Water: Essential to life

1. Water: Essential to life Chapter 10

2. A molecule essential to life • Water is the most abundant liquid on earth, covering over 70% of the planet

3. The water cycle

4. The reactions of life • Photosynthesis 6H20(l) + 6CO2 (g) C6H1206 (s) + 6O2 (g) • Respiration C6H1206 (s) + 6O2 (g) 6H20(l) + 6CO2 (g) Without these two reactions life on earth could not be sustained. Water is essential for both of these reactions Chlorophyll Sunlight

5. Other functions of water • It provides a system to transport nutrients and soluble wastes. • It dissolves a range of substances which are transported around the body. • It provides a system to transfer heat. • Water can store large amounts of heat energy, it transports heat energy from cells to the body surface where it can be removed. • It cools the body. • When water evaporates from the skin, it absorbs a large amount of heat energy from the body allowing the body to cool down.

6. Properties of water • Water is a covalent molecular compound. • Molecular formula H2O. • The O-H bonds are polar, with the O atom having the larger share of the bonding electrons. • The forces between water molecules are hydrogen bonds, which are strong in comparison to other types of intermolecular bonds

7. Water is unique • Water is the only substance commonly found in all three states on earth. Looking at this graph can you think of another reason that water is so unique.? • Water exists as a liquid over • a temperature range commonly • found on earth. • The melting and boiling temp • of water are significantly higher • than those of other molecules • of similar size.

8. Why is this so? • In ice each molecule forms hydrogen bonds to FOUR other molecules. • A lot of energy is required to break these four bonds. • When ice melts enough energy is added to break some of these bonds. • When water is boiled, all the hydrogen bonds are broken. • A significant amount of energy to overcome these strong hydrogen bonds.

9. High latent heat values • Latent heat measures the energy needed to change the state of a substance at its melting or boiling temperature: • Latent heat of fusion is the amount of energy needed to change a fixed amount of water from a solid to a liquid at 0°C • Latent heat of vaporisation of water is the amount of energy needed to change a fixed amount of water from liquid to a gas at 100°C

10. Your Turn • Page 194 • Question 3 • Question 4 • You may have to think about 4c

11. 4c • As it freezes, water expands, unlike most liquids. This is because of hydrogen bonding. Each molecule is surrounded by four others in what is almost a crystal-type situation. (See graph below, which shows the variation in density of water with temperature.) Therefore, ice is less dense than liquid water, and it floats on liquid water. (For most liquids, the solid is denser than the liquid.) This is good news for fish, but not good news for travellers on the Titanic!

12. Solutes and solvents • When a solid, liquid or gas dissolves in water, an aqueous solution is formed. • The dissolved substance is the solute and the water is the solvent.

13. Solutions • Solutions have the following important characteristics: • They are homogenous; that is, the solute and the solvent can not be distinguished from one another. • The dissolved particles are too small to see. • The proportion of dissolved solutes varies from one solution to another.

14. Water as a solvent • The polar nature of water molecules enables water to dissolved a large number of substances. • Due to this water is not found pure in nature. • Not all substances dissolve in water, however. Oils, fats and other non-polar substances will not dissolve in water. Also many gases have low solubility in water.

15. What dissolves and what doesn’t • When one substance dissolves in another, the following process occur: • The particles of the solute are separated from one another. • The particles of the solvent are separated from one another. • The solute and solvent particles attract each other. • A solute will dissolve if the attraction between the solute and solvent particles is strong enough to compete wit the solute-solute and solvent-solvent forces of attraction.

16. Substances that dissolve • Polar covalent compounds that can form hydrogen bonds with water. • Polar covalent molecular compounds that ionise. • Ionic compounds.

17. Polar covalent compounds that can form hydrogen bonds with water • When ethanol dissolves in water: • Hydrogen bonds between water molecules are broken • Hydrogen bonds between ethanol molecules are broken • Hydrogen bonds form between ethanol and water molecules. C2H5OH(l) C2H5OH(aq) H2O

18. Non-Polar molecules • Non-polar substances are insoluble in water because the water molecules form hydrogen bonds with each other in preference to the weak attractions with non-polar molecules. • The larger the non-polar molecule is the less soluble it is in water.

19. Comparing Vitamins

20. Polar covalent molecular compounds that ionise • Some compounds have one of more covalent bonds that are so highly polarised that they break when the compounds is placed in water. • Such bonds break as a result of the electrostatic forces of attraction between the solute molecules and the water molecules.

21. Polar covalent molecular compounds that ionise • This attraction results in the H-Cl bond breaking, both electrons go with the chlorine atom and a H+ ion forms a covalent bond with water. • This process is called ionisation and can be represented by the following equation. HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) • A H3O+ ion is called a hydronium ion.

22. In summary when polar covalent molecules ionise in water. • Polar covalent bonds within molecules are broken, producing hydrogen ions and anions. • A covalent bond forms between each H+ and a H2O molecule giving H3O+ ions. Ion-dipole attractions between the newly formed ions and the polar water molecules are formed. • Other polar covalent molecular compounds that ionise in water include the common acids nitric acid, sulfuric acid and ethanoic acid.

23. Ionic compounds • Ionic solids contain +ve and –ve ions held in a 3D lattice by strong electrostatic forces. • When an ionic solid such as NaCl is placed in water, the +ve ends of the water molecule are attracted to the –ve chloride ions. The –ve ends of the water molecule are attracted to the +ve sodium ions.

24. Ionic Compounds • The partially charged water molecules start pulling the sodium and chloride ions on the outer part of the ionic lattice apart and dragging them into the surrounding solution.

25. Dissociation • When an ionic compound dissolves in water, positive and negative ions in an ionic lattice are separated from one another. • This process is known as dissociation. • It can be represented by the equation: NaCl(s) Na+(aq) + Cl-(aq) H20

26. Dissociation Equations • Dissociation equations must balance. • The number of elements on each side must balance. • The charges on each side must balance. • The equation breaks into its cation and anion in the presence of water. • For example: K2CO3(s) 2K+(aq) + CO32-(aq) H20

27. Insoluble ionic substances • Not all ionic substances are soluble, limestone (CaCO3) is not soluble. • For insoluble ionic substances, the energy required to separate the ions from the lattice is much greater than the energy released when the ions are dissolved in water. • The ions tend to stay in the lattice.

28. So what is soluble? All group 1 metals are soluble.

29. Worked example 10.4a • Predict, giving reason, whether each of the following substances is likely to be water soluble. • Copper(II) nitrate (Cu(NO3)2) • Glycerol (CH2(OH)CH(OH)CH2(OH) • Octane (C8H18) • Nitric acid (HNO3) • Barium carbonate (BaCO3)