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Equilibrium
Chapter 17
MRE’s
•The idea behind a flameless heater is to use the
oxidation of a metal to generate heat. Magnesium
metal works better than iron because it rusts much
more quickly.
• To make a flameless heater, magnesium dust is
mixed with salt and a little iron dust in a thin, flexible
pad about the size of a playing card.
• To activate the heater, a soldier adds a little water.
Within seconds the flameless heater reaches the
boiling point and is bubbling and steaming.
•To heat the meal, the soldier simply inserts the
heater and the MRE pouch back in the box that the
pouch came in.
•Ten minutes later, dinner is served!
How long does a reaction take?
The amount of time it takes for a chemical
reaction to come to completion can vary
depending on the reaction
Strike a match and it erupts
instantaneously.
Coal made from dead plants takes
millions of years
Rates
Rates measure the speed of any change
that occurs within an interval of time.
In chemistry, rates of chemical change
usually are expressed as the amount of
reactant changing per unit time.
Different Reactions
Which occurs
fastest? Which
reaction occurs
slowest?
Collision Theory
Rates of chemical reactions are related to
the properties of atoms, ions, and
molecules through a model called collision
theory.
According to collision theory, atoms, ions,
and molecules can react to form products
when they collide, provided they have
enough kinetic energy.
Think clay
If you throw 2 clay balls together gently,
they do not stick together, kinda like the
lack of reaction between colliding particles
with low energy.
If you throw the same balls of clay
together with great force, they stick
together.
Activation Energy
Activation energy is a barrier that
reactants must cross to be converted to
products.
Activated Complex
An activated complex is the arrangement
of atoms at the peak of the activationenergy barrier
Also called transition
state.
Factors Affecting Reaction Rates
Temperature
Concentration
Particle Size
Catalysts
Pressure
Temperature
Usually, raising the
temperature speeds up
the reactions, while
lowering the temperature
slows down the reactions.
This will increase the
number of particles that
have enough kinetic
energy when they collide.
Concentration
Cramming more particles into a fixed
volume increases the concentration of
reactants.
Particle Size
The smaller the particle size, the larger the
surface area for a given mass of particles.
Which burns faster, a piece of wood or a
kindle? Why?
Catalyst
A catalyst is a substance that increases
the rate of a reaction without being used
itself.
Catalyst permit reactions to proceed at a
lower energy than required.
Reversible Reaction
Do reactions only go one way or do
reactions go 2 ways?
Let’s look at this equation:
– 2SO2(g) + O2-------------- 2SO3
What happens when sulfur dioxide and
oxygen gas are mixed in a sealed
container?
Reversible Reactions
As the SO3 concentration increases, a small
amount slowly reverts to SO2 and oxygen by the
reverse direction!
As the concentration of SO3 becomes higher and
higher, the reverse reaction speeds up!
Eventually SO3 decomposes to SO2 and O2 as
fast as it forms SO3.
Chemical Equilibrium
Equilibrium occurs when opposing
reactions occur at equal rates.
Reactants and products are not
necessarily equal in amount.
Le Chatelier’s Principle
Le Chatelier’s Principle
– If a stress is applied to a system in dynamic
equilibrium, the system changes to relieve
stress.
Stresses include changes in the
concentration of reactant or products,
changes in temperature, and changes in
pressure