Hemet High  Chemistry
18  Chemical Equilibrium
Section 18.2 - Shifting Equilibrium
Chapter Notes
Reversible Reactions
 _________________________: one in which the conversion of reactants to products and the
conversion of products to reactants occur simultaneously.
 The ____________ arrow tells you that the reaction is reversible.

2 SO2(g) + O2(g) ⇄ 2 SO3(g)
_________________________: when the rates of the forward and reverse reactions are
equal, the reaction has a reached a state of balance.
 At chemical equilibrium, ____ net change occurs in the ____________ amounts of the components
of the system.
 The relative concentrations of the reactants and products at equilibrium constitute the
_____________________________ of a reaction.
 Think of a 2-story mall with an escalator. You have 100 people on the top floor and 50 people
on the bottom floor. If 10 people want to move to the top floor, how can this happen while still
keeping 100 people on the top and 50 people on the bottom?
Factors Affecting Equilibrium: Le Châtelier’s Principle
 Le Châtelier’s Principle: If a _________ is applied to a system in dynamic equilibrium, the
system changes in a way that _____________ the stress.
There are three factors that we will talk about that can affect equilibrium.
CONCENTRATION
 If you add more of a _____________, the reaction goes toward the ________________.
 If you take away some of a _____________, the reaction goes toward the _______________.
 If you add more of a _____________, the reaction goes toward the ___________________.
 If you take away some of a ______________, the reaction goes toward the ________________.
TEMPERATURE
 If heat is a ____________ and you add heat, the reaction shifts toward the ______________.
 If heat is a ____________and you add heat, the reaction shifts toward the ______________.
 If heat is a ____________and you take away heat, the reaction shifts toward the _____________.
 If heat is a ____________and you take away heat, the reaction shifts toward the ____________.
PRESSURE – ONLY GASES!!
 If you ______________pressure, the reaction shifts towards the side with _____ moles.
 If you ______________pressure, the reaction shifts towards the side with _____ moles.
VOLUME – ONLY GASES!!
 If you _____________ volume, it is the same as _______________ pressure, so the reaction
shifts towards the side with ______ moles.

If you _____________ volume, it is the same as _______________ pressure, so the reaction
shifts towards the side with ______ moles.

Because it is the same as changing pressure, we ____________ consider this to be a 4th factor.
CATALYSTS
 Adding a catalyst has no effect on the equilibrium, it will only help to reach equilibrium faster.
Reactions to Completion
 A reaction is considered to “go to completion”, when almost all of the _____ are removed from the
solution.
 This depends on the ___________ of the product formed, and if it is ___________, then on its
degree of ________________ .
Formation of a Gas
 Gases are not very _____________, so when a gas is formed and the reaction container is ______
to the air, the gas will escape and the reaction will go __________ to completion.
Formation of a Precipitate
 If a product is ____________ (a precipitate), then when the product forms, it __________ dissolve
to allow the reaction to go in the reverse direction.
Formation of a Slightly Ionized Product
 This occurs with the neutralization reactions of _________ and ____________.
 HCl(aq) + NaOH(aq)
 Water only slightly ionizes, so it exists as mainly _________ molecules.
Section 18.1 - The Nature of Chemical Equilibrium
Equilibrium Expressions
 ______________________: Keq is the ratio of product concentrations to reactant
concentrations at equilibrium.
Keq = [C]x [D]y
[A]n [B]m
nA + mB ⇄ xC + yD
*Remember: it is
products over reactants!
Write Equilibrium Expressions:

H2 + I2 ↔ 2HI

2HgO ↔ 2Hg + O2

2SO2 + O2 ↔ 2SO3

N2 + 3H2 ↔ 2NH3

Equilibrium Constants
To find an Equilibrium Constant (Keq), plug in the __________________ of the reactants and
products into the equilibrium expression and solve!
 Keq > 1, _____________ favored at equilibrium
 Keq < 1, _____________ favored at equilibrium
 Keq does not have any units!

Calculating Keq
A liter of a gas mixture at equilibrium at 10°C contains 0.0045 mol of N2O4 and 0.030 mol of NO2.
Write the expression for the equilibrium constant and calculate Keq.
N2O4(g) ⇄ 2NO2(g)
 An equilibrium mixture of N2, O2, and NO gases is determined to consist of 6.4 mol/L of N2,
1.7 mol/L of O2, and 1.1 mol/L of NO. What is the Keq for this system?
N2 + O2 ⇄ 2NO
Hemet High  Chemistry
21  Nuclear Chemistry
Chapter Notes
Section 21.1 – Radioactivity
•
Wilhelm Roentgen made a big discovery in 1895. He found that invisible rays were emitted when
electrons bombarded materials. He named these rays,
•
.
At the same time, Henri Becquerel was studying minerals that emitted light after being exposed to
sunlight, a phenomenon called phosphorescence.
•
and her husband Pierre were working with Becquerel and took his mineral
sample and were able to isolate the components emitting the rays.
•
Marie named the process by which materials give off such rays
•
•
.
: the penetrating rays and particles emitted by a radioactive source.
Marie Curie was the first
to win the Nobel Prize and is the only person to
receive Nobel Prizes in two different sciences--physics and chemistry!
Nuclear Reactions vs. Normal Chemical Changes
•
Marie Curie discovered that:
•
Nuclear Reactions involve the
, which changes the type of element.
•
Chemical reactions involve
, not protons and neutrons.
•
When a substance emits
•
A
•
Isotopes are atoms of the same element that have
•
, it changes its identity.
element has an unstable nucleus.
: isotopes of atoms with unstable nuclei
Nuclear Stability and Decay
•
: the attractive force that acts between all nuclear particles that are
extremely close together, such as neutrons and protons in a nucleus.
•
: the location of stable nuclei on a neutron vs. proton plot.
Types of Radiation
•
The three types of nuclear radiation are
radiation,
radiation, and
radiation.
•
There is also
and they can be separated by an electric field.
Alpha Decay Radiation
•
: a positively charged helium isotope.
•
Written with the following symbol:
•
It contains 2 protons and 2 neutrons and has a 2+ charge
Beta Decay Radiation or Electron Capture
•
: a very fast moving electron, resulting from the breaking apart of a
neutron.
•
Written with the following symbols:
Positron Emission Radiation


: a particle with the same mass as an electron, but has a positive charge.
Emitted from the nucleus during some kinds of radioactive decay. Symbol:
Gamma Emission Radiation
•
•
: high energy electromagnetic radiation.
The emission of gamma rays
the atomic number or mass number of a
nucleus.
•
Written with the following symbol:
EXAMPLES:
Write the nuclear reaction for the following:
Alpha Decay – Symbol:
Positron Emission – Symbol:
1.
1.
2.
2.
3.
3.
Beta Decay – Symbol:
Gamma Emission
1.
1.
2.
2.
3.
3.
Electron Capture – Symbol:
1.
2.
3.
Section 21.2 - Penetrating Abilities and Half-Life
Penetrating Abilities
•
: stopped by piece of paper.
•
: stopped by thin metal
•
: stopped by thick lead and concrete.
Half-Life
•
: the time required for one-half of the nuclei of a radioisotope sample to decay to
products.
•
After each half-life,
of the existing radioactive atoms have decayed into atoms of a
element.
Half-Life Calculation
•
Carbon-14 emits beta radiation and decays with a half-life of 5730 years. Assume you start with
a mass of 2040 g of Carbon-14. How long is three half-lives and how many grams of the isotope
remain at the end of three half-lives?

The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would
be left after 9.6 minutes has elapsed?

Os-182 has a half-life of 21.5 hours. How many grams of a 10.0 gram sample would have
decayed after exactly two half-lives?