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Reaction RatesEquilibrium Water, Solutions, and Colloids

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Title: Reaction RatesEquilibrium Water, Solutions, and Colloids

1
Reaction Rates/EquilibriumWater, Solutions, and
Colloids
2
Reaction Mechanisms and Rates

Reactions do not all take place at the same
speed, and the reasons for the rate of each
reaction can be deduced from the chemicals
involved and the theory of how reactions take
place
Collision model of reactionsreactions take place
as atoms and/or molecules collidein order for a
reaction to occur there must be
1. Sufficient energy to break bonds
2. Correct orientation of molecules to each
other

3
Activation Energy, Eact

For a reaction to begin
Colliding particles must overcome repulsions
Colliding particles must form activated
complex
Energy diagrams

4
Energy Yield From Reactions

Endothermicenergy is required to make the
reactants change to products, the energy of the
products is higher than the reactants
Exothermicenergy is released in making the
reactants change to products, the energy of the
products is lower than the reactants
Net heat of reaction, ? Hthe change in energy
content between reactants and products, if
endothermic, - if exothermic

5
Factors Affecting Rate of Reaction

1. Nature of reactantswhat the reactants are and
how stable their bonds are
2NO O2 ? 2NO2 very fast at 25?
2CO O2 ? 2CO2 very slow at 25?
2. Concentration of reactantsthe more particles
there are, the more often they can collide
3. Surface area of solidsthe more molecules
available at the surface, the more collisions
which can occur

6
Factors Affecting Rate of Reaction

4. Temperature of the reactionhigher temperature
means higher speed of molecules thus more
energetic collisions
5. Presence of catalystschemicals which help a
reaction go faster, but which are not affected in
the course of the reaction
a. In living systems, enzymes do this job
b. Enzymes are specific, catalysts much less so

7
Homework-8a

CYU p. 191 All
p. 204ff 4, 5, 7, 11

8
Chemical Equilibrium

Reactions not totally one way, balance between
reactants and productsdynamic
The state where the concentrations of all
reactants and products remain constant with time.
On the molecular level, there is frantic
activity. Equilibrium is not static, but is a
highly dynamic situation.

9
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10
The Law of Mass Action

For
jA kB ? lC mD
The law of mass action is represented by the
equilibrium expression

11
Equilibrium Expression

4NH3(g) 7O2(g) ? 4NO2(g) 6H2O(g)

12
Le Châteliers Principle

. . . if a change is imposed on a system at
equilibrium, the position of the equilibrium will
shift in a direction that tends to reduce that
change.

13
Changing Equilibrium

Concentration
a. increase pushes equilibrium away from that
increased
b. decrease pushes reaction toward that
decreased
Temperature
a. increase favors endothermic direction of
equilibrium
b. decrease favors exothermic direction
Catalysts
Dont affect equilibrium, just the rate at which
it is established

14
Effects of Changes on the System

1. Concentration The system will shift away
from the added component.
2. Temperature K will change depending upon the
temperature (treat the energy change as a
reactant).

15
Effects of Changes on the System (continued)

3. Pressure
a. Addition of inert gas does not affect the
equilibrium position.
b. Decreasing the volume shifts the
equilibrium toward the side with fewer moles.

16
Example

Consider the reaction
2 NO2 (g) ?? N2O4 (g) 13.6 kcal
What effect would each of these changes have on
the equilibrium (final) concentration of NO2 ?
Removing N2O4 from the container
Putting the cylinder in an ice-water bath
Putting the cylinder in a boiling water bath
Decreasing the volume of the cylinder

17
Homework 8b

p.204 CYU all
p. 205ff 13, 14, 16, 18, 20, 22, 29, 30, 31

18
Water and its properties

Good solvent for ionic and polar substances
Melting and boiling point high for its MM
Solid density less than liquid state
Surface tension very high
High heats of fusion and vaporization
High specific heat
Hydrogen bonding in water explains most of these

19
Suspensions

Suspensions- mixtures which have particles too
large to be supported by the solvent and which
settle out of the mixtureriver water

20
Colloids

Colloids- mixtures where the particles are small
enough to be supported by the solvent, but which
are not truly dissolved
Classes of colloids-depend on what phase
dispersing medium and dispersed medium aresee
table 9.3
Properties i. Brownian movement the continual
motion of colloid particles due to bombardment by
solvent particles
ii. Tyndall effectthe scattering of light so
as to see a beam of light passing through a
transparent colloidal suspension

21
Solutions

Solutethe component which is dissolved
Solventthe component which dissolves the solute
Process of solvation i. In water, water
molecules pull substance particles into
solution ii. water molecules surround substance
particles (hydration)

22
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23
A Solute

dissolves in water (or other solvent)
changes phase (if different from the solvent)
is present in lesser amount (if the same phase as
the solvent)

24
A Solvent

retains its phase (if different from the solute)
is present in greater amount (if the same phase
as the solute)

25
Electrolytes

Strong - conduct current efficiently
NaCl, HNO3
Weak - conduct only a small current
vinegar, tap water
Non - no current flows
pure water, sugar solution

26
Dissolving of Electrolytes

Fe2Fe(CN)63 ---gt 2 Fe3 3 Fe(CN)6-2
H2SO4 --gt 2H SO4-2
BaCl2 ---gt Ba2 2 Cl-
All strong electrolytes break up completely into
their respective ions.

27
Power Source
04_43

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-

-
-

-
-

(a)
(b)
(c)
28
Solubility

The extent to which a solute will dissolve in a
solvent
Factors which affect solubility
Naturelike dissolves likepolar vs nonpolar
Temperature
Solids usually more in warm
Gases more in cold
PressureHenrys Law from gases
Ionic substances solubility Table page 222

29
Simple Rules for Solubility

1. Most nitrate (NO3?) salts are soluble.
2. Most alkali (group 1A) salts and NH4 are
soluble.
3. Most Cl?, Br?, and I? salts are soluble (NOT
Ag, Pb2, Hg22)
4. Most sulfate salts are soluble (NOT BaSO4,
PbSO4, HgSO4, CaSO4, Ag2SO4)
5. Most OH? salts are only slightly soluble
(NaOH, KOH are soluble, Ba(OH)2, Ca(OH)2 are
marginally soluble)
6. Most S2?, CO32?, CrO42?, PO43? salts are only
slightly soluble.

30
Net Ionic Equations

In reactions between soluble ionic substances,
not all of the ions get involved in the reaction.
Those which do not change between reactant and
product sides of the reaction are called
spectator ions and should be eliminated from the
equation. An equation where only active ions are
shown is called a net ionic equation

31
Examples
sodium phosphate calcium chloride?
potassium chloride lead(II) nitrate?
lithium carbonate strontium iodide?
32
Homework-9