Chapter 6 Chemical Reactions and Change
In a chemical change, reacting substances
form new substances with different
compositions and properties;
a chemical reaction takes place
In a chemical reaction,old bonds are broken
and new bonds formed;
atoms in the reactants are rearranged to form
one or more different substances
A chemical equation gives the chemical formulas of the reactants on
the left of the arrow and the products on the right.
Since matter in a chemical reaction is conserved, the number of atoms
you begin with must equal the number oand type you end up with.
In a balanced chemical equation, there must be the same number of
each type of atom on the reactant side and on the product side
numbers called coefficients are used in front of one or more formulas.
Al + S
Al2S3
2Al + 3S
2Al
3S
Not balanced
Al2S3 Balanced
=
=
2Al
3S
Coefficients that can be changed
2Al + 3S
1Al2S3
Balance the following reaction:
P4(s) + Br2(l)
=
P4(s)
PBr3(g)
4PBr3(g)
P4(s) + 6Br2(l) = 4PBr3(g)
Coefficients that can’t be changed
2Al + 3S
Al2S3
Al(s) + Fe2O3(s) = Fe(s) + Al2O3(s)
2Al(s)
Al2O3(s)
2Al(s) + Fe2O3(s) = 2Fe(s) + Al2O3(s)
Thermite reaction
Burning of natural gas, CH4
CH4 + O2 = CO2 +
H2O
CH4 + O2 = CO2 + 2H2O
Burning of butane, C4H10
C4H10 + O2 = CO2 +
C4H10 + O2 = 4CO2 +
C4H10 + 13/2O2 =
H2O
5H2O
4CO2 +
2C4H10 + 13O2 = 8CO2 +
5H2O
10H2O
Magnesium phoshate, Mg3(PO4)2, is insoluble
while sodium phosphate, Na3PO4, and
magnesium chloride, MgCl2, are water
soluble
Na3PO4(aq) + MgCl2(aq) = Mg3(PO4)2(s) + NaCl(aq)
6 NaCl (aq)
Reactions of some metals with water
Reactions of some elements with oxygen
Mg(s) + O2 (g) = MgO (s)
Na(s) + H2O (l) = NaOH(aq) + H2 (g)
2Mg
+ O2
= 2MgO
Na + 3/2H2O = NaOH + H2
2Na + 3H2O = 2NaOH + 2H2
K(s) + H2O (l) = KOH(aq) + H2 (g)
K + 3/2H2O = KOH + H2
2K + 3H2O = 2KOH + 2H2
Fe(s) + O2(g) = Fe2O3 (s)
2Fe + 3/2O2 = Fe2O3
4Fe + 3O2 = 2Fe2O3
P4 (s, white phosphorus) + O2 (g) = P4O10(s)
P4 + 5O2 = P4O10 (s)
Note that when Mg and Fe reacted with oxygen, they lost electrons and became positively
charged in the process; the loss of electrons is called oxidation.
The oxygen gained electrons and this is called reduction. All elements in their elemental state
are neutral and are assigned an oxidation state of zero. Similarly, when Na and K reacted with
H2O they became positively charged. In this case the hydrogen in water, H+1 was reduced to its
elemental state H20
Quantities
Definition of terms.
1 ream of paper
= 500 sheets
1 case = 24 cans
1 dozen donuts
= 12 donuts
Definition of a new quantity, the mol
1 mol = 6.03*1023
Why choose this number?
Several lectures ago we discussed the mass of various particles
mass of a proton (p)
mass of a neutron (n)
mass of an electron (e)
1.672*10-24 g
1.674*10-24 g
9.1*10-28 g
Consider the element H; it consists of a proton and electron; the mass of the elctrom is
small in comparison so lets ignore it.
What is the mass of 1 mol of H atoms?
The mass of 1 mol of H atoms:
(6.03*1023)*(1.67*10-24g) = 10.0*10-1 = 1g
12C
6
What is the mass of 1 mol of
atoms?
1.672*10-24 g
1.674*10-24 g
mass of a proton (p)
mass of a neutron (n)
mass of a proton ≈ mass of a neutron ≈ 1.67*10-24 g
What is the mass of one
12C
6
atom?
C = 6 p + 6 n = 12(1.67*10-24) g
The mass of 1 mol of 12C6 atoms?
12 (6.03*1023)* (1.67*10-24g) = 12*10.0*10-1 = 12g
The mass of 1 mol of yMx atoms? y (6.03*1023)* (1.67*10-24g) = y*10.0*10-1 = yg
where y = atomic mass of M
The atomic mass of any element is also numerically equal to the mass in grams of 6.03*1023
atoms of that element
How many mols of O are in 1 mole of aspirin, C9H8O4?
4 moles
How many O atoms are in 1 mole of aspirin, C9H8O4?
4 oxygen atoms/aspirin* 6.03*1023
=
24*1023
What is the mass in grams of a mol
of aspirin, C9H8O4?
9*12+8*1+4*16= 180 g/mol
180 is also referred to as the molar mass
6*12+12*1+6*16 = 180
To calculate the molar mass simply add up the (atomic masses of each element times the
number of times they appear) in the molecule
The Law of Conservation of Mass indicates that in an ordinary chemical reaction,
matter cannot be created or destroyed
no change in total mass occurs in a reaction
mass of products is equal to mass of reactants
Reactants
2 moles of Ag + 1 mole of S
2 (107.9 g) +
=
1(32.1 g) =
Products
1 mole Ag2S
1 (247.9 g)
Ag = 215.8g
S
= 32.1g
Total = 247.9
Consider the following equation:
4Fe(s) + 3O2(g)
=
2Fe2O3(s)
4 atoms of iron react with 3 molecules of oxygen to produce 2 molecules of iron(III) oxide
This equation can be read in “moles” by placing the words “moles of” between each
coefficient and formula.
4 moles of Fe + 3 moles of O2
equals
2 moles of Fe2O3
Energy Changes in Chemical Reactions
Ever seen water go up hill on it own?
Why do chemical reactions occur?
Consider the reaction of paper which is mainly cellulose, (C6H10O5)n with oxygen
Does a reaction occur?
What are the products?
Why doesn’t paper react spontaneously with oxygen? or does it?
The driving force for most chemical
reaction is the conversion of potential
energy to kinetic energy (ie the release
of heat) similar to water going over a
dam
Far less common is the situation illustrated in the photo and described by the diagram on
the right
Identify each of the following reactions as exothermic or endothermic
A. N2(g) + 3H2(g)
= 2NH3(g) + 22 kcal
B. CaCO3(s) + 133 kcal = CaO(s) + CO2(g)
C. 2SO2(g) + O2(g)
= 2SO3(g) + heat
A and C are exothermic; B is endothermic and it proceeds because the CO2 is driven off by
heating and escapes into the environment