chapter 13
TRANSCRIPT
Chapter 13
Solutions
What is the solution?
A solution is just a Homogeneous Mixture!
One material (solute) is completely dissolved in another (solvent)
How do Solutions Work?
Compounds are either polar or non-polar.
A polar solvent only will dissolve a polar solute.
A non-polar solvent will only dissolve a non-polar solute
“Like dissolves like”
Why “Like dissolves like”?
Polar compounds will hang tightly to their own kind (like magnets).
They will not surround (as a solvent) or allow themselves to be surrounded (as a solute) by non-polar compounds
Concentrate on the Solution
Concentration is a measure of how much solute is present in the solution.– Qualitative: Described without
numbers– Quantitative: Described WITH
numbers.
Qualitative Concentrations
Unsaturated: The solvent can still dissolve more solute.
Saturated: The solution contains the maximum amount of solute.
Supersaturated: The solution has dissolved more than the normal maximum amount of solute– VERY unstable!
Chapter 13
Separating Mixtures
Separating Mixtures
Mixtures are separated based on their physical properties.
Mixtures may be separated by decanting, pouring off the liquid, with a centrifuge, filtering, or evaporation
Mixtures
If the boiling points of the components are different, distillation can separate them based on their boiling points.
As one component reaches its boiling point, it evaporates from the mixture and is allowed to cool and condense. This is called a distillate.
– This process continues until all the desired components have been separated from the mixture.
Chapter 13
Concentration and Molarity
Concentration
In a solution, the solute is distributed evenly throughout the solvent. This means that any part of a solution has the same ratio of solute to solvent as any other part of the solution.
– This ratio is the concentration of the solution.
– The concentration is the amount of a particular substance in a given quantity of a solution
Chapter 13
Physical Properties of Solutions
Electrical Conductivity
Some substances conduct electricity and some cannot.
The conductivity of a substance is described as its ability to conduct an electric current.
The conductivity of a substance depends on whether it contains charged particles, and these particles must be able to move.
Electrical Conductivity
Electrons move freely within a metal, thus allowing it to conduct electricity.
An aqueous solution of ionic compounds such as NaCl contains charged ions, which can move about. Solutions of ionic compounds conduct electricity.
Pure water does not conduct electricity.
Electrical Conductivity
An electrolyte is a substance that dissolves in a liquid solvent and provides ions that conduct electricity.
Strong electrolytes completely dissociate into ions and conduct electricity well.
Weak electrolytes provide few ions in solution.
Covalent compounds may be strong electrolytes, weak electrolytes, or nonconductors.
Electrical Conductivity
The extent to which electrolytes dissociate into ions is indicated by the conductivity of their solutions.
The sugar sucrose does not ionize at all in solution. – It is a nonelectrolyte and does not conduct electricity.
A nonelectrolyte is a liquid or solid substance that does not allow the flow of an electric current, either in solution or in its pure state, such as water or sucrose.
Colligative Properties
The physical properties of water are changed when substances dissolve in it.
Salt can be added to icy sidewalks to melt the ice. – The salt actually lowers the freezing point of
water. – Ice is able to melt at a lower temperature than it
normally would.
This change is called freezing-point depression.
Colligative Properties
Nonvolatile solutes such as salt also increase the boiling point of a solvent.
This change is called boiling-point elevation. – For example, glycol in a car’s radiator
increases the boiling point of water in the radiator, which prevents overheating.
– It also lowers the freezing point, preventing freezing in cold weather.
Colligative Properties
Any physical effect of the solute on the solvent is a colligative property.
A colligative property is a property of a substance or system that is determined by the number of particles present in the system but independent of the properties of the particles themselves.
Colligative Properties
Any solute, whether an electrolyte or a nonelectrolyte, contributes to the colligative properties of the solvent.
The degree of the effect depends on the concentration of solute particles (either molecules or ions) in a certain mass of solvent.
– The greater the particle concentration is, the greater the boiling-point elevation or the freezing-point depression is.
Chapter 14
Reversible Reactions and Equilibriums
Complete Reactions
If enough oxygen gas is provided for the following reaction, almost all of the sulfur will react:
S8 + 8O2 → 8SO2
Reactions such as this one, in which almost all of the reactants react, are called completion reactions.
In other reactions, called reversible reactions, the products can re-form reactants.
Reversible Reactions
2 2-4 4Ca ( ) SO ( ) CaSO ( ) aq aq s
Solid calcium sulfate, the product, can break down to make calcium ions and
sulfate ions in a reaction that is the reverse of the previous one.
Reversible Reactions
The reactions occur at the same rate after the initial mixing of CaCl2 and Na2SO4.
The amounts of the products and reactants do not change.
Chemical equilibrium is a state of balance. the rate of a forward reaction equals the rate of the reverse reaction.
Equilibrium
In equilibrium, an atom may change from being part of the products to part of the reactants many times.
But the overall concentrations of products and reactants stay the same.
For chemical equilibrium to be maintained, the rates of the forward and reverse reactions must be equal.
Chapter 14
Systems and Stress
STRESS
Le Chatelier’s Principle
Stress is another word for something that causes a change in a system at equilibrium.
Chemical equilibrium can be disturbed by a stress, but the system soon reaches a new equilibrium.
Le Châtelier’s principle states that when a system at equilibrium is disturbed, the system adjusts in a way to reduce the change.
Le Chatelier’s Principle
Chemical equilibria respond to three kinds of stress: – changes in the concentrations of reactants or
products– changes in temperature– changes in pressure
When a stress is first applied to a system, equilibrium is disturbed and the rates of the forward and backward reactions are no longer equal.
Le Chatelier’s Principle
The system responds to the stress by forming more products or by forming more reactants.
A new chemical equilibrium is reached when enough reactants or products form.
At this point, the rates of the forward and backward reactions are equal again.
Concentrate
Increase the amount of reactants, the equilibrium will shift towards products.
Increase the amount of products, the equilibrium will shift towards the reactants.
Getting Hot!
If the forward reaction is exothermic, cooling the system will force the reaction forward.
If the forward reaction is endothermic, heating the system will force the reaction forward.
Feel the Pressure
Increasing pressure will shift the equilibrium to the side with less atoms.
Practical Le Chatelier
The chemical industry makes use of Le Châtelier’s principle in the synthesis of ammonia by the Haber Process.
High pressure is used to drive the following equilibrium to the right.
2 2 3N ( ) 3H ( ) 2NH ( ) g g g
Chapter 15
Acids and Bases
Acids
These substances can recognized as acidic by their tart, sour, or sharp taste.
These substances contain dissolved compounds that chemists describe as acids.
Many other acids, such as sulfuric acid or hydrochloric acid, are highly caustic and should not be put to the taste test.
Acids
Acids are electrolytes, so their solutions in water are conductors of electric current.
Like other electrolytes, hydrogen chloride dissociates to produce ions.
HCl(g) + H2O(l) H3O+(aq) + Cl−(aq)
The hydronium ion, H3O+, is able to transfer charge through aqueous solutions much faster than other ions do.
Acids
Another property shared by aqueous solutions of acids is that they react with many metals.
All metals that are above hydrogen in the activity series react with acids to produce hydrogen gas.
2H3O+(aq) + Zn(s) 2H2O(l) + H2(g) + Zn2+(aq)
Acids
Some electrolytes are strong and others are weak, depending on whether they dissociate completely or partially.
When a weak acid is dissolved in water, only a small fraction of its molecules are ionized at any given time.
Bases
Bases are another class of electrolytes. Unlike acids, which are usually liquids or gases, many common bases are solids.
Solutions of bases are slippery to the touch, but touching bases is an unsafe way to identify them. – The slippery feel comes about because
bases react with oils in your skin, converting them into soaps.
Bases
Some bases, such as magnesium hydroxide, Mg(OH)2, are almost insoluble in water.
Other bases, such as potassium hydroxide, are so soluble that they will absorb water vapor from the air and dissolve in the water.
A base that is very soluble in water is called an alkali, a term that describes the Group 1 metals of the periodic table.
Bases
The alkali metals react with water to form hydroxides that are water-soluble alkalis. These are called basic or Alkaline.
Just as acids may be strong or weak depending on whether they ionize completely or reach an equilibrium between ionized and un-ionized forms, bases are also classified as strong or weak.
Bases
Both strong and weak bases generate hydroxide ions when they dissolve in water.
Many oxides, carbonates, and phosphates are bases, too.
Classifications
Arrhenius acid – Produces Hydronium ions
Arrhenius Base – Produces Hydroxide ions
Classification
Brønsted-Lowry acids – Donate Protons
Brønsted-Lowry acids – Accept Protons
Conjugate Acids and Bases
Look again at the equation for the reversible reaction of ammonia, NH3, with water:
-3 2 4NH ( ) H O ) NH ( ) OH ( ) aq (l aq aq
base acid c. acid c.
base
Water donates a proton to ammonia, so it is an acid.
Ammonia accepts the proton, so it is a base.
Amphoteric
Some species are both an acid and a base and can both donate and accept protons.
Such species are described as amphoteric.
Amphoteric describes a substance, such as water, that has the properties of an acid and the properties of a base.
Self-Ionization
Water is both an acid and a base. It both gives and receives protons.
-2 2 3H O( ) H O( ) H O ( ) OH ( ) l l aq aq
[H3O+] = [OH−] = 1.00 × 10−7 M
Self-Ionization
An equilibrium-constant expression relates the concentrations of species involved in an equilibrium.
The relationship for the water equilibrium is simply
[H3O+][OH−] = Kw
The value of Kw can be found from the known concentrations of the hydronium and hydroxide ions in pure water.
Kw = (1.00 × 10−7)(1.00 × 10−7) = 1.00 × 10−14
The product of these two ion concentrations is always a constant.
Chapter 15
pH, or more math!
Meaning of pH
When acidity and basicity are exactly balanced such that he numbers of H3O+ and OH− ions are equal, we say that the solution is neutral.
Pure water is neutral because it contains equal amounts of the two ions.
Meaning of pH
A solution made by dissolving 0.100 mol of NaOH in 1.00 L of water has a hydroxide ion concentrationof 0.100 M.
The hydronium ion concentration can be calculated using Kw.
+ - 14
+ 1333 -
[H O ][OH ] 1.00 10[H O ] 1.00 10
[OH ] 0.100
Meaning of pH
In 1909, Danish chemist Søren Sørensen proposed using the negative logarithm of [H3O+] as the index of basicity and acidity.
He called this measure the pH or power of Hydrogen.
pH can be calculated by the following mathematical equation:
pH = −log [H3O+] OR [H3O+] = 10−pH
The Meaning of pH, continuedCalculating pH from [H3O+], continued
• The pH equation may be rearranged to calculate the hydronium ion concentration from the pH.
[H3O+] = 10−pH
• Because pH is related to powers of 10, a change in one pH unit corresponds to a tenfold change in the concentrations of the hydroxide and hydronium ions.
Section 2 Acidity, Basicity, and pHChapter 15
