Balancing Chemical Equations

Chemical Equations & Reactions : Chemical Equations & Reactions

Chemical Reactions : Chemical Reactions You should be able to Classify reactions by type. Write a balanced molecular equation, complete ionic equation, and a net ionic equation. Balance oxidation-reduction reactions. Predict if a precipitate will form using the solubility rules. Predict products of reactions given the chemical names of the reactants.

Organize Your Thoughts : Organize Your Thoughts Chemical reactions Chemical equations Chemical equations Balancing equations Predicting products from reactants Synthesis Decomposition Single replacement Double replacement Combustion

Describing a Chemical Reaction : Describing a Chemical Reaction Indications of a Chemical Reaction Evolution of heat, light, and/or sound Production of a gas Formation of a precipitate Color change

Signs of Chemical Reactions : Signs of Chemical Reactions There are five main signs that indicate a chemical reaction has taken place: change in color change in odor production of new gases or vapor input or release of energy difficult to reverse release input

Chemical Equations : Chemical Equations Depict the kind of reactants and products and their relative amounts in a reaction. The numbers in the front are called stoichiometric coefficients. The letters (s), (g), and (l) are the physical states of compounds. reactants product aluminum oxide

Chemical Equations : Chemical Equations This equation means: 4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3 4 Al moles + 3 O2 moles yield 2 moles of Al2O3 or 4 g Al + 3 g O2 yield 2 g Al2O3 4 mol Al@27g/mol 3 mol O2@32g/mol 2 mol Al2O3@102g/mol 108 g + 96 g = 204 g aluminum oxide sandpaper

Chemical Equations : Chemical Equations Because the same atoms are present in a reaction at the beginning (reactants) and at the end (products), the amount of matter in a system does not change. The Law of Conservation of Matter

Chemical Equations : Chemical Equations Because of the principle of the conservation of matter, An equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788

Characteristics of Chemical Equations : Characteristics of Chemical Equations The equation must represent known facts. The equation must contain the correct formulas for the reactants and products. The law of conservation of mass must be satisfied.

Chemical Equations : Chemical Equations Reactants – the substances that exist before a chemical change (or reaction) takes place. Products – the new substance(s) that are formed during the chemical changes. CHEMICAL EQUATION indicates the reactants and products of a reaction. REACTANTS  PRODUCTS

Word Equations : Word Equations A WORD EQUATION describes chemical change using the names of the reactants and products. Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water. methane + oxygen Reactant Product CH4 O2 CO2 H2O + + 2 2 carbon dioxide + water

Slide 13 : Cl Cl Cl H H H H2 + Cl2  HCl H2 + Cl2  2 HCl 2 2 2 2 2 2 1 1 (unbalanced) (balanced) Unbalanced and Balanced Equations

Visualizing a Chemical Reaction : ? Visualizing a Chemical Reaction Na + Cl2 NaCl ___ mole Cl2 ___ mole NaCl ___ mole Na 2 10 5 10 2 10 5 10

Visualizing a Chemical Reaction : Visualizing a Chemical Reaction Na + Cl2 NaCl 2 2

Meaning of Chemical Formula : Meaning of Chemical Formula Chemical Symbol Meaning Composition H2O One molecule of water: Two H atoms and one O atom 2 H2O Two molecules of water: Four H atoms and two O atoms H2O2 One molecule of hydrogen peroxide: Two H atoms and two O atoms

Balancing Chemical Equations : Balancing Chemical Equations Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product What is the relationship between conservation of mass and the fact that a balanced equation will always have the same number of atoms of each element on both sides of an equation? Determine whether the following equation is balanced. 2 Na + H2O  2 NaOH + H2 2 Na + 2 H2O  2 NaOH + H2

Balancing Chemical Equations : Balancing Chemical Equations Write a word equation for the reaction. Write the correct formulas for all reactants and products. Determine the coefficients that make the equation balance.

Slide 19 : Guidelines for Balancing Chemical Equations 1) polyatomic ions first 2) even / odd (make all even) 3) H2O Mg(OH)2 2 4) single elements last Example: need 13 oxygen atoms Multiply by O2 = 13 2 H-OH vs. ?

Slide 20 : (NH4)3PO4 + Mg(OH)2  Mg3(PO4)2 NH4OH ? ammonium phosphate magnesium hydroxide magnesium phosphate ammonium hydroxide NH41+ OH1- + 6 2 3 Now you try… AlCl3 + Li2CO3  Al2(CO3)3 + LiCl 3 2 6

Slide 21 : 1) Write a word equation for the reaction. Write a balanced equation for the reaction between chlorine and sodium bromide to produce bromine and sodium chloride. 2) Write the correct formulas for all reactants and products. 3) Determine the coefficients that make the equation balance. chlorine + sodium bromide  bromine + sodium chloride Cl2 + NaBr  Br2 + NaCl Cl2 + 2 NaBr  Br2 + 2 NaCl

Slide 22 : 1) Write a word equation for the reaction. 2) Write the correct formulas for all reactants and products. 3) Determine the coefficients that make the equation balance. aluminum sulfate + calcium chloride  calcium sulfate Al2(SO4)3 + CaCl2  CaSO4 + AlCl3 Write the balanced equation for the reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate. Al2(SO4)3 + 3 CaCl2  3 CaSO4 + 2 AlCl3 + aluminum chloride ? ?

CH4 + 2 O2  CO2 + 2 H2O : CH4 + 2 O2  CO2 + 2 H2O Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms

Reactants  Products : Reactants  Products Unbalanced 2 2 catalyst – speeds up reaction

Showing Phases in Chemical Equations : Showing Phases in Chemical Equations Solid Phase – the substance is relatively rigid and has a definite volume and shape. NaCl(s) Liquid Phase – the substance has a definite volume, but is able to change shape by flowing. H2O(l) Gaseous Phase – the substance has no definite volume or shape, and it shows little response to gravity. Cl2(g) H2O(s) H2O(l) H2O(g)

Additional Symbols Used in Chemical Equations : Additional Symbols Used in Chemical Equations “Yields”; indicates result of reaction Used to indicate a reversible reaction A reactant or product in the solid state; also used to indicate a precipitate Alternative to (s), but used only to indicate a precipitate A reactant or product in the liquid state A reactant or product in an aqueous solution (dissolved in water) A reactant or product in the gaseous state (s) (l) (aq) (g)

Slide 27 : Additional Symbols Used in Chemical Equations Alternative to (g), but used only to indicate a gaseous product Reactants are heated Pressure at which reaction is carried out, in this case 2 atm Pressure at which reaction is carried out exceeds normal atmospheric pressure Temperature at which reaction is carried out, in this case 0 oC Formula of catalyst, in this case manganese (IV) oxide, used to alter the rate of the reaction 2 atm pressure 0 oC MnO2 D

Slide 28 : Solubility Ionic Equations Cover the answers, work the problem, then check the answer. 1. Dissolve ammonium nitrate: 2. Precipitate cupric hydroxide: 3. Dissolve chromium thiocyanate: 4. Precipitate lead arsenate: 5. Dissolve silicon permanganate: 6. Precipitate zinc phosphate: NH4NO3 (s) ---> NH4+1 (aq) + NO3-1 (aq) Cu+2 (aq) + 2OH-1 (aq) ---> Cu(OH)2 (s) Cr(SCN)3 (s) ---> Cr+3 (aq) + 3SCN-1 (aq) 3Pb+2 (aq) + 2AsO4-3 (aq) ---> Pb3(AsO4)2 (s) Si(MnO4)4 (s) ---> Si+4 (aq) + 4MnO4-1 (aq) 3Zn+2 (aq) + 2PO4-3 (aq) ---> Zn3(PO4)2 (s)