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NOTES: 11.1 – Writing and Balancing Chemical Equations
Effects of chemical reactions:
• Chemical reactions rearrange atoms in the reactants to form new products.
• The identities and properties of the products are completely different from that of the reactants.
What is a Chemical Reaction? • Chemical Reaction – one or more substances
change into new substances• Process involves reactants and products• Reactant – a starting substance • Product – a substance formed • Example:
Nitrogen and hydrogen gas can react to form ammonia under certain conditions.
Reactants Yield Products
N2 (g) + H2 (g) NH3 (g)
How Can You Tell Whether or Not a
Chemical Reaction Has Taken Place?
• Chemical Change – alters a given material by changing its chemical composition
• Production of gases and color changes are signs of chemical reactions
• Examples: burn, rust, decompose, corrode, explode
Chemical Reactions:
• we can describe a chemical reaction with words:
“Iron metal reacts with oxygen gas to produce iron(III) oxide, or rust.”
• we can then write a word equation:
Iron + oxygen iron(III) oxide
Chemical Equations:• Chemical equations are used to represent or describe chemical reactions.
• Chemical equations use chemical symbols and formulas for the reactants and products
• For example when hydrogen, H2,burns, it reacts with oxygen, O2, in the air to form water. We write the chemical equation for this reaction as follows:
2H2 + O2 2H2O
Chemical Equations:
An equation shows…
Chemical formulas of reactants;
Chemical formulas of products;
Molecule / Mole ratios of all compounds in the reaction.
Chemical Equations:
We read the (+) sign as “reacts with” and the arrow ( ) as “produces” or “yields”.
2H2 + O2 2H2O
Reactants Products
To show physical states of each substance:
• (s) = solid • (l) = liquid• (g) = gas• (aq) = aqueous
**aqueous means dissolved in water
• Consider again the reaction of iron with oxygen to form iron(III) oxide, or rust.
Fe(s) + O2 (g) Fe2O3 (s)
(unbalanced)
**this is a skeleton equation in that is NOT “balanced” and does not show the relative amounts of reactants and products
Coefficients & Subscripts
COEFFICIENTS: numbers in front of compound that represents the number of molecules/moles of that compound
SUBSCRIPTS: small numbers within a formula that help define the compound.
2H2SO4
Coefficient Subscript
H2O: One molecule of water
2H2O: Two molecules of water
H2O2: One molecule of Hydrogen Peroxide
• During a chem. rxn. atoms are rearranged (NOT created or destroyed!)• Chemical equations must be BALANCED to show the relative amounts of all substances.• Balanced means: each side of the equation has the same # of atoms of each element.
CH4 + O2 H2O + CO2
Unbalanced
CH4 + 2O2 2H2O + CO2 Balanced
In order to balance…
• Write correct formulas for all reactants and products
• Reactants Products• Count the number of atoms of each
element in reactants & products.• Balance one at a time using
coefficients.• Check for balance• Are the coefficients in the lowest
possible ratio?
Balancing EquationsBalancing Equations
NOTE: When balancing equations, you may change coefficients as much as you need to, but you may
never change subscripts because you can’t change what substances are involved.
Examples:
CuCl2 (aq) + Al (s) Cu (s) +AlCl3 (aq)
3CuCl2 (aq) + 2Al (s) 3Cu (s) + 2AlCl3 (aq)
(3:2:3:2)
Examples:Propane, C3H8, burns in oxygen, O2, to form carbon dioxide and water.
C3H8 + O2 CO2 + H2O
Balance C – then H – then O
C3H8 + 5O2 3CO2 + 4H2O
(1:5:3:4)
Examples:Pentane, C5H12, burns in oxygen, O2, to form carbon dioxide and water.
C5H12 + O2 CO2 + H2O
Balance C – then H – then O
C5H12 + 8O2 5CO2 + 6H2O
(1:8:5:6)
Examples:Silver nitrate reacts with copper to produce silver and copper (II) nitrate.
AgNO3 + Cu Ag + Cu(NO3)2
2AgNO3 + Cu 2Ag + Cu(NO3)2
(2:1:2:1)
Examples:Phosphorus reacts with oxygen gas to produce diphosphorus pentoxide.
P + O2 P2O5
4P + 5O2 2P2O5
(4:5:2)
Examples:
C7H14 + O2 CO2 + H2O
Balance C – then H – then O
C7H14 + 10½O2 7CO2 + 7H2O
2C7H14 + 21O2 14CO2 + 14H2O
(2:21:14:14)