead React moderately with acid copper silver platinum gold Unreactive in acid

Activity series of metals potassium sodium React vigorously with cold water  calcium React slowly with cold water magnesium aluminum zinc chromium React very slowly with steam but are quite reactive in acid increasing reactivity iron nickel Tin Lead React moderately with acid copper silver platinum gold Unreactive in acid

: Activity Series A list organized by the ease at which certain elements will undergo a chemical reaction.(p.266) The order is determined by single-replacement reactions.

Activity Series Most Active Least Active The most active element is placed at the top and can replace each of the elements below it.

An element further down the list can only replace an element below it –not above it. The Activity Series will predict whether or not a reaction will take place.

Activity Series Examples Al will replace Zn: 2Al (s) + 3ZnCl2(aq)  3Zn(s) + 2AlCl3 (aq) Co will not replace Na Co(s) + NaCl(aq)  no rx.

Double Replacement Reactions The ions of two compounds exchange places in an aqueous solution to form two new compounds. AX + BY  AY + BX

Double Replacement Examples 2KI(aq) + Pb(NO3)2(aq)  2 KNO3(aq) + PbI2 (s) K cation replaces Pb cation I anion replaces nitrate anion FeS(s) + 2HCl(aq)  H2S(g) + FeCl2(aq) Fe cation replaces H cation S anion replaces Cl anion

…but there is more!! Double displacement reactions only occur if at least one product is a solid liquid or a gas! If both products are aqueous, then no reaction occurs!! Check your solubility rules! Write in the state of your previous products

Some simple solubility rules All Acids and alkali metal salts and NH4+ salts are soluble. All nitrate and acetate salts are soluble. All chlorides except AgCl and Hg2Cl2 PbCl2 are soluble. All sulfates are soluble except PbSO4, Hg2SO4, SrSO4, BaSO4. Ag2SO4 and CaSO4. All sulfides are insoluble except those of the Group IA (1), IIA (2) and ammonium sulfide. All hydroxides are insoluble except those of the group IA(1) and Ba(OH)2. Sr(OH)2 and Ca(OH)2 are slightly soluble.

Mixed Practice State the type, predict the products, and balance the following reactions: BaCl2 + H2SO4  Zn + CuSO4  Cs + Br2  FeCO3 

Mixed Practice State the type, predict the products, and balance the following reactions: BaCl2 + H2SO4  2HCl +BaSO4 Zn + CuSO4  Cu + ZnSO4 Cs + Br2  CsBr CS2  C + S