Chemical Reactions
Chapter 17
I. Physical Change
Changes which affect the size or shape of the substance
BUT NOT its chemical properties or formulas.
e.g.; ice melting. Water is still water
II. Chemical Change
Affects the chemical properties of one or more
substances (can also affect the size or shape).
A physical change is like printing the same
word in a different style of type.
stampedes → stampedes
A chemical change, or chemical reaction,
is like scrambling the letters of a word to
make new words.
stampedes → made + steps
Evidence for Chemical Reactions
Color change
Gas production
Precipitation
Changes in temperature
Endothermic reactions- absorbs energy
(drop in temperature)
Exothermic reactions- release energy
(raise in temperature)
Changes in properties
The key characteristic of a chemical reaction
is the production of new materials that are
chemically different from the starting
materials.
Chemical reactions occur when chemical
bonds are either formed or broken.
All chemical bonds involve valence electrons
Conservation of Mass
In a chemical reaction, no atoms just disappear and
no new atoms just appear.
The Law of conservation of mass and energy
tells us this can not happen.
Therefore the number of atoms on each side of the
arrow in a chemical reaction equation MUST be equal.
Remember even a gas has mass.
III. Chemical Equations
Use formulas (equations) and symbols (elements)
to show what happens during a chemical reaction.
Molecules are separated by either a + sign or the →
→ = shows the direction a reaction moves in; yields.
↓ = shows that the substance is a solid or a precipitate.
All other molecules are assumed to be in liquid
form (aqueous form)
NaI + Pb(NO3)2 → NaNO3 + PbI2 ↓
Chemical Symbols
Water
H2O
The “little” 2 is a subscript
This means there are 2 atoms of H and
1 atom of O in this molecule.
If there is NO subscript it is 1
Hydrogen peroxide
H2O2
This means there are 2 atoms of H and
2 atoms of O in this molecule.
IV. Balancing Chemical Reactions
REMEMBER- In a chemical reaction, no atoms just
disappear and no new atoms are just formed.
To balance a chemical reaction equation you can only
change the number of molecules in the reaction by
changing the COEFFICIENT in front of the molecule.
2Mg + O2 → 2MgO
The big “2” is the COEFFICIENT on both sides sides of
the arrow.
This means you have 2 Mg and 2 MgO
REMEMBER- The little “2” is a subscript
Structure of a Chemical Equation
A chemical equation uses symbols to show the
reactants and products of a chemical reaction.
Reactant + Reactant → Product + Product
The → means “yields”
H2
+
O2
→.
H2O
Hydrogen + Oxygen yields
Reactants
yields
Water
Products
REMEMBER
you can not change, in any way:
the subscripts
formulas
symbols in the equation
http://www.wfu.edu/~ylwong/balanceeq/balanceq.html
2 H2O2 → 2 H2O +
Zn + 2 HCl →
O2 ↑
ZnCl2 +
I2 + 2 KCl → 2 KI +
H2 ↑
Cl2 ↑
Ba(OH)2 +
Ba + 2 H2O →
2 H2 +
O2 → 2 H2O
H2 ↑
2 NaI + Pb(NO3)2 → 2 NaNO3 + PbI2 ↓
Cu + 2 H2SO4 →
CuSO4 + 2 H2O+
SO2
http://proton.csudh.edu/lecture_help/startbalancerxns.html
AgNO3 + NaCl → AgCl ↓+ NaNO3
AgNO3 + NaOH→ Ag2O ↓ + NaNO3 + H2O
AgNO3 + Na2CO3→ Ag2CO3↓ + NaNO3
AgNO3 + KI →
AgI ↓ +
KNO3
Pb(NO3)2 + KI → PbI2↓ + KNO3
Pb(NO3)2 + NaOH → Pb(OH)2↓ + NaNO3
Pb(NO3)2 + H2SO4 → PbSO4↓ + HNO3
Pb(NO3)2 + Na2CO3 → PbCO3↓ + NaNO3
Pb(NO3)2 + CuSO4 → PbSO4↓ + Cu(NO3)2
Pb(NO3)2 + NaCl → PbCl2↓ + NaNO3
CaCl2 + NaOH → Ca(OH)2↓ + NaCl
CaCl2 + Na2CO3 → CaCO3↓ + NaCl
CaCl2 + H2SO4 → CaSO4↓ + HCl
V. Types of Reactions
1. Synthesis- two or more elements unite to form one
compound.
2 Na + Cl2 → 2 NaCl
2. Decomposition- break down of one compound into
two or more elements.
ZnCl2 → Zn + Cl2 ↑
3. Single displacement- one element changes places
with one cation in a molecule.
BeF2 + Mg → MgF2 +Be
4. Double displacement- the cation in each of two
molecules change places.
AgNO3 + NaCl → AgCl + NaNO3 ↓
VI. Controlling Chemical Reactions
Every chemical reaction involves a change in energy.
Some reactions release energy:
Exothermic
Gives off heat
Some reactions absorb energy:
Endothermic
Take in heat
VII. Rates of Chemical Reactions
Four ways to affect the rate of a chemical reaction (SPEED).
A. Number of particles (reactant side of the equation)
↑ the concentration of molecules; This will give
you more collisions between the molecules.
B. Temperature
↑ the temperature and then the molecules move
faster; therefore you increase the number of
collisions.
C. Surface size of reactant molecules
Smaller size will ↑ reaction speed
D. Catalyst
A substance that effects the rate of a chemical
reaction without itself being changed or used up in
the reaction.
ENZYMES- catalyst in living things (CELLS).
Inhibitor- a material used to slow down the rate of a
reaction.