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Solids and Liquids
Kinetic-Molecular Theory of Matter
Gas Solid Liquid
Kinetic-Molecular Theory
According to the Kinetic-Molecular Theory, the state of a substance at room temperature depends on the strength of the attraction between its particles.
Gas – weakLiquid-moderateSolid- strong
Kinetic Theory of Gases
Particles are independent from one another and move in straight lines.
Change of direction occurs only when particles strike each other or container.
Movement is random.
Kinetic Theory of Liquids
Particles much closer together.
Particles do not act independently from one another.
Have a vibratory type of motion.
Movement occurs when particles shift between the spaces of particles.
Kinetic Theory of Liquids
Kinetic Theory of Solids
Particles close together and do not act independently.
Particles occupy a relatively fixed position in relation to surrounding particles.
Vibration around a fixed point.
Properties of Solids
All true solid substances are composed of crystals.
Solids have low kinetic energy.
They have a definite volume and assume their own shape.
High densities and Melting points.
Crystal Structure Crystals are made from
small units called unit cells. These units are repeated
over and over again as the crystal grows.
The arrangement of these unit cells is determined by the bonds between the particles.
Early crystallographers classified crystals on the basis of their external shapes into seven crystal systems.
Crystal SystemsEX: Cubic Hexagonal
Unit cell
Allotropes Allotropes are different chemical states of same element in same physical state.
EX: Allotropes of carbon are diamonds and graphite.
Diamond: pyradmidal lattice held together by strong covalent bonds.
Graphite: layers of hexagonal units held together by weak forces.
Amorphous Substances
Amorphous substances, sometimes referred to as amorphous solids, appear to be in the solid state but have no crystal structure.
Glass is the best known example. Glass does not have a sharply defined
melting point (like other solids). Viscosity (resistance to flow) is used
to describe the texture of glass. High viscosity, high resistance to flow
(thick).
AMORPHOUS CRYSTALLINE SOLID
Properties of Liquids
Liquids occupy a definite volume but they do not have their own shape.
Liquid movement is considered fluid. Bodies of nonmoving liquids are always
flat on top.
Surface Tension and Capillary Action
A liquid molecule is attracted to other liquid molecules. This is called cohesion.
Surface tension is a function of cohesion.
A liquid molecule can also be attracted to other materials. This is called adhesion.
Capillary action is a function of adhesion and cohesion.
Surface Tension
Surface tension is the force at the surface of a liquid due to the cohesive forces of the molecules of liquid for each other.
A paper clip will “float” on water despite having a higher density.
The cohesive forces of the water molecules keep the paper clip from pushing through.
An insect takes a walk on water
Surface Tension
Surface Tension
Surface tension can also be affected by adhesion.
Example: Adhesive forces of the liquid molecules for the walls of the container.
The smaller the container the greater the cohesion.
EX: A belly flop would be more painful in a small pool of water.
Capillary Action
Capillary action is related to the adhesive properties of liquids.
Liquid molecules are attracted to the straw. As they “climb” up the straw, other molecules follow.
Capillary action is limited by gravity and the size of the straw.
The thinner the straw or tube the higher up capillary action will pull the liquid.
Last slide
Capillary Action
Phase Changes
Phase Changes
Phase changes are due to changes in temperature, which affects kinetic energy; or pressure, which establishes how close the molecules of a substance are.
Melting point- temperature of solid liquidFreezing point (crystallization)- temp of
liquid solidCondensation- gas molecules cool and
condense (come close together) to form a liquid
Sublimation- the direct change from a solid to a gas (dry ice, frozen CO2 CO2 gas)
Vaporization- change from a liquid gasEvaporation- vaporization of liquid gas
below the boiling pointBoiling point- temp at which liquid gas
Some Important Vocab
Heating Curve A graph that represents a substance’s
change from solid to gas.
Boiling point
Melting point
Cooling Curve A graph that represents a substance’s
change from a gas to a solid.
Condensation point
Freezing point
Melting Point
Freezing Point
Vaporization Point
Condensation Point
http://www.kentchemistry.com/links/Matter/HeatingCurve.htm
Phase DiagramsPhase diagrams show how the states of matter in a system are affected by changes in temperature and pressure.
This phase diagram shows the relative temperatures and pressures at which each of the three states of matter can exist.
Note the point labeled triple point. This is the point at which
all three states of matter can exist.
Note the point labeled critical point. This is the point at which,
no matter how much pressure is applied, the substance will exist only in the gas state.
Phase Diagrams
Boiling Point
__________
Freezing Point
__________
Triple Point
__________
Critical Point
__________
Phase Diagram for CO2
Dynamic Equilibrium
Equilibrium
Remember, before a chemical reaction begins, you have only reactants (only N2O4 is present).Once the reaction starts, products are then being made (both N2O4.and NO2 are present)
37
N2O4(g) 2NO2(g)
Equilibrium: the extent of a reaction
In stoichiometry we talked about theoretical yields of products, and the many reasons actual yields may be lower.
Another critical reason actual yields may be lower is the reversibility of chemical reactions: some reactions may produce only 70% of the product you may calculate they ought to produce.
MOST REACTIONS DO NOT GO TO COMPLETION AND DO NOT PRODUCE 100% OF THEIR THEORETICAL YEILD
38
Equilibrium
In principle, every chemical reaction is reversible ... capable of moving in the forward or backward direction.
Some reactions are easily reversible ...Some not so easy ...
The double arrow implies the reaction is dynamic.
39
N2O4(g) 2NO2(g)
Equilibrium is the state in a system in which there is no net change in that system.The rate of the forward reaction is equal to the rate of the reverse reaction.
At that point, the concentrations of all species are constant.
Equilibrium
In a state change system, for example, a beaker of water that is closed off will have two phases.
1. Liquid2. Gas
The point at which there is no increase in the gas particles or no increase in the liquid particles is called equilibrium.
H2O(l) H2 (g) + O2(g)
Equilibrium
When a chemical reaction reaches Equilibrium, it DOES NOT mean that there are the same amount of reactants as there are products.
It just means that the concentrations of the substances remain constant. Ex. H2O(l) H2 (g) + O2(g)
Molecules Evaporating = Molecules Condensing
Equilibrium
A System at Equilibrium
Once equilibrium is achieved, the amount of each reactant and product remains constant.
Chemical Equilibrium Analogy
Let us introduce the idea of chemical equilibrium by an analogy, seemingly far-fetched at first sight, but actually mathematically correct.
Imagine that a crabapple tree sits on the dividing line between two homes, one inhabited by a grouchy old man, and the other by a father who has told his son to go out and rid the back yard of crabapples.
Chemical Equilibrium Analogy
The boy quickly realizes that the easiest way to dispose of the crabapples is to throw them into the neighbouring yard. He does so, arousing the ire of the old man.
The boy and the man start throwing crabapples back and forth across the fence as fast as they can.
Who will win?Let’s see…
Chemical Equilibrium Analogy Assuming that the
boy is more energetic and agile than the old man, you might think at first that the conflict would end with all of the apples on the old man's side…
Chemical Equilibrium Analogy
It is true that with equal numbers of crabapples on either side, the boy will throw apples across the fence faster than the old man can return them… But this only means that apples will become more plentiful on the old man's side, and easier for him to reach.
Chemical Equilibrium Analogy
The apples will become scarcer on the boy's side, and require more running around to locate. Eventually a standoff, or equilibrium, will be reached, in which the number of apples crossing the fence is the same in both directions.
The old man will throw less quickly but will have less trouble finding apples; the boy will throw more rapidly but will waste time scurrying around hunting for the relatively few crabapples on his side.
The ratio of apples on the two sides of the fence ultimately will be determined by the relative agility of the two combatants, but all of the apples will not end up on one side.
Le Chatelier’s Principle
If stress is applied to a system at equilibrium, the system will tend to readjust so that the stress is reduced and the reaction goes back to equilibrium.
The stress may be a change in temperature, pressure, concentration of products or reactants, or other external factors.
Equilibrium shifts occur after a stress has been applied.
LeChatelier’s Principle
H2O (l) H2O (s)
If we add heat to this system at equilibrium, what will happen?
We call that an equilibrium shift to the left because we will form more of the substance on the left side of the yield sign.
If we place this system in a freezer, what type of equilibrium shift will occur?
LeChatelier’s Principle
Shifts
Shift to the left to make more reactants. Everything on the left increases, everything on the right decreases.
Shift to the right to make more products. Everything on the right increases and reactants decrease.
Stresses to Equilibrium
Only gases and aqueous substances can induce a stress. Addition or elimination of solids or liquids will not affect
equilibrium. They can however, increase or decrease when other stresses occur.
If a stress is added to a system, the system shifts in the direction to relieve the stress.
HINT… Adding a stress – shift to the other side Subtracting a stress – shirt to same side
Changing Concentrations and the Effect on Equilibrium
N2(g) + H2(g) NH3(g)
1. What shift will occur when the [NH3] is increased?
2. What happens to [N2], and [H2]?3. What shift will occur when some H2 is
taken out of the system?4. What happens to [N2], and [NH3]?5. What shift will occur when some NH3 is
taken out of the system?6. What happens to [N2], [H2], and [NH3]?
If temp or energy (J, kJ, cal, kcal) is a reactant then the reaction is endothermic (requires heat) Treat temp as any reactant.
If temp or energy is a product then the reaction is exothermic (releases heat) Treat as any product.
Changing Temperatureand the Effect on Equilibrium
N2(g) + H2(g) NH3(g) + heat
1. What shift will occur when heat is added to the system?
2. What happens to [N2], [H2], and [NH3]?
3. What shift will occur when some NH3 is taken out of the system?
4. What happens to temperature?5. What shift will occur when heat is taken
away from the system?6. What happens to [N2], [H2], and [NH3]?
Changing Temperatureand the Effect on Equilibrium
Applies to gases only. If increasing pressure, shift will occur
in the direction of the least moles. EX: 2 mol 1 mol = right
shift If decreasing pressure, shift will occur
in the direction of the most moles. EX: 2 mol 1 mol = left
shift If moles are equal then there is no
shift and everything remains the same.
Changing Pressureand the Effect on Equilibrium
N2(g) + 3H2(g) 2NH3(g) + heat
1. What shift will occur when pressure is increased?
2. What happens to [N2], [H2], [NH3], and temperature?
3. What shift will occur when pressure is reduced?
4. What happens to [N2], [H2], [NH3], and temperature?
Changing Pressureand the Effect on Equilibrium
K value
The equilibrium constant (K) is a unitless quantity characterizing a chemical equilibrium in a chemical reaction which is a useful tool to determine the concentration of various reactants or products in a system where chemical equilibrium occurs.
It is only affected by temp changes. Use the general equation: K=
[products]
[reactants]
Equilibrium
What Does the Value of K Mean?
• If K >> 1, the reaction is product-favored; product predominates at equilibrium.
• If K << 1, the reaction is reactant-favored; reactant predominates at equilibrium.
Note that during a right shift the concentration of products increases while the concentration of reactants decrease. Mathematically, this increases the K value. (K>>1)
During a left shift the concentration of the reactants increase while the concentration of the products decrease, decreasing the K value. (K<<1)
K value
N2(g) + H2(g) NH3(g) + heat1. What shift will occur when heat is taken
out of the system?2. What happens to the K value?3. What shift will occur when some NH3 is
taken out of the system?4. What happens to the K value?5. What shift will occur when heat is
added to the system?6. What happens to the K value?
Review: EquilibriumConcentration
•only (g) & (aq) cause a shift•(L) and (s) = NO SHIFT
•add… shift to other side
•subtract… shift to same side
Temperature*add… shift to other side*subtract… shift to same side
• ← K << 1• → K >> 1
*K values change only with temperature shifts
*Endothermic – energy on react side (left)*Exothermic – energy on product side (right)
Pressure• ↑Increase P… shift toward lower moles•↓Decrease P… shift toward greater moles•if moles are equal = NO SHIFT
*only gases are affected by pressure
changes