E. Chemical Systems and Equilibrium · Chemical EquilibriumEquilibrium in Living Things, the Environment and Technology
E1.2 E3.3 E3.1 A1.9 A2.1About 70 minutes
Learning goals- I can explain oxygen transport by hemoglobin, and the effects of altitude and carbon monoxide, using Le Châtelier's principle.
- I can describe how CO₂ equilibria affect blood chemistry and the oceans.
- I can explain how equilibrium shifts affect tooth enamel and kidney stones.
- I can assess the impact of these equilibrium processes on health, society and the environment.
Words to know
hemoglobin (Hb)
The iron-containing protein in red blood cells that binds oxygen reversibly.
carboxyhemoglobin
Hemoglobin bound to carbon monoxide; it cannot carry oxygen.
ocean acidification
The ongoing decrease in seawater pH caused by the ocean absorbing CO₂ from the atmosphere.
demineralization
The dissolving of minerals (such as hydroxyapatite in tooth enamel) when conditions shift the solubility equilibrium toward the dissolved ions.
hydroxyapatite
Ca₅(PO₄)₃OH, the main mineral in tooth enamel and bone.
Your body runs on equilibria
Every breath you take depends on a reversible reaction. So does the fizz in pop, the strength of your teeth and the shells of animals in the sea. In this lesson you will use Le Châtelier's principle to explain several equilibria that matter for health and the environment, and to judge what actions people and governments take because of them. (Calculations with buffers, pH and Ksp for these systems are in the unit Acid-Base and Solubility Equilibria.)
Oxygen transport: hemoglobin
Hemoglobin in red blood cells binds oxygen reversibly. A simplified equation is:
Hb(aq) + 4 O₂(aq) ⇌ Hb(O₂)₄(aq)
- In the lungs, the concentration of O₂ is high, so the equilibrium shifts right: hemoglobin loads oxygen.
- In active tissues, cells use up O₂ quickly, so [O₂] is low; the equilibrium shifts left and oxygen is released where it is needed.
The same molecule picks up and drops off oxygen at the right places because the equilibrium responds to concentration.
High altitude
At high altitude the air pressure is lower, so the partial pressure of O₂ is lower and less dissolves in the blood. The hemoglobin equilibrium shifts left and less oxygen is carried: people feel short of breath, tired or dizzy (altitude sickness). Over several weeks, the body adapts by making more red blood cells and hemoglobin. More Hb (a reactant) shifts the equilibrium back to the right, so more oxygen is carried. Some athletes train at altitude for this reason.
Carbon monoxide: a dangerous competitor
Carbon monoxide binds to hemoglobin far more strongly than oxygen (more than 200 times more strongly):
Hb(O₂)₄(aq) + 4 CO(aq) ⇌ Hb(CO)₄(aq) + 4 O₂(aq)
Even a small amount of CO pushes this equilibrium right, so hemoglobin carries CO instead of O₂. Treatment is to remove the person from the CO and give high concentrations of oxygen, sometimes in a pressurized (hyperbaric) chamber. Adding O₂ (a product) shifts the equilibrium left, releasing CO.
Because CO is colourless and odourless, Ontario's Fire Code requires carbon monoxide alarms in homes that have a fuel-burning appliance, a fireplace or an attached garage: beside sleeping areas and, since January 1, 2026, on every storey.
Worked exampleExample 1: Explaining a treatment
A person exposed to carbon monoxide from a faulty furnace is given 100 % oxygen through a mask. Use Le Châtelier's principle to explain why this helps.
- Equilibrium: Hb(O₂)₄ + 4 CO ⇌ Hb(CO)₄ + 4 O₂. The CO exposure has shifted it right.
- Giving 100 % oxygen greatly increases [O₂], a product of this equilibrium.
- The system shifts left to use up some of the added O₂, which releases CO from hemoglobin and forms Hb(O₂)₄.
- The released CO is breathed out, and the person must stay away from the CO source so it does not shift back.
Raising [O₂] shifts the equilibrium left, freeing hemoglobin from CO so it can carry oxygen again. Hyperbaric oxygen (higher pressure) raises the dissolved [O₂] even more.
Carbon dioxide in blood and in the oceans
CO₂ dissolves in water and reacts to form carbonic acid, which partly ionizes:
CO₂(g) ⇌ CO₂(aq); CO₂(aq) + H₂O(l) ⇌ H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq)
- In blood: breathing out removes CO₂, which shifts these equilibria left and lowers [H⁺]. Holding your breath raises [CO₂], shifting them right and increasing [H⁺]. Your body uses this: breathing rate changes help keep blood acidity in a narrow healthy range (the buffer chemistry is studied in Acid-Base and Solubility Equilibria).
- In the oceans: the ocean absorbs a large share of the CO₂ released by burning fossil fuels. More dissolved CO₂ shifts the equilibria right, producing more H⁺. The H⁺ combines with carbonate ions (H⁺ + CO₃²⁻ → HCO₃⁻), lowering [CO₃²⁻]. Animals such as clams, mussels, corals and some plankton build shells from calcium carbonate: CaCO₃(s) ⇌ Ca²⁺(aq) + CO₃²⁻(aq). Less carbonate shifts this equilibrium toward dissolving, so shells are harder to build and maintain. This is ocean acidification. Cold water dissolves more CO₂ than warm water, so polar seas, including Canada's Arctic waters, are especially affected.
Teeth and kidney stones: solubility equilibria in the body
Tooth enamel is mostly hydroxyapatite, which is in equilibrium with ions in saliva:
Ca₅(PO₄)₃OH(s) ⇌ 5 Ca²⁺(aq) + 3 PO₄³⁻(aq) + OH⁻(aq)
Bacteria in plaque turn sugars into acids. The H⁺ reacts with OH⁻ (and with PO₄³⁻), removing products, so the equilibrium shifts right and enamel dissolves (demineralization), which can lead to cavities. Saliva, which contains Ca²⁺ and phosphate, pushes the equilibrium back left between meals (remineralization). Fluoride (in toothpaste and in many communities' drinking water) can replace OH⁻ to form fluorapatite, Ca₅(PO₄)₃F, which resists acid better.
Kidney stones are often calcium oxalate: Ca²⁺(aq) + C₂O₄²⁻(aq) ⇌ CaC₂O₄(s). When the urine is concentrated (for example, from not drinking enough water), the ion concentrations rise until the ion product, Q = [Ca²⁺][C₂O₄²⁻], is greater than the solubility product Ksp, and crystals form. Drinking plenty of water lowers the concentrations and shifts the equilibrium toward the dissolved ions, which is one reason doctors give that advice.
Worked exampleExample 2: Assessing an action
A city council debates whether to keep adding fluoride to its drinking water. Use equilibrium ideas to explain the dental benefit, and name one other factor council should weigh.
- Enamel dissolves when acids remove OH⁻ and shift Ca₅(PO₄)₃OH(s) ⇌ 5 Ca²⁺ + 3 PO₄³⁻ + OH⁻ to the right.
- Fluoride ions replace OH⁻ in the enamel surface, forming fluorapatite, which is less affected by acid, so less enamel dissolves.
- Other factors include: the cost; the dose (too much fluoride in childhood can cause white spots on teeth, called dental fluorosis); evidence from health agencies; access to dental care in the community; and residents' views.
Fluoride makes enamel more resistant to acid attack. A good decision also weighs dose, cost, health-agency evidence and community values. Communities in Canada have reached different decisions on this question.
Assessing impacts
When you assess an equilibrium issue, address: the chemistry (which equilibrium, which way it shifts); the effects on people, other living things and the environment; the evidence and its reliability (who produced it, how recent, peer-reviewed?); and actions that individuals, industries or governments can take.
Just for funHemoglobin is the ultimate delivery driver: it picks up oxygen where it is plentiful, drops it off where it is scarce, and never needs a map. Just Le Châtelier's principle.
Hands-on activityModel ocean acidification with eggshells
You need: shell pieces from two hard-boiled eggs (calcium carbonate), three clear glasses, white vinegar, unflavoured carbonated water, tap water, a kitchen scale if you have one. Safety: wash your hands after handling eggshells; do not drink the test liquids.
- Rinse and dry the shell pieces. Split them into three equal piles (weigh them if you can).
- Cover one pile with vinegar, one with carbonated water and one with tap water. Label the glasses.
- Observe after 10 minutes, 1 hour and 24 hours. Look for bubbles and for softening or thinning. If you have a scale, rinse, dry and reweigh the shells at the end.
- Explain your results using CaCO₃(s) ⇌ Ca²⁺(aq) + CO₃²⁻(aq) and the idea that acid removes CO₃²⁻.
What to look for: vinegar (a stronger acid solution) makes many bubbles (CO₂) and dissolves much of the shell within a day. Carbonated water (carbonic acid from dissolved CO₂) acts much more slowly, with little visible change; tap water does almost nothing. Acid removes carbonate ions, shifting the equilibrium toward dissolving. Seawater is only very slightly less basic than before, but over years, even small shifts affect shell-building animals. Sources of error: unequal shell amounts, and shells that are not fully dry when weighed.
Your turn
1Explain, using Le Châtelier's principle, why hemoglobin releases oxygen in active muscles.
Active muscles use O₂ quickly, so [O₂] is low. The equilibrium Hb + 4 O₂ ⇌ Hb(O₂)₄ shifts left to replace O₂, releasing oxygen from hemoglobin.
2Why do climbers feel short of breath at high altitude, and how does the body adapt?
The partial pressure of O₂ is lower, so less O₂ dissolves and the hemoglobin equilibrium shifts left; less oxygen is carried. Over weeks the body makes more hemoglobin, and the extra Hb (a reactant) shifts the equilibrium right again.
3Why is carbon monoxide so dangerous, and why does breathing pure oxygen help?
CO binds to hemoglobin much more strongly than O₂, so even a little CO shifts Hb(O₂)₄ + 4 CO ⇌ Hb(CO)₄ + 4 O₂ to the right and blocks oxygen transport. Pure oxygen raises [O₂], a product, shifting the equilibrium left and freeing hemoglobin.
4Explain how increasing CO₂ in the atmosphere makes it harder for clams to build shells.
More CO₂ dissolves in seawater, forming carbonic acid and H⁺. H⁺ reacts with CO₃²⁻, lowering its concentration. With less CO₃²⁻, CaCO₃(s) ⇌ Ca²⁺ + CO₃²⁻ shifts toward dissolving, so shells are harder to form and maintain.
5How do acids from plaque bacteria damage teeth?
H⁺ removes OH⁻ (and PO₄³⁻), products of the hydroxyapatite equilibrium, so it shifts right and enamel dissolves.
6Why does drinking more water help prevent calcium oxalate kidney stones?
It dilutes the urine, lowering [Ca²⁺] and [C₂O₄²⁻]. The ion product [Ca²⁺][C₂O₄²⁻] falls below Ksp, so the equilibrium favours the dissolved ions and crystals are less likely to form.
More practice: warm-up
1What protein carries oxygen in the blood?
2Name the main mineral in tooth enamel.
Hydroxyapatite, Ca₅(PO₄)₃OH.
3What gas, absorbed by the ocean, causes ocean acidification?
4Why must homes with gas furnaces in Ontario have CO alarms?
CO is colourless and odourless; alarms detect it before it builds up to dangerous levels. The Ontario Fire Code requires them in homes with fuel-burning appliances, fireplaces or attached garages.
5What compound does fluoride form in tooth enamel?
Fluorapatite, Ca₅(PO₄)₃F.
More practice: on your own
1A person breathes very fast (hyperventilates). Predict the effect on [CO₂] and [H⁺] in the blood.
Fast breathing removes CO₂, so CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ shifts left: [CO₂] and [H⁺] decrease (the blood becomes less acidic).
2Why are cold Arctic waters especially affected by ocean acidification?
Gases are more soluble in cold water, so cold seawater absorbs more CO₂, which shifts the carbonate equilibria further toward H⁺ and away from CO₃²⁻.
3Some sports drinks and pops are acidic. Explain why sipping them slowly all day may harm teeth more than drinking one quickly with a meal.
Each sip lowers the pH in the mouth and shifts the enamel equilibrium toward dissolving. Frequent sipping keeps the mouth acidic for longer, giving saliva less time to shift the equilibrium back (remineralize).
4Evaluate this claim from a website: "Ocean acidification means the oceans will soon be as acidic as vinegar."
Not supported. Seawater is still basic (above pH 7); acidification means its pH is falling (becoming less basic). The concern is that even small shifts lower carbonate levels enough to harm shell-building organisms. Check claims against reliable sources such as government science agencies or peer-reviewed research.
5Describe two careers that deal with the equilibria in this lesson.
Examples: a respiratory therapist (oxygen therapy, CO poisoning), a dental hygienist (enamel and fluoride), an oceanographer or marine biologist (ocean chemistry), a medical laboratory technologist (blood gas tests).
Challenge
1Fetal hemoglobin binds oxygen more strongly than adult hemoglobin. Use equilibrium ideas to explain why this helps a fetus get oxygen from its mother's blood.
In the placenta, both hemoglobins compete for the same dissolved O₂. Because fetal hemoglobin's equilibrium for binding O₂ lies further to the right (larger K), oxygen released by the mother's hemoglobin is taken up by the fetal hemoglobin, moving oxygen from mother to fetus.
2Explain how ocean acidification and Le Châtelier's principle connect a gas in the atmosphere to the shells of animals, in at least four linked steps.
(1) More CO₂(g) shifts CO₂(g) ⇌ CO₂(aq) right. (2) More CO₂(aq) shifts CO₂ + H₂O ⇌ H₂CO₃ right. (3) More H₂CO₃ shifts H₂CO₃ ⇌ H⁺ + HCO₃⁻ right, raising [H⁺]. (4) H⁺ reacts with CO₃²⁻, lowering [CO₃²⁻]. (5) Lower [CO₃²⁻] shifts CaCO₃(s) ⇌ Ca²⁺ + CO₃²⁻ toward dissolving.
3A CO alarm sounds in a home in January. Propose an action plan that uses equilibrium reasoning for the immediate response and prevention.
Immediately: everyone goes outside into fresh air (low CO, normal O₂ shifts the CO-hemoglobin equilibrium back toward oxygen) and calls 911 or the fire department from outside; seek medical help for symptoms (oxygen therapy). Prevention: have fuel-burning appliances and chimneys inspected yearly, never run cars or generators in a garage or indoors, and test CO alarms regularly.
Quiz
1. In the lungs, hemoglobin loads oxygen because:
2. Giving pure oxygen to a person with CO poisoning works by:
3. Ocean acidification makes shell building harder mainly because:
4. Acid from plaque bacteria damages enamel by:
5. Over weeks at high altitude, the body makes more hemoglobin. This:
6. Drinking more water helps prevent kidney stones because:
Big idea (E1.2, E3.3): many biological, environmental and technological systems are equilibria. Le Châtelier's principle explains oxygen transport, altitude adaptation, CO poisoning and its treatment, CO₂ in blood and oceans, tooth decay and kidney stones.
How to help: connect the chemistry to everyday choices: CO alarms (test them together), brushing with fluoride toothpaste, drinking water, limiting sipping of acidic drinks. Ask your teen to name the equilibrium and the direction of the shift each time.
Common difficulties:
- Describing the effect without naming the equilibrium or the direction of the shift.
- Thinking acidified seawater is "acidic" (it is still basic, just less so).
- Mixing up K changes (temperature only) with shifts (concentration).
Safety: if a CO alarm sounds, everyone leaves the home immediately and calls 911 from outside. In the eggshell activity, wash hands and do not drink the test liquids.