Week 2 · 1-2 Chemical Properties of Water: the water molecule, polarity, hydrogen bonds and boiling point, water as a universal solvent (NaCl), ionic equilibrium, hydrolysis of salts, pH and its measurement · 3 lessons
1
The water molecule: shape, polarity and hydrogen bonds
Integrated Science Sec 1 Term 1, Unit 1: pp. 8–15
GoalDescribe the shape of the water molecule, explain why it is polar, and say what a hydrogen bond is and why it makes water boil at a high temperature.
New words
covalent bond · a bond in which two atoms share electronspolar molecule · a molecule with a slightly negative end and a slightly positive end, because its electrons are shared unequallyhydrogen bond · an attraction between the slightly negative oxygen of one water molecule and the slightly positive hydrogen of a neighbouring molecule
Explanation
A water molecule has one oxygen atom and two hydrogen atoms joined by two single covalent bonds, and the angle between the bonds is about 104.5°, so the molecule is bent, not straight. By mass, oxygen makes 16 of every 18 parts (88.89%) and hydrogen 2 of every 18 parts (11.11%), so hydrogen and oxygen are in a 1 : 8 ratio by mass, not the 2 : 1 you get by counting atoms. Oxygen has a higher electronegativity: it pulls the shared electrons closer to itself. So oxygen carries a slight negative charge (δ−) and each hydrogen a slight positive charge (δ+), and the molecule is polar, although it has no charge overall. The δ− oxygen of one molecule attracts the δ+ hydrogen of a neighbour: this is a hydrogen bond. It is much weaker than a covalent bond and it works between molecules, while covalent bonds are inside them. Each molecule can make up to four hydrogen bonds, so liquid water is a network. This network explains why water (molecular mass 18) boils at 100 °C, while hydrogen sulphide (mass 34) boils at about −61 °C.
Worked examples
How much oxygen and hydrogen are in 36 g of water? The mass ratio hydrogen : oxygen is 1 : 8, so water is 9 parts in all. One part is 36 ÷ 9 = 4 g. Hydrogen is 1 part = 4 g and oxygen is 8 parts = 32 g. Check: 4 g + 32 g = 36 g. In the same way 90 g of water holds 10 g of hydrogen and 80 g of oxygen.
Water has a smaller mass (18) than hydrogen sulphide (34), yet it boils 161 degrees higher: 100 − (−61) = 161 °C. Usually a heavier molecule boils higher, so something else must hold water molecules together. It is the hydrogen bonds, which need much energy to break.
Is each of these a covalent bond or a hydrogen bond? (a) between O and H inside one water molecule; (b) between the oxygen of one molecule and a hydrogen of the next. (a) is covalent, (b) is a hydrogen bond. A way to remember: covalent is inside the molecule, hydrogen bonds are between molecules.
Class activity
Bent-molecule model, with a grown-up. Make a water molecule from one large bead (oxygen) and two small beads (hydrogen) on two paper strips bent at about 105°. Make three of them. Put them side by side so that the oxygen of one is near a hydrogen of the next, and join those near pairs with dotted lines: these are hydrogen bonds. Then put a drop of water on a coin and add drops one at a time. Describe how many drops stay on the coin and what shape the water makes.
Practice
1
In the water molecule, which atom carries the slight negative charge, and why?
Oxygen, because it has the higher electronegativity and pulls the shared electrons closer.
2
Find the mass of hydrogen and of oxygen in 90 g of water.
90 ÷ 9 = 10, so hydrogen is 10 g and oxygen is 8 × 10 = 80 g.
3
A water molecule is polar. Does that mean it has an overall electric charge? Explain.
No. It has a slightly negative end and a slightly positive end, but the two cancel, so the whole molecule is neutral.
4
Why does water boil at 100 °C although hydrogen sulphide, a heavier molecule, boils at about −61 °C?
Hydrogen bonds hold water molecules together, so much more energy is needed to separate them.
