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Solutions

Chemistry

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Solutions Objectives

1. Describe the characteristics of solutions.

2. Relate intermolecular forces and process of solvation.

3. Define solubility and identify factors affecting it.

4. State the concentrations of solutions in different ways.

5. Calculate the concentration of solutions.

6. Describe four colligative properties of solutions.

7. Identify the properties of suspensions and colloids.

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What are solutions?

solution: homogeneous mixture of two or more substances in a single phase

-has the same composition and properties throughout

What makes up a solution?

1. solute: substance being dissolved

-usually the lesser quantity

-particles are so small they cannot be seen

-remains mixed indefinitely as long as the conditions

do not change

2. solvent: dissolving medium; what does the

dissolving

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There are different types of solutions:

1. gas

-gas dissolved in another gas (air: O2, CO2, H2O, and other gases in N2)

2. liquid

-gas in liquid (carbonated water: CO2 in water)

-liquid in liquid (vinegar: acetic acid in water)

-solid in liquid (sugar water, salt water)

3. solid

-liquid in solid (dental filling-solid Ag in liquid Hg)

-solid in solid (steel-solid Fe and solid C)

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Not all substances form solutions:

1. soluble: when a solute dissolves in a solvent

-ex: sugar in water

2. insoluble: when a substance does not dissolve in a

solvent

-ex: oil in water

When two liquids do not mix, they are said to be immiscible (oil in water, oil in vinegar) and the less dense liquid floats on top of the more dense one.

-if two liquids do mix, they are said to be miscible (vinegar in water)

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Is there a way to tell if a solute will dissolve in a solvent?

Yes, remember:

“Like dissolves like”

A polar solute will dissolve in a polar solvent and a nonpolar solute will dissolve in a nonpolar solvent.

To determine polarity, you must look at bonding:

1. Most ionic compounds are polar because of difference in electronegativity and the formation of ions in solution

2. In covalent compounds you have to look at the type of bond formed and the shape of the molecule.

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Polarity of covalent compounds:

1. intermolecular forces (van der Waals forces): relatively weak attractive force between individual molecules.

-weaker than ionic bonding

-several types of IF

a. dispersion force: attraction between nonpolar

molecules

b. dipole-dipole force: attraction between polar

molecules

-strength depends on electronegativity

c. hydrogen bond: forms between a hydrogen end of a

polar molecule and a F, O, or N from another dipole

-strong force

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2. shape of the molecule -asymmetrical molecules, such as water (bent), are polar -symmetrical molecules, such as CCl4 (tetrahedral with the same terminal atoms) are nonpolar

Factors that affect the rate of dissolving:1. increasing surface area -dissolving process occurs at the surface of the solute2. agitating a solution -solute particles are dispersed more quickly3. heating a solvent -increases the kinetic energy of the solvent, making more contact between the solvent/solute particles

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Energy is required to overcome the attractive forces within the solute and solvent molecules (endothermic); when the solute and solvent particles mix, energy is released (exothermic)

heat of solution: overall energy change that occurs when a specific amount of solute dissolves in a solvent.

-negative (exothermic)

-positive (endothermic)

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Solubility

For a given amount of solvent at a given temperature, there is only a certain amount of solute dissolves.

1. saturated solution: solution that contains the maximum amount of dissolved solute, for the given conditions

-once saturated, the solute falls to the bottom2. unsaturated solution: solution that contains less

solute than a saturated solution at existing conditions3. supersaturated solution: solution that contains more

dissolved solute than a saturated solution -produced by heating the solution -as it cools, crystals begin to form, which continues until equilibrium is reestablished at the lower temp

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solubility: amount of a given substance required to form a saturated solution with a specific amount of solvent at a specified temperature.

-varies with temperature (as in supersaturation) and

pressure

Effect of Temperature

Many, but not all, substances are more soluble at high temperatures than lower temperatures, as shown by Fig 15-7.

-depends if the solute is a gas or solid

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1. gas solutes

-increasing temperature decreases solubility

-at a higher temperature, gas molecules want to

escape from a solution

2. solids

-most solids have an increased solubility with

increasing temp

-the amount of increased/decreased solubility changes

from one solid to the next

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Effect of PressurePressure does not affect liquid and solid solutes.Pressure does affect gas solutes. -increasing the pressure puts stress on the equilibrium of the solution, allowing more gas to dissolve

Henry’s Law: solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the surface of the liquid

-example: coca-cola

~during bottling, CO2 is forced into the flavored water and held there by an enclosed system (can/bottle) ~when the cap is removed, the gas escapes from the liquid, which is known as effervescence

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Interesting Tidbits

Other examples of how pressure affects gases dissolving is in scuba diving:

1. When scuba divers remain underwater too long, nitrogen dissolves in the blood, leading to nitrogen narcosis, which causes sleepiness and poor judgment, as well as oxygen toxicity, which occurs when the oxygen concentration becomes toxic

-to prevent this, divers use a mixture of helium with less than 20% oxygen2. When a diver gets ‘the bends’, the increased amount

of dissolved gases in the blood is released too quickly while flowing through blood vessels and blood causing extreme pain, paralysis or death

-prevented by the diver coming up slowly

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Solubility Review

1. Describe the two components that make up a solution.2. Give all possible combinations of solutions (ex: gas/gas)3. What does immiscible mean?4. Will all substances dissolve in any solvent? Explain5. How do we determine polarity?6. What 2 things does the polarity of covalent compounds

depend on? 7. Describe all types of intermolecular forces.8. Describe the three things that affect rate of dissolving.9. What is the difference between a saturated, unsaturated,

and supersaturated solution?10. What is the effect of temperature and pressure on solubility?

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Solution Concentration

concentration: measure of the amount of solute in a given amount of solvent or solution

There are several ways of expressing concentrations, but we will discuss only two: molarity (most used) and molality.

molarity (M): number of moles of solute in 1 liter of solution

-you must know the molar mass of the solute

-ALWAYS add only the amount of solvent to make 1.00 L of solution AFTER the solute is dissolved

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Formula:

molarity (M) = amount of solute (mol)

amount of solution (L)

= 0.500 mol NaOH

1.00 L solution

= 0.500 M NaOH, or

0.5 molar solution of NaOH

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Example 1: You have 3.50 L of solution that contains 90.0 g of NaCl. What is the molarity of the solution?

-First, you calculate the number of moles of NaCl.

90.0 g NaCl x 1 mol of NaCl = 1.54 mol NaCl 58.44 g NaCl

-Next, you calculate the molarity. M = 1.54 mol NaCl = 0.440 mol/L 3.50 L solution = 0.440 M NaCl

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Example 2: You have 0.8 L of a 0.5 M HCl solution. How many moles of HCl does this solution contain

-First, setup the general equation to solve for # moles.

M = # moles solute

volume solution

# mol solute = (concentration)(volume solution)

-Second, plug in the given values.

# moles HCl = 0.5 mol HCl x 0.8 L soln

1 L soln

= 0.4 mol HCl

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Example 3: How many grams of LiOH would need to be dissolved in water to make 1.0 L of 2.0 M solution?

-First, calculate the number of moles of LiOH needed.

M = # moles solute

volume soln

# moles of solute = (concentration)(volume soln)

# moles LiOH = 2.0 mol LiOH x 1.0 L soln

1 L

=2.0 mol LiOH

-Second, convert the moles of LiOH into grams.

2.0 mol LiOH x 23.95 g LiOH = 47.9 g LiOH

1 mol LiOH

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molality (m): concentration of a solution expressed in moles of solute per kilogram of solvent.

-since mass of solvent is expressed in grams, the mass of solvent must be converted to kilograms using the conversion:

1000g = 1 kg

Molality (m) = amount of solute A (mol)

mass of solvent (kg)

= 0.500 mol NaOH

1 kg H2O

= 0.500 m NaOH,

= 0.5 molal solution of NaOH

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Concentration Review

1. What are two ways to express concentration? Discuss both, be sure to include the units.

2. How many grams of CO2 would need to be dissolved in water to make 0.75 L of 2.0 M solution?

3. Define concentration.

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Colligative Properties

colligative property: property that depends on the amount of solute particles, not the identity of the particles

1. vapor pressure lowering

-the greater the number of solute particles, the lower

the vapor pressure

2. boiling point elevation

-there is a relationship between vapor pressure and

boiling point: increase vapor pressure, increase bp

-due to the difference in the solutions bp and the pure

solvent’s bp

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Colligative Properties

3. freezing point depression

-decreasing temperature interferes with dissolving

-solute particles move closer together to form a solid

4. osmotic pressure

-osmosis: diffusion of solvent across a

semipermeable membrane from an area of higher

concentration to an area of lower concentration

-if there is a large amount of water molecules

(pure solvent) crossing the membrane to the solution

side, it creates pressure that pushes water molecules

back across the membrane.

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Electrolytes and Nonelectrolytes

Substances that dissolve in water are classified according to whether they yield molecules or ions in solution.

electrolyte: substance that dissolves in water to give a solution that conducts electric current

-all soluble ionic compounds forms an electrolyte

-conducts due to ions ( + and - ) in the solution

nonelectrolyte: substance that dissolves in water to give a solution that does not conduct electric current

-molecular compounds, such as sugar

-molecular compounds do not form ions in solution

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Heterogeneous Mixtures

1. suspension: when the particles in a solvent are so large that they settle out of solution unless the mixture is constantly stirred or agitated.

-example: muddy water -particles can be separated from the mixture using a

filter2. colloid: the particles are intermediate in size between

those in solutions and suspensions; form colloid dispersions

-mixture often remains cloudy, even when large particles are filtered out as some are small enough to pass through a filter, but large enough to see

-examples: paint, geletin, milk, mayo, smoke, cheese

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Many colloids are emulsions, a mixture that consists of colloidal-sized particles that are held together by an emulsifier.

-example: lecithin is used in margarine, mayo, salad dressings to bond the oil/water particles together.

Tyndall EffectThe Tyndall effect can be used to distinguish between a

solution and a colloid. -colloidal particles are large enough to scatter light,

making the beam visible in a transparent medium

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Colligative Properties and Heterogeneous Mixtures Review

1. Define colligative property.

2. List the 4 colligative properties.

3. What is the difference between electrolytes and nonelectrolytes?

4. Describe the two types of heterogeneous mixtures. Give an example of each.

5. Define heterogeneous mixtures.

6. What is the Tyndall effect?

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Solubility Review Answers 1. Describe the two components that make up a solution.

solute: what is being dissolved

solvent: what is doing the dissolving

2. Give all possible combinations of solutions (ex: gas/gas)

gas/gas, gas/liquid, liquid/liquid, solid/liquid, solid/solid

3. What does immiscible mean?

two liquids will not mix and the less dense floats on top

4. Will all substances dissolve in any solvent? Explain

will only dissolve when polarity allows; “like dissolves like”

5. How do we determine polarity?

look at bonding (ionic vs covalent, etc.)6. What 2 things does the polarity of covalent compounds depend

on? shape of the molecule, intermolecular forces

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Solubility Review Answers7. Describe all types of intermolecular forces.

a. dispersion force: attraction between nonpolar molecules

b. dipole-dipole force: attraction between polar molecules

c. hydrogen bond: forms between a hydrogen end of a

polar molecule and a F, O, or N from another dipole

8. Describe the three things that affect rate of dissolving.

1. increasing surface area (make particles smaller)

2. agitating a solution (solute particles are dispersed more

quickly)

3. heating a solvent (increases the kinetic energy of the solvent,

making more contact between the solvent/solute particles)

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Solubility Review Answers9. What is the difference between a saturated, unsaturated, and

supersaturated solution? 1. saturated solution: solution that contains the maximum amount of dissolved solute, for the given conditions 2. unsaturated solution: solution that contains less solute than a saturated solution at existing conditions 3. supersaturated solution: solution that contains more dissolved solute than a saturated solution; produced by heating the solution 10. What is the effect of temperature and pressure on solubility?

gas: increasing temperature decreases solubility; increasing

pressure puts stress on the equilibrium of the solution. solids: most solids have an increased solubility with increasing temp; no pressure effect liquids: no pressure or temperature effects

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Concentration Review Answers1. What are two ways to express concentration? Discuss both, be

sure to include the units. molarity (M): number of moles of solute in 1 liter of solution;

mol solute/L solution molality (m): concentration of a solution expressed in moles of solute per kilogram of solvent; mol solute/kg solvent

2. How many grams of CO2 would need to be dissolved in water to make 0.75 L of 2.0 M solution?

2.0 mol/L = mol solute/ 0.75 L (.75L)(2.0 mol/L) = (mol solute/ 0.75 L)(0.75L)

mol CO2 = 1.5mol 1.5 mol x 44 g/mol = 66 g3. Define concentration.

measure of the amount of solute in a given amount of solvent or solution

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Colligative Properties and Heterogeneous Mixtures Review Answers

1. Define colligative property. property that depends on the amount of solute particles, not the

identity of the particles2. List the 4 colligative properties. vapor pressure lowering, boiling point elevation, freezing point

depression, osmotic pressure3. What is the difference between electrolytes and nonelectrolytes? electrolytes conduct, nonelectrolytes do not4. Describe the two types of heterogeneous mixtures. Give an

example of each. suspensions: particles in a solvent are so large that they settle

out of solution unless the mixture is constantly stirred or agitated; muddy water

colloids: the particles are intermediate in size between those in solutions and suspensions; form colloid dispersions; mayo

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5. Define heterogeneous mixtures.

mixture that is not the same throughout

6. What is the Tyndall effect?

when a light is shown through a colloid that you can

see