Chemical Equations & Reactions. Describing a Chemical Reaction Indications of a Chemical Reaction...
-
Upload
ruby-stalker -
Category
Documents
-
view
240 -
download
11
Transcript of Chemical Equations & Reactions. Describing a Chemical Reaction Indications of a Chemical Reaction...
Chemical Equations & Reactions
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
Chemical EquationsDepict the kind of reactantsreactants and productsproducts and their relative amounts in a reaction.
4 Al(s) + 3 O2(g) 2 Al2O3(s)
The numbers in the front are called stoichiometric coefficientsstoichiometric coefficients.
The letters (s), (g), and (l) are the physical states of compounds.
Chemical Equations
This equation means:
4 Al(s) + 3 O2(g) 2 Al2O3(s)
4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3
4 Al moles + 3 O2 moles yield 2 moles of Al2O3
or
Chemical Equations
Because the same atoms are present in a reaction at the beginning (reactants) and at the end (reactants), the amount of matter in a system does not change.
The Law of Conservation of MatterLaw of Conservation of Matter
Chemical Equations
Because of the principle of the conservation of matterconservation of matter,
An equation must be balancedequation must be balanced.
It must have the same number of atoms of the same kind on both sides.
Lavoisier, 1788
Signs of Chemical ReactionsThere 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
rele
ase
inp
ut
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.
• 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.
methane + oxygen carbon dioxide + water
Reactant Product
Meaning of Chemical FormulaChemical 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
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
2 atm
pressure
0 oC
MnO2
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)
Types of Chemical ReactionsSynthesis (Combination) reaction
Decomposition reaction
ASingle-replacement reaction
BDouble-replacement reaction
Acid base reaction
Combustion reaction (of a hydrocarbon)
A + B AB
AB A + B
A + BC AC + B
AB + CD AD + CB
HX + BOH BX + HOH
CH + O2 CO2 + H2O
Ause activity series to predict
Bdriving force…water, gas, or precipitate
Synthesis Reaction
Direct combination reaction (Synthesis)
General form: A + B AB
element or element or compound compound compound
Na+Cl -
Na
ClNa
Cl
Na+ Cl -
2 Na + Cl2 2 NaCl
H
H
H
H
Decomposition Reaction
H
H
H
HO
Decomposition reaction
2 H2O 2 H2 O2
General form: AB A B
+
+
+compound two or more elements
or compounds
OO
O
H
H
H
H
Decomposition Reaction
H
H
H
HO
Decomposition reaction
2 H2O 2 H2 O2
General form: AB A B
+
+
+compound two or more elements
or compounds
OO
O
Single and Double Replacement Reactions
Double-replacement reaction
CaCO3 + 2 HCl CaCl2 + H2CO3
General form: AB + CD AD + CB
Single-replacement reaction
Mg + CuSO4 MgSO4 + Cu
General form: A + BC AC + B
CombustionCombustion ReactionsReactionsA substance combines with oxygen, releasing a large amount of energy in the form of light and heat.
Reactive elements combine with oxygen
P4(s) + 5O2(g) P4O10(s)
(This is also a synthesis reaction)
The burning of natural gas, wood, gasoline
C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)
Unbalanced and Balanced Equations
Cl
ClClCl
ClCl
ClH
H
H
HHH
H
H2 + Cl2 HCl (unbalanced) H2 + Cl2 2 HCl (balanced)
2 12 1 2 2
reactants products reactants products 2 2H
Cl
H
Cl
Meaning of Chemical FormulaChemical 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.
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
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 + aluminum chloride
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
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