Chapter 15 Water and Aqueous Systems. Water Water makes plant and animal life on Earth possible...

Chapter 15Water and Aqueous Systems

Water

Water makes plant and animal life on Earth possible

• Water is present on the earth’s surface.

• Water reserves are deep underground.

• Ice and snow dominate the polar regions.

• Water vapor from evaporation of surface water and steam from geysers and volcanoes is always present in the atmosphere.

Water’s Properties

H2O – the oxygen atom forms a covalent bond to each of the hydrogen atoms

Because of its greater electronegativity, oxygen attracts the electron pair of the covalent O – H bond to a greater extend than hydrogen.

As a result, the Oxygen atom acquires a partial negative charge (δ-)

The less electronegative hydrogen atoms acquire partial positive charges (δ+)


The O – H bonds are highly polar.

Polar bondPolar bond – a covalent bond between atoms in which the electrons are shared unequally.

How do the polarities of the two O – H bonds affect the polarity of the molecule?

The shape of the molecule is the determining factor.


The bond angle of water is approximately 105 which give it a bent shape.

Polar moleculePolar molecule – a molecule in which one side of the molecule is slightly negative and the opposite side is slightly positive.

The water molecule as a whole is polar. The water molecule as a whole is polar.

PolarityPolarity – refers to the net molecular dipole resulting from electronegativity differences between covalently bonded atoms


In general, polar molecules are attracted to one another by dipole interactions.

Dipole interactionsDipole interactions – intermolecular forces resulting from the attraction of oppositely charged regions of polar molecules.

The negative end of one molecule attracts the positive end of another molecule


The intermolecular attractions among water molecules result in the formation of hydrogen bonds.

Hydrogen bondsHydrogen bonds – attractive forces in which a hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom.

Many unique and important properties of water, including its high surface tension and low vapor pressure, result from hydrogen bonding.

Surface Tension

The surface of water acts like a skin.

The skinlike property of water’s surface is explained by water’s ability to form hydrogen bonds.

The molecules within the body of the liquid form hydrogen bonds with other molecules that surround them on all sides.

The attractive forces on each of these molecules are balanced.

Surface Tension

Water molecules at the surface of the liquid experience an unbalanced attraction.

Water molecules are hydrogen-bonded on only one side of the drop.

As a result, water molecules at the surface tend to be drawn inward.

Surface tensionSurface tension – the inward force, or pull that tends to minimize the surface area of a liquid

Spherical Shape of Water Drops

All liquids have surface tension, but water’s is higher than most.

The surface tension of water tends to hold a drop of liquid in a spherical shape.

The drop is not a perfect sphere, because the force of gravity tends to pull it down, causing it to flatten.

This is why, on some surfaces, water tends to bead up rather than spread out.

Why A Spherical Shape?

Nature tends to find the path of least resistance

Moving molecules takes work

A spherical shape provides the minimum surface area for a given volume.

Molecules expend the least energy possible to move into a spherical arrangement while maximizing their interactions with one another.

To adopt any other shape, more work would have to be done. The worked saved is the surface tension.

Surfactants

It is possible to decrease the surface tension of water by adding a surfactant.

SurfactantSurfactant – any substance that interferes with the hydrogen bonding between water molecules and thereby reduces the surface tension.

Examples of surfactants are soaps and detergents.

Adding a detergent to beads of water on a greasy surface reduces the surface tension causing the beads of water to collapse and spread out.

Vapor Pressure

Hydrogen bonding also explains water’s unusually low vapor pressure.

Vapor pressure is the result of molecules escaping the surface of the liquid & entering the vapor phase.

Hydrogen bonds hold water molecules to one another. The tendency to escape is low, thus evaporation is slow.

It is a good thing because all the lakes and oceans would tend to evaporate.

Water in the Solid State

Water is the solid state exhibits unique properties the same as a liquid.

Ice cubes float in your glass of iced tea because solid water has a lower density than liquid water.

As a typical liquid cools, it begins to contract and its density increases gradually.

Increasing density means the molecules of the liquid move closer together so that a given volume of a liquid contains more molecules and thus more mass.


Eventually the liquid solidifies and it continues to cool, having a greater density than the liquid

Because the density of a typical solid is greater than that of its liquid, the solid sinks in its own liquid.

As water begins to cool, it behaves initially like a typical liquid.

It contracts slightly and its density gradually increases.


When the temperature of water falls below 4º C, the density of water actually starts to decrease.

Below 4º C, water no longer behaves like a typical liquid.

Hydrogen bonds hold the water molecules in place in the solid phase.

The structure of ice is a regular open framework of water molecules arranges like a honeycomb.


Extensive hydrogen bonding in ice holds the water molecules farther apart in a more ordered arrangement than in liquid water.

When ice melts, the framework collapses and the water molecules pack closer together, making liquid water more dense than ice.


The fact that ice floats has important consequences for organisms.

A layer of ice on the top of a pond acts as an insulator for the water beneath, preventing it from freezing solid except in extreme conditions.

The liquid water at the bottom is warmer than 0ºC, fish and other aquatic life are better able to survive.


Ice melts at 0ºC. This is a high melting temperature for a molecule with such a low molar mass.

A considerable amount of energy is required to return water in the solid state to the liquid state.

The heat absorbed when 1g of water changes from a solid to a liquid is 334 J.

Question

What causes the high surface tension and low vapor pressure of water?

Water molecules are hydrogen-bonded to each other, but not to air molecules. Net attraction is inward, minimizing the water surface area. Hydrogen bonding makes it more difficult for water molecules to escape from the liquid phase to the vapor phase.

Questions

How would you describe the structure of ice?

Honeycomb-like structure of hydrogen-bonded water molecules.

What effect does a surfactant have on the surface tension of water?

Surfactants lower the surface tension by interfering with hydrogen bonding.

Questions

What are two factors that determine how spherical a drop of liquid will be?

Surface tension of a liquid tend to hold a drop of liquid in a spherical shape; gravity tends to flatten the drop.

The molecules water (H2O) and methane (CH4) have similar masses, but methane changes from a gas to a liquid at -161 C. Water becomes a gas at 100ºC. What could account for the difference?

Water has intermolecular hydrogen bonding between its molecules; methane does not.

Questions

Why is the surface tension of water so high compared to that of other liquids?

Water molecules form a large number of hydrogen bonds, in addition to dipole-dipole forces between molecules.

Why does water form a meniscus in a narrow tube?

Water molecules have a greater attraction to the molecules on the surface of the glass than they do to each other.

End of Section 15.1

Solvents and Solutes Water dissolves so many of the substances that it comes in contact with that you won’t find chemically pure water in nature.

Even the tap water you drink is a solution that contains varying amounts of dissolved minerals and gases.

Aqueous solutionAqueous solution – water that contains dissolved substances.

Solvent Solvent – the dissolving medium

SoluteSolute – the dissolved particles

Solvents and Solutes

Solutions are homogeneous mixtures. They are also stable mixtures.

Example: salt (NaCl) does not settle out of the solution when allowed to stand. (provided other conditions, like temperature remain constant)

Solute particles can be atoms, ions, or molecules and their average diameter are usually less than 1nm.

If you filter a solution through filter paper, both the solute and the solvent pass through the filter.

Solvents and Solutes

Ionic compounds and polar covalent molecules dissolve most readily in water.

Ionic compoundsIonic compounds – composed of a positive and negative ion (ex: metal and non metal)

Polar covalent moleculesPolar covalent molecules – electrons are shared equally between atoms (covalent) and one side of the molecule is slightly negative and the opposite side is slightly positive.

Nonpolar covalent molecules, such as methane and compounds found in oil, grease & gasoline, do not dissolve in water.

The Solution Process

Water molecules are in constant motion because of their kinetic energy.

When a crystal of NaCl is place in water, the water molecules collide with it.

Since the water molecule is polar, the partial positive charge on the H+ attracts the negative solute ion Cl-

The partial negative charge on the O2- attracts the positive solute ion Na+

Solvation

As individual solute ions break away from the crystal, the negatively (Cl-) and positively (Na+) charged ions become surrounded by solvent molecules and the ionic crystal dissolves.

SolvationSolvation – the process by which the positive and negative ions on an ionic solid become surrounded by solvent molecules.

Insoluble Ionic Compounds

In some ionic compounds, the attractions among the ions in the crystals are stronger than the attractions exerted by water.

These compounds cannot be solvated to any significant extent and are therefore nearly insoluble.

Barium sulfate (BaSO4) and calcium carbonate (CaCO3) – nearly insoluble ionic compounds

The Solution Process

As a rule, polar solvents such as water dissolve ionic compounds and polar compounds.

Nonpolar solvents such as gasoline dissolve nonpolar compounds.

Like dissolves likeLike dissolves like

Questions

What must happen for an ionic solid to dissolve?

The molecules of the solvent must be able to overcome the attractive forces between ions that hold the solid together.

What part of a water molecules is attracted to a negatively charges solute ion?

The hydrogen atoms

Reminders

Solutions are homogeneous mixtures containing a solvent and one or more solutes.

Usually the solvent is defined as the component in the system that is present in the greatest amount.

A water-soluble solute can be a solid, liquid, or a gas.

AnionAnion is a negatively charged atom or group of atoms

CationCation is a positively charges atom or group of atoms.

Electrolytes & Nonelectrolytes

ElectrolyteElectrolyte – compound that conducts electric current when it is in an aqueous solution or in the molten state.

All ionic compounds are electrolytes because they dissociate into ions.

NaCl Na+ + Cl-

Nonelectrolyte Nonelectrolyte – compound that does not conduct electric current in aqueous solutions or in the molten state

Many molecular compounds are nonelectrolyes because they are not composed of ions.


Some polar molecular compounds are nonelectrolytes in the pure state, but become electrolytes when they dissolve in water.

This occurs because they ionize in solution.

Ex: neither ammonia or hydrogen chloride is an electrolyte in the pure state.

NH3 + H2O NH4+ + OH-

HCl + HHCl + H22O HO H33OO++ + Cl + Cl--

Both conduct electricity in aqueous solutions because ions form.

Strong Electrolytes

Not all electrolytes conduct an electric current to the same degree.

Strong ElectrolyteStrong Electrolyte – a solution that is a good conductor of electricity because a large portion of the solute exists as ions.

Strong AcidsHCl, HBr, HI, HNO3, HClO3, HClO4, and H2SO4

Strong BasesNaOH, KOH, LiOH, Ba(OH)2, and Ca(OH)2

Salts NaCl, KBr, MgCl2 …


Weak electrolyteWeak electrolyte – solution that conducts electricity poorly because only a fraction of the solute exists as ions.

Weak AcidsHF, HC2H3O2 (acetic acid), H2CO3 (carbonic acid), H3PO4 (phosphoric acid) …..

Weak BasesNH3 (ammonia), C5H5N (pyridine), and several more, all containing "N"


A solution conducts electricity if it A solution conducts electricity if it contain ions.contain ions.

Electrolytes are excreted through the skin via sweat, and they must be replenished or cramps and heat stroke may occur.

Sports drinks are a good source of electrolytes; they contain Na+, K+ and Ca+

Hydrates

When an aqueous solution of copper(II) sulfate (CuSO4) is allowed to evaporate, deep blue crystals of copper(II) sulfate pentahydrate are deposited.

The chemical formula for this compound is

CuSO4 · 5H5H22OO

Water of Hydration or Water of CrystallizationWater of Hydration or Water of Crystallization – the water contained in a crystal.

Hydrates

HydrateHydrate – a compound that contains water of hydration

When writing the formula of a hydrate, use a dot to connect the formula of the compound and the number of water molecules per formula unit.

CuSO4 · 5H5H22OO

Crystals of copper(II) sulfate pentahydrate always contain five molecules of water for each copper and sulfate ion pair.

Hydrates

When CuSO4 · 5H5H22OO is heated above 100ºC, the crystals lose their water of hydration and crumble to a white anhydrous powder of CuSO4.

If anhydrous CuSO4 is treated with water, the blue

CuSO4 · 5H5H22OO is regenerated.

+ heat

CuSO4 · 5H5H22O O CuSO4(s) + 5 H2O (g)

- heat

Each hydrate contains a fixed quantity of water and has a definite composition.

Efflorescent Hydrates

The forces holding the water molecules in hydrates are not very strong, so the water is easily lost and regained.

Because the water molecules are held by weak forces, it is often possible to estimate the vapor pressure of the hydrates.

If a hydrate has a vapor pressure higher than the pressure of water vapor in the air, the hydrate will lose its water of hydration – effloresceeffloresce.

Hygroscopic Hydrates

Hydrated salts that have a low vapor pressure remove water from moist air to form higher hydrates.

These hydrates and other compounds that remove moisture from air are called hygroscopichygroscopic.

CaCl2 · H2O CaCl2 · 2H2O

Calcium chloride monohydrate spontaneously absorbs a second molecule of water when exposed to moist air.

Hygroscopic Hydrates

CaCl2 · H2O is used a a desiccant in the laboratory.

DesiccantDesiccant – a substance used to absorb moisture from the air and create a dry atmosphere.

Desiccants can be added to a sealed container to keep substances inside the container dry.

Desiccants can be added to liquid solvents to keep them dry.

When a desiccant has absorbed all the water it can hold, it can be returned to its anhydrous state by heating.

Percent Water

Suppose you are measuring a mass of Na2CO3 for a chemical reaction and you want to use the hydrate form of the compound because it is less expensive than the anhydrous compound.

To determine what percent of the hydrate is water.

1.Determine the mass of the number of moles of water in one mole of hydrate.

2.Determine the total mass of the hydrate.

% Water = mass of water / mass of hydrate x 100%% Water = mass of water / mass of hydrate x 100%

Question

What is the percent by mass of water in Na2CO3 · 10H2O?

Mass of 10 moles of H2O = 18 x 10 = 180 g

Mass of Na2CO3 · 10H2O = 46 + 12 + 48 + 180 = 286g


% Water = 180 / 286 = 62.9 %% Water = 180 / 286 = 62.9 %

Question

What is the percent by mass of water in CuSO4 · 5H2O?

Mass of 5 moles of H2O = 18 x 5 = 90 g.

Mass of CuSO4 · 5H2O = 63.5 + 32.1 + 64 + 90 = 249.6


% Water = 90 / 249.6 = 36.1 %% Water = 90 / 249.6 = 36.1 %

Question

What is the percent by mass of water in CaCl2 · 6H2O?

Mass of 6 moles of H2O = 18 x 6 = 108 g

Mass of CaCl2 · 6H2O = 40.1 + 71 + 108 = 219.1g


% Water = 108 / 219.1 = 49.3 %% Water = 108 / 219.1 = 49.3 %

Questions

In the formation of a solution, how does the solvent differ from the solute?

The dissolving medium is the solvent, and the dissolved particles are the solute

Describe what happens to the solute and the solvent when an ionic compound dissolves in water.

As individual solute ions break away from the crystal, the negatively and positively charged ions become surrounded by solvent molecules and the ionic crystal dissolves.

Questions

Why are all ionic compounds electrolytes?

Because they dissociate into ions.

How do you write the formula for a hydrate?

Use a dot to connect the formula of the compound with the number of water molecules per formula unit.

Why are all ionic compounds electrolytes?

Because they dissociate into ions.

Questions

Which of the following substances dissolve to a significant extent in water? Explain your answer.

CH4 – insoluble, nonpolarKCl – soluble, ionic

He – insoluble, nonpolarMgSO4 – soluble, ionic

Sucrose – soluble, polarNaHCO3 – soluble, ionic

Questions

Distinguish between efflorescent and hygroscopic substances.

Efflorescent compounds lose water to the air. Hygroscopic compounds remove moisture from air, sometimes forming hydrates.

Identify the solvent and the solute in vinegar, a dilute aqueous solution of acetic acid.

Water is the solvent, acetic acid is the solute

Questions

What types of substances will dissolve in water to form aqueous solutions?

Ionic compounds and polar covalent molecules.

What type of electrolyte is nickel(II) chloride (NiCl2)? Explain.

NiCl2 is an ionic compound and is soluble in water, therefore, it is a strong electrolyte.

Question

Get into groups of 2 and find the percent by mass of water in the compounds in Table 15.2 on page 455

End of Section 15.2

Heterogeneous Aqueous Systems

Heterogeneous mixtures are not solutions.

If you shake a piece of clay with water, the clay breaks into fine particles.

The water becomes cloudy because the clay particles are suspended in the water.

If you stop shaking, the particles begin to settle out.

SuspensionSuspension – a mixture from which particles settle out upon standing.

Suspensions

A suspension differs from a solution because the particles of a suspension are much larger and do not stay suspended indefinitely.

The larger size of suspended particles means that gravity plays a larger role in causing them to settle out of the mixture.

Cooks use suspensions of flour or cornstarch in water to thicken sauces and gravies.

Suspensions

Suspensions are heterogeneous because at least two substances can be clearly identified.

You can clearly see the dispersed phase (clay) in the dispersion medium (water).

If muddy water is filtered, the filter traps the suspended clay particles and clear water passes through.

Suspensions

How does the filtration of a suspension compare with the filtration of a solution?

Filtration of a solution – both the solvent and the solute will pass through the filter. (small size of the solute particles)

Filtration of a suspension – the filter will trap the suspended particles and the liquid will pass through (large size of the suspended particles)

Colloids

Gelatin is referred to as a heterogeneous mixture called a colloid.

What does that tell you about the composition of gelatin?

It is not uniform throughout.

Would you expect a colloid to have smaller or larger particles than a solution?

Larger

Colloids

Colloid – a heterogeneous mixture containing particles that range in size from 1nm to 1000 nm.

The particles are spread throughout the dispersion medium, which can be a solid, liquid or gas.

glues gelatin paint milksmog smoke cream asphalt

Ink sea foam aerosols

Colloids

A colloid is a type of mixture that appears to be a solution but it is actually a mechanical mixture.

A colloidal system consists of two separate phases: a dispersed phase (internal phase) and a continuous phase (dispersion medium).

In a colloid, the dispersed phase is made of tiny particles or droplets that are distributed evenly throughout the continuous phase.

The Tyndall Effect

Ordinarily you can’t see a beam of sunlight unless the light passes through particles of water or dust in the air.

A beam of light is visible as it passes through a colloid.

Tyndall effectTyndall effect – the scattering of visible light by colloidal particles

Suspensions also exhibit the Tyndall effect, but solutions do not. (particles are too small to scatter light)

Brownian Motion

Brownian MotionBrownian Motion – The chaotic movement of colloidal particles (first observed by Robert Brown 1773 – 1858)

Brownian motion is caused bycollisions of the molecules of the dispersion medium with the small, dispersed colloidal particles.

These collisions help prevent the colloidal particles from settling.

Digital video microscopy

Coagulation

Colloidal particles also tend to stay suspended because they become charged by adsorbing ions from the dispersing medium onto their surface.

AdsorptionAdsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a molecular or atomic film (the adsorbate).

It is different from absorptionabsorption, in which a substance diffuses into a liquid or solid to form a solution


Coagulation

Some colloidal particles become positively charged by adsorbing positively charged ions

Other colloidal particles become negatively charged by adsorbing negatively charged ions.

All the particles in a particular colloidal system will have the same charge.


Coagulation

The repulsion between the like-charged particles prevents the particles from forming heavier aggregates that would have a greater tendency to settle out.

Thus a colloidal system can be destroyed or coagulated by the addition of ions having a charge opposite to that of the colloidal particles.

The added ions neutralize the charged colloidal particles.

The particles can clump together to form heavier aggregates and precipitate from the dispersion.

Coagulation Example

Milk is a colloid of protein and fat.

The curdling of milk occurs when bacteria produce enough lactic acid to cause dispersed protein and fat particles to coagulate into larger particles, which separate from the rest of the mixture.

Comparable to the clotting of blood


Quick Review

Main difference between solutions, suspensions, and colloids is particle size.

Solution particles – typically less than 1 nm diameter

Colloid particles – between 1 nm and 1000 nm

Suspension particles - typically larger than 1000nm


Quick Review

Smaller particles are less susceptible to the effects of gravity and are influenced more by the effects of Brownian movement.

The collisions of molecules with extremely small colloidal particles are sufficiently energetic to move colloidal particles in a random fashion that prevents their settling to the bottom.


Emulsions

EmulsionEmulsion – a colloidal dispersion of a liquid in a liquid.

An emulsifying agent is essential for the formation of an emulsion and for maintaining the emulsion’s stability.

Ex. Oils and greases are not soluble in water.

However, the readily form a colloidal dispersion if soap or detergent is added to the water.


Emulsions

Soaps and detergents are emulsifying agent

One end of a large soap or detergent molecule is polar and is attracted to water molecules.

The other end of the soap or detergent molecule is nonpolar and is soluble in oil or grease.

Emulsifying agents thus allow the formation of colloidal dispersions between liquids that do not ordinarily mix.


Emulsions

An example of an emulsion ismayonnaise

Mayonnaise is a heterogeneous mixture of oil and vinegar, which would quickly separate without the presence of egg yolk (the emulsifying agent.)

Milk, margarine and butter are also emulsions.

Cosmetics, shampoos, and lotions are formulated with emulsifiers to maintain consistent quality.


Questions

In what way are colloids similar to solutions?

In both, dispersed particles are small enough to pass through standard filter paper and to withstand the pull of gravity.

In what way are colloids similar to suspensions?

Both types of mixtures produce the Tyndall effect.


ReviewProperties of Solutions

SolutionsSolutions• Particle type – ions, atoms, small molecules

• Particle size – 0.1 – 1 nm

• Effect of light – no scattering

• Effect of gravity – stable, does not separate

• Filtration – particles not retained on filter

• Uniformity - homogeneousDigital video microscopy

ReviewProperties of Colloids

ColloidsColloids• Particle type – large molecules or particles

• Particle size – 1 – 1000 nm

• Effect of light – exhibits Tyndall effect

• Effect of gravity – stable, does not separate

• Filtration – particles not retained on filter

• Uniformity - borderlineDigital video microscopy

ReviewProperties of Suspensions

SuspensionSuspension• Particle type – large particles or aggregates

• Particle size – 1000nm and larger

• Effect of light – exhibits Tyndall effect

• Effect of gravity - unstable, sediment forms

• Filtration – particles retained on filter

• Uniformity – heterogeneousDigital video microscopy

Questions

How does a suspension differ from a solution?

Particles of a suspension are much larger and do not stay suspended indefinitely.

What distinguishes a colloid from a suspension and a solution?

Colloids have particles smaller than those in suspensions and larger than those in solutions.

Questions

How can you determine through observation that a mixture is a suspension?

The particles in a suspension will settle out over time.

Could you separate a colloid by filtering? Explain

Particles in a colloid such as gelatin are smaller than the holes in filter paper and cannot be removed by filtering.

Question

How can the Tyndall effect be used to distinguish between a colloid and a solution?

A beam of light is visible as it passes through a colloid; it is invisible as it passes through a solution.

QuestionWhat causes Brownian motion?

Collisions of the molecules of the dispersion medium with the small, dispersed colloidal particles.

Can the presence of Brownian motion distinguish between a solution and a colloid? Explain.

Flashes of light (scintillations) are seen when colloids are studied under a microscope.

Colloids scintillate because the particles reflecting and scattering the light move erratically.

Particles in a solution are too small to be seen under a microscope and do not cause scintillations.

End of chapter 15

Chapter 15 Water and Aqueous Systems. Water Water makes plant and animal life on Earth possible...

Documents

Transcript of Chapter 15 Water and Aqueous Systems. Water Water makes plant and animal life on Earth possible...