E. Solutions and Solubility · Solutions and SolubilityWater: A Polar Molecule with Hydrogen Bonds
E3.1 E2.1 A1.12About 70 minutes
Learning goals- I can explain why a water molecule is polar, using electronegativity and its bent shape.
- I can describe hydrogen bonding between water molecules.
- I can use polarity and hydrogen bonding to explain water's high boiling point, high specific heat capacity, surface tension and the floating of ice.
- I can use the terms polar, dipole, intermolecular force and hydrogen bond correctly.
Words to know
polar molecule
A molecule with a slightly negative end and a slightly positive end, because electrons are shared unequally and the shape does not cancel the pulls.
partial charge (δ⁺, δ⁻)
A small charge, less than a full ion's charge, on part of a molecule. δ is the Greek letter delta.
dipole
A pair of opposite partial charges separated in space, as in a polar molecule.
intermolecular force
An attraction between separate molecules. It is much weaker than a covalent bond inside a molecule.
hydrogen bond
A strong intermolecular attraction between an H atom bonded to O, N or F in one molecule and a lone pair on O, N or F in a neighbouring molecule.
specific heat capacity
The energy needed to warm 1 g of a substance by 1 °C. For liquid water it is 4.18 J/(g·°C).
surface tension
The 'skin' on a liquid's surface, caused by molecules at the surface being pulled inward by their neighbours.
The most unusual ordinary liquid
In January, a lake in Algonquin Park can be covered by ice thick enough for a skating trail, while fish swim in liquid water below. That only works because solid water floats on liquid water. Almost every other substance is denser as a solid than as a liquid, so it would sink.
Water also dissolves a wider range of substances than most other liquids, which is why this whole unit, Solutions and Solubility, starts with water. To understand dissolving, you first need to understand the water molecule itself.
Remember from the Matter, Chemical Trends, and Chemical Bonding unit
Electronegativity (EN) is how strongly an atom pulls on shared electrons in a bond. When two atoms with different EN share electrons, the bond is polar covalent: the atom with the higher EN gets a small negative charge. In Grade 10 you also learned that water is a molecular compound made of two hydrogen atoms covalently bonded to one oxygen atom.
Step 1: the bonds are polar
Oxygen has an electronegativity of 3.44 and hydrogen has 2.20. The difference is
ΔEN = 3.44 − 2.20 = 1.24
That is large enough for each O–H bond to be polar covalent. The shared electrons spend more time near the oxygen. So the oxygen end gets a partial negative charge (δ⁻) and each hydrogen gets a partial positive charge (δ⁺).
Step 2: the shape is bent
Oxygen also has two lone pairs of electrons. The lone pairs push the two bonds down, so the molecule is bent (V-shaped), with an H–O–H angle of about 104.5°.
Because the molecule is bent, the two bond pulls do not cancel. Compare carbon dioxide, O=C=O: its bonds are polar too, but the molecule is straight (linear), so the two pulls point in opposite directions and cancel. CO₂ is a non-polar molecule even though it has polar bonds.
A molecule is polar when it has polar bonds AND a shape that does not cancel them. Water is polar. It is a dipole: one negative end, one positive end.
Step 3: water molecules stick to each other
Opposite charges attract. The δ⁺ hydrogen of one water molecule is attracted to a lone pair on the δ⁻ oxygen of a neighbouring molecule. This special, strong intermolecular attraction is called a hydrogen bond.
Hydrogen bonds form only when hydrogen is bonded to one of the three most electronegative small atoms: N, O or F. That makes the hydrogen very positive and lets it get very close to a lone pair on another molecule.
Keep two ideas separate:
- The O–H covalent bond is inside one molecule. It is strong. Boiling water does not break it; steam is still H₂O.
- A hydrogen bond is between molecules. It is roughly 5 to 10 percent as strong as a covalent bond, but there are a huge number of them.
Each water molecule can form up to four hydrogen bonds (two through its H atoms and two through its lone pairs). In liquid water these bonds are constantly breaking and re-forming. You will study intermolecular forces in more detail in the Gases and Atmospheric Chemistry unit.
What hydrogen bonding explains
1. A very high boiling point for such a small molecule. To boil water you must pull molecules apart, which means breaking hydrogen bonds. Methane (CH₄) has almost the same molar mass as water but no hydrogen bonds and boils at −162 °C. Hydrogen sulfide (H₂S) is heavier than water, but sulfur is not electronegative enough for strong hydrogen bonds, and it boils at −60 °C. Water boils at 100 °C.
2. A high specific heat capacity: 4.18 J/(g·°C). Energy added to water goes partly into breaking hydrogen bonds instead of speeding up molecules, so water warms and cools slowly. Large lakes like Lake Ontario keep nearby towns cooler in early summer and milder in early winter.
3. Surface tension. A molecule inside the liquid is pulled equally in all directions. A molecule at the surface is pulled inward and sideways only. The surface acts like a stretched skin, strong enough to hold up a water strider.
4. Ice floats. As water freezes, hydrogen bonds lock the molecules into an open, hexagonal pattern with a lot of empty space. So ice (about 0.92 g/mL) is less dense than liquid water (about 1.00 g/mL). Liquid water is densest at about 4 °C, so in winter the 4 °C water sinks to the bottom of a lake and ice forms on top, insulating the water below.
5. Water dissolves many substances. Its δ⁺ and δ⁻ ends can attract ions and other polar molecules. That is the topic of the next three lessons.
Worked exampleExample 1: Is the molecule polar?
Decide whether each molecule is polar: (a) H₂O (bent), (b) CO₂ (linear), (c) NH₃ (pyramid shape, N has one lone pair). Electronegativities: H 2.20, C 2.55, N 3.04, O 3.44.
- Step 1: check the bonds. O–H: ΔEN = 1.24, polar. C=O: ΔEN = 3.44 − 2.55 = 0.89, polar. N–H: ΔEN = 3.04 − 2.20 = 0.84, polar.
- Step 2: check the shape. Do the bond pulls cancel?
- (a) H₂O is bent. Both O–H pulls point partly 'down' toward the H atoms, so they add up. Polar.
- (b) CO₂ is linear: O=C=O. The two pulls point in exactly opposite directions and cancel. Non-polar, even though the bonds are polar.
- (c) NH₃ is a pyramid with the lone pair on top. The three N–H pulls all point partly downward and do not cancel. Polar.
H₂O and NH₃ are polar; CO₂ is non-polar because its linear shape cancels its bond dipoles.
Worked exampleExample 2: Explaining a property
Explain, using hydrogen bonding, why it takes a lot of energy to boil a kettle of water compared with boiling the same mass of a liquid like hexane (C₆H₁₄), which has no O–H bonds.
- Name the particles and forces: water molecules are held to each other by hydrogen bonds (plus weaker attractions). Hexane molecules are non-polar and attract each other only through weak London dispersion forces (temporary attractions all molecules have).
- Boiling separates molecules, so it breaks the intermolecular forces, not the covalent bonds inside the molecules.
- Water's many hydrogen bonds need more energy to break per gram than hexane's weaker attractions.
- Write a clear claim-evidence-reasoning answer.
Boiling means separating molecules. Water molecules are held together by many strong hydrogen bonds, so more energy per gram is needed to separate them than for hexane, whose molecules attract each other only weakly. (Water's covalent O–H bonds are not broken; steam is still H₂O.)
Just for funA hydrogen atom walks into a bar and says, "I think I lost an electron." The bartender asks, "Are you sure?" It replies, "I'm positive."
In water, hydrogen is only partly positive, so it should say "I'm δ⁺ sure."
Hands-on activityHow many drops fit on a coin?
You need: a quarter (or a loonie), a medicine dropper or a drinking straw, a glass of tap water, dish soap, a paper towel and a notebook.
- Dry the coin and lay it flat on the paper towel.
- Predict how many drops of water will fit on the coin before the water spills over.
- Add water one drop at a time, counting, and watch from the side. Record the count when the water spills.
- Dry the coin. Stir one drop of dish soap into the glass of water, and repeat with the soapy water.
- Repeat each trial three times and find the average (that is good practice for reliable data).
What to look for: plain water builds into a high dome, often 20 or more drops on a quarter (your count depends on drop size). Soapy water spills much sooner. Explanation: hydrogen bonds pull surface molecules inward, giving water a strong surface tension. Soap molecules crowd onto the surface and get between water molecules, so fewer hydrogen bonds hold the surface together. (Wash your hands after handling coins.)
Your turn
1Calculate ΔEN for an O–H bond and classify the bond. (EN: O 3.44, H 2.20)
ΔEN = 3.44 − 2.20 = 1.24, so it is a polar covalent bond.
2Why is CO₂ non-polar even though each C=O bond is polar?
CO₂ is linear. The two bond dipoles are equal and point in opposite directions, so they cancel.
3Which atom in water carries the partial negative charge, and why?
Oxygen, because it is more electronegative and pulls the shared electrons closer to itself.
4What is a hydrogen bond? Is it inside a molecule or between molecules?
An attraction between a δ⁺ H (bonded to N, O or F) on one molecule and a lone pair on N, O or F of another molecule. It is between molecules.
5When water boils, which is broken: O–H covalent bonds or hydrogen bonds?
Hydrogen bonds (and other intermolecular attractions). The molecules stay as H₂O; steam is still water.
6Explain why a lake freezes from the top down.
Water is densest at about 4 °C, so the coldest water near 0 °C stays on top, and ice is less dense than liquid water because hydrogen bonds hold it in an open pattern. So ice forms and floats on the surface.
7Why does a large lake warm up slowly in spring?
Water has a high specific heat capacity (4.18 J/(g·°C)) because added energy goes into breaking hydrogen bonds, so a lot of energy is needed to raise its temperature.
More practice: warm-up
1What does the symbol δ⁺ mean?
A partial positive charge: smaller than the charge of an ion.
2What is the H–O–H bond angle in water?
3How many lone pairs does the oxygen in water have?
4Name the three elements that H must be bonded to for hydrogen bonding.
Nitrogen, oxygen and fluorine (N, O, F).
5True or false: ice is denser than liquid water.
False. Ice is about 0.92 g/mL, less dense than liquid water (about 1.00 g/mL), so it floats.
More practice: on your own
1Calculate ΔEN for the N–H bond (N 3.04, H 2.20). Is NH₃ polar? Explain.
ΔEN = 0.84, polar bonds. NH₃ is a pyramid with a lone pair on N, so the bond dipoles do not cancel: NH₃ is polar.
2Methane (CH₄) has a molar mass of 16.05 g/mol and water 18.02 g/mol. Explain why their boiling points differ by about 260 °C.
Water molecules form hydrogen bonds with each other; methane molecules are non-polar and have only weak dispersion forces. Much more energy is needed to separate water molecules, so water boils at 100 °C and methane at −162 °C.
3A water strider walks on a pond. Explain using particles.
Surface water molecules are pulled inward and sideways by hydrogen bonds to their neighbours, creating surface tension, a skin strong enough to support the light insect.
4Explain why adding soap lets a floating paper clip sink.
Soap molecules gather at the surface and come between water molecules, reducing the hydrogen bonding at the surface, so surface tension drops and can no longer hold the clip up.
5Use the properties of water to explain why coastal towns near Lake Superior often have cooler summers than towns farther inland.
The huge lake has a high specific heat capacity, so it warms slowly in spring and summer and keeps the air near it cooler. (It also cools slowly in fall, keeping early winter milder.)
Challenge
1Hydrogen fluoride, HF, boils at about 20 °C; hydrogen chloride, HCl, boils at about −85 °C. HCl has the larger molar mass. Explain the difference.
F is the most electronegative element, so H–F is very polar and HF molecules form hydrogen bonds. Cl is not electronegative enough (and larger), so HCl has no strong hydrogen bonds, only weaker dipole and dispersion attractions. Stronger intermolecular forces mean a higher boiling point, even though HCl is heavier.
2Explain why a water molecule can form up to four hydrogen bonds but an HF molecule forms, on average, fewer.
Water has two δ⁺ H atoms and two lone pairs on O, so it can donate two and accept two hydrogen bonds. HF has only one H per molecule. Although F has three lone pairs, each molecule can donate only one hydrogen bond, so on average each HF molecule takes part in only about two (one donated, one accepted), forming zig-zag chains instead of a 3-D network.
3Sketch-in-words: describe what would happen to lakes in a Canadian winter if ice were denser than liquid water.
Ice would sink as it formed, so lakes would fill with ice from the bottom up. Many shallow lakes could freeze solid, with no insulating ice layer on top, and fish and other organisms could not survive the winter in liquid water underneath.
Quiz
1. Why is a water molecule polar?
2. Which is a hydrogen bond?
3. When ice melts, which bonds are partly broken?
4. Which molecule is non-polar even though its bonds are polar?
6. Water's specific heat capacity is 4.18 J/(g·°C). What does this mean?
Big idea (E3.1): water's special properties come from two facts: its bonds are polar and its shape is bent, so molecules attract each other through hydrogen bonds.
How to help: Ask your teen to explain one everyday thing using hydrogen bonding: ice cubes floating, a kettle taking time to boil, beads of water on a car. A good answer names the particles (water molecules), the force (hydrogen bonds between molecules) and links it to the observation.
Common difficulties:
- Mixing up the covalent O–H bond (inside a molecule) with the hydrogen bond (between molecules). Boiling does not split water into hydrogen and oxygen.
- Thinking any molecule with polar bonds is polar. Shape matters: CO₂ is non-polar.
- Calling any bond with H in it a 'hydrogen bond'.
The coin activity is safe; just wash hands afterwards.