Unit notes and practice questions
Everything here is for unit 5 of the Grade 12 Chemistry textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.
The cards filed under this chapter on the Grade 12 Chemistry page. Open one at a time.
When you use it
Use when identifying which atmospheric gases contribute to the greenhouse effect.
Watch out
Nitrogen and oxygen make up roughly 99 percent of the atmosphere but do not absorb infrared radiation and do not cause the greenhouse effect.
Drafted from Grade 12 Chemistry, page 241 onwards, then checked twice before it went up
When you use it
Use this formula to evaluate the efficiency of a chemical reaction according to green chemistry principles.
Watch out
Divide by the total mass of all products formed, not just the mass of reactants or byproducts alone.
Drafted from Grade 12 Chemistry, page 241 onwards, then checked twice before it went up
| Parameter | Definition | Standard value or condition |
|---|---|---|
| Dissolved Oxygen (DO) | Oxygen dissolved in water vital for aquatic life | is good quality, is polluted |
| Biological Oxygen Demand (BOD) | Capacity of dissolved organic matter to consume oxygen | Measured experimentally over a 5 day period in a sealed sample |
| Threshold Limit Value (TLV) | Permissible level of toxic pollutant in the atmosphere | Safe for exposure over an 8 hour day without adverse effects |
When you use it
Use when assessing water quality levels or safe atmospheric exposure limits for chemical species.
Watch out
A higher BOD value indicates more organic pollution and lower water quality, whereas high DO indicates healthy water.
Drafted from Grade 12 Chemistry, page 241 onwards, then checked twice before it went up
| Greenhouse gas | Atmospheric lifetime | Contribution to global warming |
|---|---|---|
| 80% lasts 200 years, 20% up to 30000 years | 52.92% | |
| 12 years | 14.88% | |
| Halogenated compounds (CFCs and HCFCs) | Months to tens of thousands of years | 10.78% |
| Tropospheric ozone () | Few months | 10.72% |
| Nitrous oxide () | 114 years | 10.70% |
When you use it
Use when comparing the sources, persistence, and warming impact of major anthropogenic greenhouse gases.
Watch out
Carbon dioxide contributes the largest percentage to global warming even though other gases like CFCs may persist longer or have higher intrinsic warming potential.
Drafted from Grade 12 Chemistry, page 241 onwards, then checked twice before it went up
| Control measure | The problem it works on | The pollutant and the harm it does |
|---|---|---|
| Cut the emission at source: efficient furnaces and engines, and electricity from hydro, wind, solar and geothermal instead of coal and oil. Afforestation is a supporting sink that stores part of the carbon. | Global warming | Burning fossil fuels releases , which absorbs the infrared radiation leaving the Earth's surface and warms the lower atmosphere. |
| Electrostatic precipitators and bag filters take out the particles, wet scrubbers take out the acidic gases, catalytic converters treat vehicle exhaust, and the authority enforces emission limits at the chimney. | Respiratory illness | Industrial and vehicle emissions carry particulate matter, , and . Fine particles and the acidic gases inflame the airways and bring on bronchitis and asthma attacks, while carbon monoxide binds to haemoglobin and blocks oxygen transport in the blood. |
| Burn low sulfur fuel, strip out of the flue gas with lime or limestone, which fixes it as calcium sulfite and then calcium sulfate, and fit catalytic converters to cut nitrogen oxides. | Acid rain | , then , and . The acid acidifies soil and lakes, scorches leaves and lowers crop yield, corrodes metal and eats limestone and marble. |
| The Montreal Protocol of 1987 phased out CFCs worldwide, and industry moved to substitutes that carry no chlorine. | Ozone layer depletion | CFCs are unreactive in the troposphere, so they survive to drift up. Ultraviolet light splits a chlorine atom off, and it destroys ozone catalytically: , then . The chlorine comes back, so one atom destroys many ozone molecules and more UV-B reaches the ground, raising rates of skin cancer and cataracts and lowering crop yield. |
When you use it
Use for the environmental chemistry question that asks you to name an air pollution problem, the pollutant behind it and one control measure. Each row pairs the control with the chemistry it is fixing, so your answer names the equipment or the treaty instead of saying that pollution should be reduced.
Watch out
Students merge the two big problems and write that the ozone hole lets more heat in, or that carbon dioxide destroys the ozone layer. Keep them apart: ozone depletion is chlorine chemistry high in the stratosphere and what comes through is ultraviolet light, while global warming is carbon dioxide and methane low in the atmosphere holding back the infrared going out. The same slip turns up in acid rain answers, where carbon dioxide gets blamed. Acid rain is sulfur dioxide and nitrogen oxides, and carbon dioxide only accounts for the pH 5.6 that clean rain already has.
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
According to environmental chemistry, what is the optimum level of dissolved oxygen (DO) in good quality water?
Answer: A 4 to 8 mg L-1
Why
The optimum level of dissolved oxygen (DO) in good quality water is 4 to 8 mg L-1. Water with levels below 4 mg L-1 is considered polluted and unfit for consumption.
Question 2
What portion of the global water supply in the hydrosphere is fresh water?
Answer: D Only about 1%
Why
Although the oceans contain vast amounts of water, only about 1% of the global water supply is fresh water.
Question 3
Which greenhouse gas of human origin contributes the largest percentage (52.92%) to global warming?
Answer: B Carbon dioxide
Why
Carbon dioxide is the major contributor to global warming among gases of human origin, contributing 52.92%.
Question 4
In the atmosphere viewed as a solution, which gas is considered the solvent because it makes up about 78% of the total volume?
Answer: C Nitrogen (N2)
Why
Because N2 gas is present in the greatest amount (about 78% by volume), it is considered the solvent in the atmospheric solution.
Question 5
A factory near Modjo introduces a synthetic chemical that does not occur in nature into the surrounding environment. In environmental chemistry, what is a substance called if it does not occur naturally but is introduced through human activity?
Answer: B Contaminant
Why
A contaminant is defined as a substance that does not occur in nature but is introduced into the environment through human activity.
Question 6
Which step in the natural nitrogen cycle converts nitrate (NO3-) back into nitrogen gas (N2) to return it to the atmosphere?
Answer: B Denitrification
Why
Denitrification is the biological reaction that reduces nitrate and returns N2 to the atmosphere: 2NO3- + 5(CH2O) + 4H+ → 2N2 + 7H2O + 5CO2.
Question 7
In the upper layers of the atmosphere, UV light breaks chlorofluorocarbons to produce free chlorine atoms (Cl). What products are formed when a free chlorine atom reacts with ozone (O3)?
Answer: A Chlorine monoxide (ClO) and oxygen (O2)
Why
The chemical reaction in the atmosphere is Cl + O3 → ClO + O2, converting ozone to molecular oxygen and causing ozone depletion.
Question 8
Without the natural greenhouse effect absorbing infrared radiation, what would the average temperature on Earth be?
Answer: C 255 K (-18 degrees Celsius)
Why
Without the greenhouse effect, Earth's average temperature would be 255 K (-18 degrees Celsius). Greenhouse gases raise it by 33 K to an average of 288 K (15 degrees Celsius).
Question 9
In an industrial synthesis, 514 g of reactants are used to produce 206 g of a desired pain medication along with by-products. What is the atom economy of this process?
Answer: B 40%
Why
Atom economy is calculated as (Mass of desired product / Total mass of products) * 100. Here, (206 g / 514 g) * 100 = 40.08%, or about 40%, matching the original six-step synthesis of ibuprofen.
Question 10
Which greenhouse gas has an average atmospheric lifetime of 12 years and contributes about 14.88% to global warming?
Answer: C Methane
Why
According to the textbook data table, methane (CH4) lasts an average of 12 years in the atmosphere and contributes 14.88% to global warming.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
The whole Grade 12 Chemistry textbook, free as a PDF.
Ministry of Education edition, English, 298 pages
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