Evidence of Chemical Change

Report
Mark S. Cracolice
Edward I. Peters
www.cengage.com/chemistry/cracolice
Chapter 8
Chemical Reactions
Mark S. Cracolice • The University of Montana
Evidence of Chemical Change
Evidence of Chemical Change
1. Color change
2. Formation of a solid
3. Formation of a gas
4. Absorption or release of heat energy
5. Emission of light energy
Evidence of Chemical Change
Color change
Evidence of Chemical Change
Formation of a solid
Evidence of Chemical Change
Formation of a gas
Evidence of Chemical Change
Evolution of heat and light
Evidence of Chemical Change
Emission of light
Evidence of Chemical Change
Heat transfer as evidence
of physical change
Many physical changes are
accompanied by heat transfer, so
be cautious in using this form of
evidence of a chemical change.
Evolution of a Chemical Equation
When solid potassium is added to liquid water, a reaction
occurs, producing hydrogen gas, potassium hydroxide
solution, and heat:
Evolution of a Chemical Equation
Chemists have developed a standardized shorthand
method for describing chemical reactions, such
as the reaction of sodium and water, in writing.
State Symbols and Their Meanings
Symbol
Meaning
(s)
solid
(l)
liquid
(g)
gas
(aq)
aqueous (dissolved in water)
Evolution of a Chemical Equation
Solid potassium
K(s)
plus
+
liquid water
H2O(l)
yields
®
hydrogen gas
H2(g)
plus
+
potassium hydroxide solution
KOH(aq)
Evolution of a Chemical Equation
The equation
K(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 change
2 K(s) + 2 H2O(l) ® H2(g) + 2 KOH(aq)
Interpreting Chemical Equations
The particulate-level interpretation of a chemical equation:
2 H2(g) + O2(g) ® 2 H2O(g)
Two molecules of hydrogen react with one oxygen molecule
to form two water molecules.
Interpreting Chemical Equations
The particulate-level interpretation of a chemical equation:
Interpreting Chemical Equations
The molar-level interpretation of a chemical equation:
2 H2(g) + O2(g) ® 2 H2O(g)
Two moles of hydrogen react with one mole of oxygen
to form two moles of water.
Writing Chemical Equations
1. Write the formulas of the given reactants to the left of an
arrow and the formulas of the products to the right.
2. Balance the equation by adding coefficients, but do not
change the qualitative description of the reaction by adding,
removing, or altering any chemical formula.
Combination Reactions
Combination Reaction
Two or more substances combine to form a single product:
Combination Reactions
Example:
Potassium combines with oxygen to form potassium oxide.
Solution:
Write the qualitative description:
K + O2 ® K2O
Balance the O:
K + O2 ® 2 K2O
Balance the K:
4 K + O2 ® 2 K2O
Combination Reactions
Reactants:
Any combination of elements and/or
compounds
Reaction type:
Combination
Equation type:
A + X ® AX
Products:
One compound
Decomposition Reactions
Decomposition Reaction
A compound breaks down into simpler substances:
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)
Decomposition Reactions
Example:
Calcium carbonate (limestone) is decomposed into carbon
dioxide and calcium oxide (lime).
CaCO3(s)  CaO (s) +
CO2 (g)
Decomposition Reactions
Reactants:
One compound
Reaction type:
Decomposition
Equation type:
AX ® A + X
Products:
Any combination of elements and
compounds
Single-Replacement Reactions
Single-Replacement Reaction
One element appears to replace another in a compound:
Single-Replacement Reactions
Example:
Potassium is able to replace hydrogen in water.
2 K(s) + 2 HOH (l)

H2 (g) + 2 KOH (aq)
Single-Replacement Reactions
Example:
A copper strip is placed in a solution of silver nitrate.
Solution:
Copper displaces silver from silver nitrate
Cu (s) + 2AgNO3 (aq)  Cu(NO3)2 (aq) + 2 Ag (s)
The reverse reaction does not occur
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
Single-Replacement Reactions
Summary:
Single-Replacement Reactions
Reactants:
Element (A) plus a solution of an acid
or an ionic compound (BX)
Reaction type:
Single-replacement
Equation type:
A + BX ® AX + B
Products:
Element (B) must be less active than
element A
Double-Replacement Reactions
Double-Replacement Reaction
Ions of two reactants appear to change partners:
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)
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) +
2 NaCl (aq)
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) + 2 Al(OH)3 (s)  Al 2(SO4)3 (aq) + 3 H2O (l)
Formation of weak acid
Reaction 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) + 2 HCl (aq)  2 NaCl (aq) +
H2S (aq)
Formation of a gas
Reaction between hydrochloric acid and sodium carbonate to
form carbon dioxide gas
Na2CO3 (aq) + 2 HCl (aq)  2 NaCl (aq) +
H2O(l) + CO2 (g)
Double-Replacement Reactions
Summary:
Double-Replacement Reactions
Reactants:
Solutions of two ionic compounds,(AX + BY)
Reaction type:
Double-replacement
Equation type:
AX + BY ® AY + BX
Products:
Two new compounds (AY + BX), one of
which may be a solid, water, weak acid,
weak base, or a gas
Summary
Homework
Homework: 9, 15, 19, 23, 27, 29, 31, 35, 39, 53, 59, 69, 76

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