UNIT 4: Solutions and Solubility Chapter 8: Solutions and Their Properties Chapter 9: Reactions in...

UNIT 4: Solutions and Solubility

Chapter 8: Solutions and Their Properties

Chapter 9: Reactions in Aqueous Solutions

How are qualitative analysis and quantitative analysis used to describe the composition of a solution, and how do humans use and affect the world’s water supply?

Chapter 10: Acids and Bases

Chapter 9: Reactions in Aqueous Solutions

All forms of life have one thing in common – water. Water’s ability to dissolve many different compounds enables it to carry the nutrients required by the vast range of life forms on Earth. However, water can also dissolve and carry harmful compounds, both natural substances and human-made pollutants.

UNIT 4 Chapter 9: Reactions in Aqueous Solutions

9.1 Net Ionic Equations and Qualitative Analysis

The reaction shown here is a double displacement reaction with a spectacular colour change. In water, ionic substances dissociate into their component ions. Non-reacting ions in an aqueous solution are called spectator ions.

Note that water is not a reactant in the chemical equation.

UNIT 4 Chapter 9: Reactions in Aqueous Solutions Section 9.1

Writing Ionic EquationsIn an ionic equation, the formulas of soluble ionic compounds are replaced with the formulas of the ions that these compounds form in water. The following equation:

2AgNO3(aq) + Na2CrO4(aq) Ag2CrO4(s) + 2NaNO3(aq)

becomes:

2Ag+(aq) + 2NO3-(aq) + 2Na+(aq) + CrO4

2-(aq)

Ag2CrO4(s) + 2Na+(aq) + 2NO3-(aq)

when the formulas for ionic substances are written in dissociated form.


Writing Net Ionic Equations

Spectator ions will appear on both sides of the equation and can be canceled to obtain the net ionic equation. This equation shows only the ions that react.

2Ag+(aq) + CrO42-(aq) Ag2CrO4(s)

It is convenient to use this form of the equation since the source of the silver and chromate ions does not matter. Changing the spectator ions does not change the reaction.


Spectators are present at a sporting event but do not take part in the event, just as spectator ions are present in a reaction but do not take part in it.

Rules for Writing a Net Ionic Equation


Qualitative Analysis

Qualitative analysis identifies elements, ions, or compounds in a sample. To identify certain ions, you can observe the:


• colour in a flame test• colour of a solution• formation of a precipitate

Fireworks are a spectacular demonstration of the different colours of light that are given off by metal ions when they are heated.

Flame TestsSince many metal ions produce a distinct colour when heated, a flame test (described below) can help identify metal ions.

A Bunsen burner and a clean wire loop are used to test an aqueous solution (dip the loop) or a solid (moisten the loop with HCl(aq)). The substance is placed on the loop and then into the flame. Electrons in the atoms of the sample absorb energy from the flame, and some of the energy as visible light.


These photographs show flame tests of strontium and copper. Notice that the colour of the copper flame is greener than a typical Bunsen burner flame.

Flame Tests


Colours of Ions in Solution

Aqueous solutions of ionic compounds of certain cations and anions have characteristic colours.


Formation of a Precipitate

Another way to identify an unknown ion in a solution is to add a known reactant to the solution and observe whether or not a precipitate forms. The solubility guidelines can be used to infer which ion must have been present in the unknown solution. This can be done multiple times to the same solution to further identify ions in the filtrate.


Formation of a Precipitate


Section 9.1 Review


9.2 Solution Stoichiometry

Solution stoichiometry is often used in quantitative analysis, which involves determining a quantity (how much) of a substance is present in a sample.

In solution stoichiometry, known volumes and concentrations of reactants or products are used to determine the volumes, concentrations, or masses of other reactants or products.


Accurate measurement is important in both cooking and chemistry. A cake made from improperly measured ingredients may not look good and may not taste good either!

Rules for Solving Solution Stoichiometry Problems


Solution Stoichiometry Problem Solving

Example

Given:

Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)

Volume of AgNO3 solution = 100 mL

Mass of dried Ag(s) precipitate = 1.65 g

What is the molar concentration of silver nitrate solution?


Solution Stoichiometry Problem Solving

1. Calculate the amount in moles of Ag(s):

n(Ag) = 1.65 g × 1 mol = 0.015 296 mol 107.87 g

2. Use moles of Ag to calculate moles of AgNO3:

n =AgNO3 = 2 mol AgNO3 × 0.0015 296 mol Ag

2 mol

= 0.015296 mol AgNO3

3. Calculate the concentration of AgNO3:

c = 0.015 296 mol = 0.153 mol/L

0.100 L


Limiting Reactant Problems

The limiting reactant is usually the one that is the most expensive. This is one way to keep the costs in industrial chemical processes as low as possible.

These calculations are much like those in Chapter 7. The amount in moles of a solution is determined using n = c × V.


Section 9.2 Review


9.3 Water Quality

The figure below illustrates the fact that a very small amount (0.01%) of Earth’s water, found in lakes and rivers as surface water, is available for safe human use. The salt water in the oceans makes up 97.5% of the total amount of water, and most of the fresh water is unavailable in ground water (0.74%) far below Earth’s surface or is frozen in polar ice caps (1.75%).


Very little of the water on Earth is fresh water, and very little fresh water is available for people to drink.

Harmful Substances

Water quality is affected by:


• naturally occurring substances, some of which are harmful

• pollutants that originate from human activities• water treatment standards

The quality of any water depends on factors that are not always immediately visible.

Naturally Occurring Harmful SubstancesIonic Compounds Containing Arsenic


• found in ground water beneath river deltas; more serious in countries such as Bangladesh

• most ground water in Canada has less than 5 ppb, considered to be the safe level

• long-term exposure to dangerous levels of these compounds may lead to cancer and diabetes

Fluoride Ions• less than 1 ppb is considered safe and actually beneficial to strengthen tooth enamel

Fluoride ions prevent tooth decay, but too much fluoride can cause brown stains on teeth.

Naturally Occurring Harmful Substances

Calcium and Magnesium Ions


• high levels of calcium and magnesium ions make the water hard, which leads to formation of insoluble soap scum as well as a buildup of lime (calcium carbonate) deposits in pipes and appliances

Many of the rocks in southern Ontario are limestone, which makes most of the water in this region hard.

The lime scale in this water pipe was caused by hard water.

Pollutants from Human Activity

Two types:


• Point Source of Pollution: a single source with a specific location

Example: wrecked tanker leaking oil • Non-point Source of Pollution: no easily defined

location and may involve many substances spread over large areas

Example: fertilizers used on farms

(A) Point source of pollution: Waste water from a factory can quickly pollute a body of water. (B) Non-point source of pollution: Run-off from farms can carry fertilizer and pesticides into nearby waterways.

Pollutants from Human ActivityUNIT 4 Chapter 9: Reactions in Aqueous Solutions Section 9.3

Lead CompoundsThe Ontario government introduced the Cosmetic Pesticide Act in 2009 to protect the environment by lowering or banning the use of harmful garden and lawn pesticides. Despite these initiatives, pollutants such as lead, mercury, nitrates, and phosphates are still a major problem. Lead is released by industrial processes such as ore smelting, car battery production, and plastics production. While environmental lead pollution has decreased significantly, exposure is still occurring and can lead to kidney failure, nerve damage, and brain disorders.


Mercury CompoundsMuch of the mercury deposited in the Great Lakes comes from emissions from coal-fired power plants, gold mines, cement plants, and smelters. Mercury is highly toxic; it affects the central nervous system, producing tremors, irritability, numbness, and tunnel vision.


Nitrate and Phosphate IonsThe source of these ions comes mainly from livestock waste and fertilizer use. These ions may leach into lakes and rivers, where they promote plant and algal growth (blooms). The bacteria that decompose this algal growth absorb the available oxygen, leaving the fish populations starved of oxygen.

Nitrate and phosphate pollution can cause excessive plant growth in lakes and rivers.



Although algal blooms appear to help plant growth at first, the overall environmental effect is negative because of the loss of both plant and animal life.



Infants less than three months old are susceptible to high concentrations of nitrate ions in the water. Nitrate ions are converted to nitrite ions in an infant’s digestive tract. These ions bond to the hemoglobin, leading to a condition known as blue-baby syndrome.



Airborne PollutionNon-metallic oxides such as carbon dioxide, sulfur dioxide, and nitrogen oxide gases dissolve in rainwater and contribute to acid rain. Sources of these pollutants include motor vehicles, refineries, and many factories. Acid rain leaches aluminum ions into ground water and surface water, which can harm aquatic life considerably.



Plastic LeachatesWater bottles and other hard plastic containers contain polycarbonates. Polycarbonates are made using the chemical bisphenol A (BPA). This substance can leach into the liquid inside the plastic container. Recently BPA has been linked to breast cancer, heart disease, and other biological changes such as those caused by the hormone estrogen. Although banned from baby bottles, BPA is still found in food containers and continues to leach into the environment from landfills.

Water Quality and Drinking Water Standards


In North America, various treatment plants ensure that our drinking water is clear, colourless, tasteless, and odourless and is free of disease-causing organisms and unsafe levels of toxic compounds. Ontario has maximum allowable concentration (MAC) standards for selected ions and compounds in drinking water.

Maximum Concentrations of Selected Ions and Compounds in Ontario’s Drinking Water


Section 9.3 Review


9.4 Water Treatment

Serious consequences result when water is polluted and then not treated properly to make it safe to drink. Here are a few examples:


• May 2000: A well for the town of Walkerton was contaminated by run-off from a nearby farm. This run-off contained E. coli bacteria from animal manure and was not tested or processed properly. Seven people died, and thousands became seriously ill.

Polluted Water


• 1962: Reed International in Dryden, Ontario began using mercury in a process to make bleaching chemicals for its pulp and paper plant. The mercury waste water was dumped into the river. Mercury compounds accumulated in local fish, which were eaten by local residents. Residents became sick with mercury poisoning, and, to this day, newborns still show clear signs of mercury poisoning.

Between 1962 and 1970, Reed International dumped over 9000 kg of mercury into the Wabigoon-English River system.

Water Treatment: Temporary Hardness


Hard water contains dissolved calcium (and magnesium) carbonates. CaCO3(s) + H2O(ℓ) + CO2(aq) + Ca(HCO3)2(aq) Calcium hydrogen carbonate Ca(HCO3)2(aq) and the comparable magnesium compound are the main causes of temporary hardness. Boiling removes carbon dioxide from the solution, so it is not available to react to make calcium hydrogen carbonate. However, the calcium carbonate precipitates out of the solution and deposits on the heating appliance. Deposits of calcium carbonate

and magnesium carbonate on water-heating elements can reduce the heater’s efficiency.

Water Treatment: Permanent Hardness


Permanent water hardness is caused by dissolved calcium (and magnesium) sulfate. This hardness cannot be removed by boiling but must be treated by chemical methods. One way is to add sodium carbonate, Na2CO3•10H2O (washing soda) as a water softener. This precipitates the calcium (and magnesium) carbonates out of solution.

CaSO4(aq) + Na2CO3(s) CaCO3(s) + Na2SO4(aq) This treatment makes the water basic. Use of an ion-exchange water softener avoids this problem.

Water Treatment: Ion-Exchange Water Softeners


Water softened by an ion-exchange softener contains a high concentration of sodium ions. People on a low-sodium diet should avoid drinking water from a sodium ion water softener.

Water Treatment: Desalination


The process of obtaining fresh water from salt water is called desalination. Once it is safe to drink, the water is called potable water. Desalination plants are commonly used in the Middle East where oil is a cheap fuel used to heat the ocean water. The salt-free water vapour is condensed and collected.

The Jubail desalination plant, on the Arabian Gulf in Saudi Arabia, is the largest desalination plant in the world. Some of the energy that is needed to boil the water comes from waste heat from an adjacent electrical power plant.

Water Treatment: Reverse Osmosis

Reverse osmosis is a more energy-efficient method for making potable water.


• Osmosis is the tendency of a solvent to move through a semi-permeable membrane to make the concentrations of solutes on both sides of the membrane equal.

• Therefore, water will flow from the more dilute side into the more concentrated side.

• In reverse osmosis, high pressure is applied to the more concentrated side to force water through the membrane in the reverse-of-the-normal direction.

Water Treatment Reverse Osmosis Desalination Plants


Large reverse osmosis desalination plants are built in coastal areas. The largest in North America is in Tampa Bay, Florida. It produces 100 million litres of potable water daily.

High pressure forces water across a semi-permeable membrane inside these vessels. The water that passes through the membranes is completely free of salt and other ions.

Water Supply Treatment


As shown in the graph, life expectancy has increased dramatically since around 1800. Many health scientists have concluded that this is due mostly to the widespread improvement in the quality of water supplies around that time.

Life expectancy in North America has increased steadily since 1800, partly because ofimprovements in water treatment.

Municipal Water TreatmentUNIT 4 Chapter 9: Reactions in Aqueous Solutions Section 9.4

Municipalities use a combination of physical and chemical processes to purify water.

Waste-Water Treatment


The goal of waste-water treatment is to remove solids, chemicals, and dangerous bacteria from sewage. The water can then be released into the environment.

Waste-water treatment plants remove possible pollutants from sewage so that those pollutants cannot enter and harm the environment.

Stages in Waste-Water Treatment


• Primary Treatment in holding tanks removes solid materials by sedimentation and by skimming scum from the surface of the water. The addition of calcium hydroxide and aluminum sulfate promotes formation of aluminum hydroxide precipitate.

• Secondary Treatment uses natural micro-organisms that feed on organic matter in the sewage. These bacteria convert organic material into carbon dioxide, water, and nitrogen compounds.

• Tertiary Treatment involves chemical precipitation of nitrogen, phosphorus, and organic compounds.

Stages in Waste-Water Treatment


Section 9.4 Review


UNIT 4: Solutions and Solubility Chapter 8: Solutions and Their Properties Chapter 9: Reactions in...

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