Chemical Equations & Reactions

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Lecture Outline – Chemical Equations& Reactions Lecture Outline – Chemical Equations & Reactions student notes outline textbook questions Lecture Outline – Chemical Equations & Reactions textbook questions Keys text http://www.unit5.org/chemistry/Equations.html

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 Chemical reactions Chemical equations Chemical equations • Synthesis • Decomposition • Single replacement • Double replacement • Combustion • Balancing equations • Predicting products • from reactants Packard, Jacobs, Marshall, Chemistry Pearson AGS Globe, page 175

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

4 Al(s) + 3 O2(g) 2 Al2O3(s) Chemical Equations aluminum oxide product reactants Depict the kind of reactants and products and their relative amounts in a reaction. The letters (s), (g), and (l) are the physical states of compounds. The numbers in the front are called stoichiometric coefficients.

4 Al(s) + 3 O2(g) 2 Al2O3(s) Chemical Equations aluminum oxide sandpaper 4 g Al + 3 g O2 yield 2 g Al2O3 This equation means: 4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3 or 4 Al moles + 3 O2moles yield 2 moles of Al2O3 2 mol Al2O3@102g/mol 4 mol Al@27g/mol 3 mol O2@32g/mol 108 g + 96 g = 204 g

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 Kotz web

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 • 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 • 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 • 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. carbon dioxide + water methane + oxygen Reactant Product O2 2 H2O 2 CO2 CH4 + +

Cl H H H Cl Cl Cl H reactants products reactants products H H Cl Cl Unbalanced and Balanced Equations H Cl Cl Cl H H H2 + Cl2 2 HCl (balanced) (unbalanced) H2 + Cl2 HCl 2 1 2 2 1 2 2 2

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

Visualizing a Chemical Reaction 2 2 Na + Cl2 NaCl

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 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 • Write a word equation for the reaction. • Write the correct formulas for all reactants and products. • Determine the coefficients that make the equation balance.

“ ” 13 13 13 2 2 2 3X + O2 2Y + Z 3X + O2 2Y + Z 6X + 13 O2 4Y + 2Z Guidelines for Balancing Chemical Equations 1) polyatomic ions first ? (make all even) 2) even / odd 3) H2O Mg(OH)2 2 H-OH vs. 4) single elements last Example: need 13 oxygen atoms Multiply by O2 = 13 2

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

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

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

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 catalyst – speeds up reaction 2 2 Unbalanced

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 “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)

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 D 2 atm pressure 0 oC MnO2

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)

Chemical Equations & Reactions