Chapter 7 Chemical Reactions
7.1 Describing Reactions
Chemical Equations
Equation states what a reaction starts with, and what it ends with.
Reactants – the starting materials that undergo change. (On the left side.)
Products – the materials resulting from the change. (On the right side.)
Reactants → Products
Conservation of Mass
Mass is neither created nor destroyed in a chemical reaction.
The mass of the products ALWAYS equals the mass of the reactants.
A balanced equation shows the conservation.
Balancing Equations
To show that mass is conserved, equations must balance – the number of atoms
of an element on the right side of the equation must equal the number of atoms
of that element on the left side. This must be true for every element in the
reaction.
NEVER change the subscripts in a formula – changing the subscripts changes
the identity of that product or reactant.
Use coefficients to balance number of atoms.
Keep checking and changing them until equation is balanced.
N2H4 + O2 → N2 + H2O
Cu + O2 → CuO
Counting With Moles
Moles are a counting unit (like dozen); except 1 moles = 6.02 x 1023 particles.
Because a chemical reaction large enough for chemists to observe and study
involves a very large number of particles (ex. atoms or compounds), we need a
very large counting unit.
6.02 x 1023 is known as Avogadro’s number.
Molar Mass
 The molar mass is equal to the mass (measured in grams) of 1 mole of a
substance.
 The molar mass is the same value as the atomic mass.
 To find the molar mass of a compound, add up the molar masses of all the
atoms in the compound.
 You can convert between moles and mass by using the mass per one mole of
the element or compound.
7.2 Types of Reactions – 5 general types:
Synthesis
 Two or more substances react to form a single substance.
 A + B → AB
 2Na + Cl2 → 2NaCl
Decomposition
 The opposite of synthesis.
 A compound breaks down into two or more substances.
 AB → A + B
 2H2O → 2H2 + O2
Single-replacement
 One element takes the place of another element in a compound.
 A + BC → B + AC
 Cu + 2AgNO3 → Cu(NO3)2 + 2Ag
Double-replacement
 Two different compounds trade positive ions and become new compounds.
 AB + CD → AD + CB
 CaCO3 + 2HCl → CaCl2 + H2CO3
Combustion
 A substance reacts rapidly with oxygen, often producing light and heat.
 CH4 + 2O2 → CO2 + 2H2O
 2H2 + O2 → 2H2O
 Notice that this second reaction could also be called a synthesis reaction –
chemical reaction types sometimes overlap.
Reactions as Electron Transfers – Reduction/oxidation reactions
Often called “redox” reactions.
Electrons are transferred from one reactant to another.
An atom is reduced when it receives/accepts electrons. (Its charge is reduced)
An atom is oxidized when it gives up electrons. The name comes from the fact
that many metals react with oxygen, and it was called oxidation. (Ex. Rust is
iron reacting with oxygen.) After subatomic particles were discovered, it was
discovered that the metals were reacting by giving electrons to oxygen.
7.3 Energy Changes in Reactions
Chemical Bonds and Energy
Chemical Energy is stored in the bonds of a substance.
Reactions involve breaking of bonds in the reactants, and forming bonds in the
products.
Breaking bonds requires energy.
Forming bonds releases energy.
Different bonds require (or release) different amounts of energy.
During a chemical reaction, energy is either released or absorbed.
Exothermic reactions
Releases energy into surroundings (often as heat).
The energy released by forming the bonds of the product was greater than the
energy needed to break the bonds of the reactants.
Example: combustion
In any reaction, the chemical energy reaches a peak before the reactants change
into products. This is the energy required to start a reaction. (Ex. You must
provide energy (a spark or a small flame) to make propane start combusting.)
Endothermic reactions
Absorbs energy from its surroundings.
More energy is needed to break the bonds of the reactants than is released by
forming the bonds of the products.
This is how chemical cold packs work. They feel cold because they are
absorbing any heat surrounding them to make the reaction go.
Conservation of Energy
The total amount of energy before and after the reaction is the same.
Any difference in the amount of chemical energy stored in the bonds is equal to
the energy lost or gained by the surroundings.
7.4 Reaction Rates - tell you how fast a reaction is going. (How fast reactants are
changing into products.)
Factors affecting reaction rates
A Temperature increase will increase reaction rate because the particles move
faster (so the reactants collide and react with each other more often).
An increase in Surface Area increases reaction rate because it increases the
exposure of reactants to each other.
Stirring increases reaction rate because it increases the exposure of the
reactants to each other.
Increasing the Concentration (the number of particles in a given volume)
increases reaction rate because there are more opportunities for collisions.
(Increasing concentration for a gas is the same as increasing pressure.)
A Catalyst is a substance that affects the reaction rate without being used up in
the reaction.
 A catalyst speeds up reaction rates, or allows reaction to occur at a lower
temperature.
 A catalyst lowers the amount of energy required to get a reaction started.
 Since it is not used up in the reaction, it is written above the arrow.
7.5 Equilibrium – The state at which the forward and reverse paths of a change take
place at the same rate (incoming equals outgoing). It applies to both physical and
chemical changes.
Physical – When a physical change does not go to completion, a physical equilibrium is
reached. ex. When water is left in a closed container, it will eventually reach equilibrium
with its water vapor – water turns to vapor at the same rate vapor turns to water.
Chemical Equilibrium
Most chemical reactions do not go to completion (all reactant turned into
product).
They are reversible to some extent (products are converted to reactants).
Equilibrium is reached when the forward and reverse reactions are happening at
the same rate.
It is shown by double arrows. (two arrows stacked, one pointing each direction)
(I cannot type this, so I will use ↔ for typed notes.)
Factors affecting chemical equilibrium
Le Châtelier’s Principle – A system changes in the direction which
will relieve the change. The reason a combustion reaction goes to completion is
that the reactants leave (since they are gasses), so the reaction tries to reach
equilibrium by making more product. CH4 + 2O2 → CO2 + 2H2O
On a test like the OGT, if a question asks why a reaction does something in
response to a change, “Le Châtelier’s Principle” is a very good guess.
N2(g) + 3H2(g) ↔ 2NH3(g) + Heat
Temperature
 If a reaction is exothermic (heat is a product), higher temperature (more
heat) will favor the reverse reaction.
 If a reaction in endothermic (heat is a reactant), higher temperature will
favor the forward reaction.
Pressure
 If pressure is increased, it will shift the reaction in the direction that will
decrease the pressure (which is the side with fewer gas molecules).
Concentration
 A change in concentration of the products or reactants will shift the
reaction in the direction to relieve the increase or decrease in concentration.
 Removing the ammonia (thereby lowering its concentration) causes the
reaction to favor the forward reaction in order to produce more ammonia.