Signs of a Chemical Reaction
Evolution of heat and light Formation of a gas Formation of a precipitate Color change
Law of Conservation of Mass mass is neither created nor destroyed
in a chemical reaction
4 H
2 O
4 H
2 O4 g 32 g
36 g
total mass stays the same atoms can only rearrange
Writing Equations
Identify the substances involved. Use symbols to show:
2H2(g) + O2(g) 2H2O(g)
· How many? - coefficient
· Of what? - chemical formula
· In what state? - physical state Remember the diatomic elements.
Writing Equations
Two atoms of solid aluminum react with three units of aqueous copper(II) chloride to produce three atoms of solid copper and two units of aqueous aluminum chloride.
• How many?• Of what?• In what state?
Al 2 (s) + 3CuCl2(aq) 3 Cu(s) + 2AlCl3(aq)
Describing Equations
Describing Coefficients:
· individual atom = “atom”
· covalent substance = “molecule”
· ionic substance = “formula unit”
3 molecules of carbon dioxide
2 atoms of magnesium
4 formula units of magnesium
oxide
3CO2
2Mg
4MgO
Describing Equations
to produce
• How many?• Of what?• In what state?
Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)
One atom of solid zinc reacts withtwo molecules of aqueous hydrochloric acid one unitof aqueous zinc chloride and onemolecule of hydrogen gas.
Balancing Steps
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Coefficient subscript = # of atoms
4. Reduce coefficients to lowest
possible ratio, if necessary.
5. Double check atom balance!!!
Helpful Tips
Balance one element at a time. If Hydrogen and Oxygen appear,
balance them last. Balance Hydrogen before Oxygen.
Update ALL atom counts after adding a coefficient.
If an element appears more than once per side, balance it last.
Al + CuCl2 Cu + AlCl3
Al
Cu
Cl
1 1
1 1
2 3
2
3
6
3
33 2
Balancing Example
Aluminum and copper(II) chloride react to form copper and aluminum chloride.
2
2
6
Balancing Example
Aqueous nitric acid reacts with solid magnesium hydroxide to produce aqueous magnesium nitrate and water
(2,1,1,2) Solid Calcium metal reacts with water
to form aqueous calcium hydroxide and hydrogen gas.
(1,2,1,1)
Balancing example (hydrocarbons)
Ethane gas reacts with oxygen gas to form carbon dioxide and water.
Ethene gas reacts with oxygen gas to form carbon dioxide and water.
Butane gas reacts with oxygen gas to form carbon dioxide and water.
Combustion
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
the burning of a hydrocarbon in O2 to produce heat
A + O2 B
C3H8(g)+ O2(g) 5 3 4
Combustion
Products:
· form CO2 + H2O· Form carbon monoxide and/or carbon if
there is limited oxygen
CO2(g)+ H2O(g)
Synthesis (Combination)
the combination of 2 or more substances to form a compound
only one product
A + B AB
Al(s)+ Cl2(g) AlCl3(s)2 3 2
Synthesis (Combination)
Products:· If products are ionic - cancel charges· If covalent - hard to tell
KBr(l) K(s) + Br2(l) 2 2
Decomposition
Products:· binary - break into elements· others - hard to tell (more on this later)
Synthesis Reactions to know
Almost all metals react with oxygen to form metal oxides.
Ex/ Magnesium reacts with oxygen to form Magnesium oxide
Synthesis reactions to know
Nonmetals also form oxides. Sulfur reacts with oxygen to form
Sulfur dioxide Carbon reacts with oxygen to form
Carbon dioxide
Synthesis reactions involving water to know
Oxides of active metals react with water to produce metal hydroxides.· Ex/ Calcium oxide reacts with water
to form calcium hydroxide.
Synthesis reactions to know involving water
Many oxides of nonmetals in the upper right portion of the periodic table react with water to produce oxyacids (acids with oxygen).· Ex/ sulfur dioxide reacts with water
to form sulfurous acid
Decomposition reactions to know
Metal carbonates break down to produce a metal oxide and carbon dioxide gas.· Ex/ calcium carbonate decomposes
to produce calcium oxide and carbon dioxide.
Decomposition reactions to know
All metal hydroxides except those containing group 1 metals decompose when heated to yield metal oxides and water.· Ex/ calcium hydroxide decomposes
to produce calcium oxide and water.
Decomposition reactions to know
Metal chlorates decompose to produce a metal chloride and oxygen.· Ex/ potassium chlorate
decomposes in the presence of the catalyst Manganese dioxide to produce potassium chloride and oxygen.
Decomposition reactions to know
Sulfites decompose into the cation oxide and sulfur dioxide gas· Calcium sulfite decomposes to form
calcium oxide and sulfur dioxide gas.
Decomposition reactions to know
Peroxides decompose into the oxide of the cation and oxygen gas.
· Hydrogen peroxide decomposes to form water (dihydrogen monoxide) and oxygen gas.
· Sodium peroxide decomposes to form . . .
Decomposition reactions to know
Certain acids decompose into nonmetal oxides and water.· Ex/ Carbonic acid is unstable and
decomposes readily to produce carbon dioxide and water.
· Sulfurous acid decomposes to produce sulfur dioxide and water.
Decomposition reactions to know
Ammonium salts decompose into ammonia and the acid of the anion.· Ex/ Ammonium acetate
decomposes to form ammonia and acetic acid.
· Ammonium nitrate decomposes to form ammonia and nitric acid.
Single Replacement
one element replaces another in a compound· metal replaces metal (+)· nonmetal replaces nonmetal (-)
A + BC B + AC
Fe(s)+ CuSO4(aq) Cu(s)+ FeSO4(aq)
Single Replacement
Products:· Metal atom metal cation(+)· Nonmetal atom nonmetal anion(-)· free element must be more active (check
activity series)
Br2(l)+ NaCl(aq) N.R.
AB + CD AD + CB
Double Replacement
Occurs between two aqueous ionic compounds
ions in two compounds “change partners” cation of one compound combines with
anion of the other
Pb(NO3)2(aq)+ KI(aq) PbI2(s)+ KNO3(aq)
Double Replacement Products:
· In order for a reaction to occur on of the following must occur- one product must be insoluble (a precipitate)
(check solubility table)- A gas is produced- A covalent compound (molecular) is produced
NaNO3(aq)+ KI(aq) N.R. 2 2
Examples
Solutions of Hydrochloric acid and Sodium hydroxide are mixed
Solutions Rubidium carbonate and acetic acid are mixed
Writing net ionic equations
1. Write the skeleton equation· Make sure charges ions are balanced.
2. Write the total ionic equation 3. Cancel out ions that appear on both
sides 4. These are called spectator ions. Spectator ions do not take part in the
overall net reaction.
Writing net ionic equations
A solution of Lead (II) nitrate is mixed with a solution of potassium iodide.
A solution of silver nitrate is mixed with a solution of sodium hydroxide.
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Predicting products from reactions
If given only reactants· Figure out what type of reaction it is· Write the products. If one of the
products is ionic, balance charges.· Balance the equation· Ex/ Hydrochloric acid is mixed with
zinc metal.
Predicting (cont’d)
Calcium metal burns in the presence of oxygen.
Sodium carbonate is heated vigorously. Potassium chlorate is heated vigorously. Pentane burns in oxygen. Aqueous silver nitrate is mixed with
aqueous sodium chloride.