8 Evidence of Chemical Change Heat transfer as evidenceof physical changeMany physical changes are accompanied by heat transfer, so be cautious in using this form of evidence of a chemical change.
9 Evolution of a Chemical Equation When solid potassium is added to liquid water, a reaction occurs, producing hydrogen gas, potassium hydroxide solution, and heat:
10 Evolution of a Chemical Equation Chemists have developed a standardized shorthandmethod for describing chemical reactions, suchas the reaction of sodium and water, in writing.State Symbols and Their MeaningsSymbol Meaning(s) solid(l) liquid(g) gas(aq) aqueous (dissolved in water)
11 Evolution of a Chemical Equation Solid potassium K(s)plus +liquid water H2O(l)yieldshydrogen gas H2(g)potassium hydroxide solution KOH(aq)
12 Evolution of a Chemical Equation The equationK(s) + H2O(l) H2(g) + KOH(aq)is said to be unbalanced because the number of atoms of each element is not the same before and after the chemical change.An equation is balanced when the number of atoms of each element is the same before and after the chemical change2 K(s) + 2 H2O(l) H2(g) + 2 KOH(aq)
13 Interpreting Chemical Equations The particulate-level interpretation of a chemical equation:2 H2(g) + O2(g) H2O(g)Two molecules of hydrogen react with one oxygen moleculeto form two water molecules.
14 Interpreting Chemical Equations The particulate-level interpretation of a chemical equation:
15 Interpreting Chemical Equations The molar-level interpretation of a chemical equation:2 H2(g) + O2(g) H2O(g)Two moles of hydrogen react with one mole of oxygento form two moles of water.
16 Writing Chemical Equations Write the formulas of the given reactants to the left of an arrow and the formulas of the products to the right.Balance the equation by adding coefficients, but do not change the qualitative description of the reaction by adding, removing, or altering any chemical formula.
17 Combination Reactions Two or more substances combine to form a single product:
18 Combination Reactions Example:Potassium combines with oxygen to form potassium oxide.Solution:Write the qualitative description:K + O K2OBalance the O:K + O K2OBalance the K:4 K + O K2O
19 Combination Reactions Reactants: Any combination of elements and/orcompoundsReaction type: CombinationEquation type: A + X AXProducts: One compound
20 Decomposition Reactions A compound breaks down into simpler substances:
21 Decomposition Reactions Example:Water is decomposed into its elements.Solution:Write the qualitative description:H2O (l) H2 (g) O2 (g)Balance the O:2 H2O (l) 2 H2 (g) O2 (g)
22 Decomposition Reactions Example:Calcium carbonate (limestone) is decomposed into carbon dioxide and calcium oxide (lime).CaCO3(s) CaO (s) CO2 (g)
23 Decomposition Reactions Reactants: One compoundReaction type: DecompositionEquation type: AX A + XProducts: Any combination of elements andcompounds
24 Single-Replacement Reactions One element appears to replace another in a compound:
25 Single-Replacement Reactions Example:Potassium is able to replace hydrogen in water.2 K(s) HOH (l) H2 (g) KOH (aq)
26 Single-Replacement Reactions Example:A copper strip is placed in a solution of silver nitrate.Solution:Copper displaces silver from silver nitrateCu (s) + 2AgNO3 (aq) Cu(NO3)2 (aq) + 2 Ag (s)The reverse reaction does not occur
27 Single-Replacement Reactions Example:A magnesium strip is placed in a solution of hydrochloric acid.Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)Magnesium displaces hydrogen from acid
28 Single-Replacement Reactions Summary:Single-Replacement ReactionsReactants: Element (A) plus a solution of an acidor an ionic compound (BX)Reaction type: Single-replacementEquation type: A + BX AX + BProducts: Element (B) must be less active than element A
29 Double-Replacement Reactions Ions of two reactants appear to change partners:
30 Double-Replacement Reactions Double-Replacement Reaction between two ionic compounds.When solutions of two ionic compounds are mixed, positive ions from one compound may combine with negative ions from the other compound to form a solid (precipitate) or a molecular compound (water, weak acid, weak base, gas)
31 Formation of a precipitate Formation of silver chloride precipitate from sodium chloride and silver nitrate.NaCl (aq) AgNO3 (aq) NaNO3 (aq) + AgCl (s)Formation of barium sulfate precipitate from barium chloride and sodium sulfate.BaCl2 (aq) + Na2SO4 (aq) BaSO4 (s) NaCl (aq)
32 Neutralization Reactions: Formation of water Neutralization reaction is a reaction between an acid and a base. One hydrogen ion from acid reacts with one hydroxide ion from a base to form one water molecule.Neutralization of hydrochloric acid by sodium hydroxide.HCl (aq) NaOH (aq) NaCl (aq) H2O (l)Reaction between sulfuric acid and solid aluminum hydroxide.3 H2SO4 (aq) Al(OH)3 (s) Al 2(SO4)3 (aq) H2O (l)
33 Formation of weak acidReaction of hydrochloric acid and sodium acetate to form acetic acid (weak acid)HCl (aq) + NaCH3CO2 (aq) NaCl (aq) + HCH3CO2 (aq)Reaction between hydrochloric acid and sodium sulfide to form hydrosulfuric acid (weak acid)Na2S(aq) HCl (aq) 2 NaCl (aq) + H2S (aq)
34 Na2CO3 (aq) + 2 HCl (aq) 2 NaCl (aq) + H2O(l) + CO2 (g) Formation of a gasReaction between hydrochloric acid and sodium carbonate to form carbon dioxide gasNa2CO3 (aq) HCl (aq) 2 NaCl (aq) + H2O(l) + CO2 (g)
35 Double-Replacement Reactions Summary:Double-Replacement ReactionsReactants: Solutions of two ionic compounds,(AX + BY)Reaction type: Double-replacementEquation type: AX + BY AY + BXProducts: Two new compounds (AY + BX), one of which may be a solid, water, weak acid, weak base, or a gas