Chapter 11 Water and Solutions

Water The universal solvent. It has the ability to dissolve most molecules. In living systems these molecules can then be transported from one place to another by diffusion or by some kind of a circulatory system. Liquid water has a higher density than ice (solid water). Ice thus floats on the surface of liquid water. Fish and other organisms can then live below the ice in natural bodies of water. Water has a very high specific heat. Therefore, large bodies of water can moderate temperature by absorbing great amounts of heat. Water has a very high latent heat. This means that a great amount of heat is needed to evaporate water. This is what occurs when people perspire, we get rid of a lot of heat from our bodies, thus enabling us to withstand high temperatures.

Bonds in the Water Molecule The H-O bonds in the water molecule are polar covalent. The oxygen atom attracts the electrons more than the hydrogen atoms because it is more electronegative. Since the molecule has a “V” shape: O Negative end H H Positive end The oxygen end of the molecule is therefore more negative and the hydrogen end of the molecule is more positive. The molecule is said to be polar and thus possesses a dipole (a negative and a positive end). As a result it can dissolve many ionic compounds, which have a positive ion and a negative ion which are attracted to the opposite charge in the two sides of the water molecule. The negative ion will be attracted to the positive side (the hydrogens), and the positive ion will be attracted to the negative side (the oxygen).

Acids, Bases, and Salts Salts are ionic compounds. The hardness of water is related to ionic compounds or salts dissolved in water. The acidity of soils determines how well plants grow. Acid rain is a by product of industry and automobiles. This is harmful for living organisms.

Acids, bases and salts Acids have sour tastes Changes litmus paper from blue to red Acids react with metals releasing hydrogen gas Acids neutralize bases forming water and salt

Bases have bitter taste (caffeine) Bases turn litmus paper blue Basic solutions feel slippery on skin. Bases neutralize acids forming water and salts Acids, bases and salts

Acid - any substance that is a proton donor when dissolved in water. Hydronium ion This is the same as: H 2 O HCl H + (aq) + Cl - (aq) A proton is a hydrogen ion, H +

Bases Many bases are ionic compounds which contain hydroxide ion (OH - ) as the negative ion. They produce OH - when dissolved in water: NaOH Na + (aq) + OH - (aq) Proton acceptor A proton is a hydrogen ion (H + ) H2OH2O

Acids, bases and salts Base - Any substance that is a proton acceptor when dissolved in water. Hydroxide ion Other bases, like ammonia, NH 3, will produce the OH - once it is dissolved in water, since a hydrogen ion gets transferred from a water molecule to the ammonia molecule. H + OH - Proton acceptor

What happens when acids and bases mix? Acids, bases and salts This is called neutralization H + (aq) + OH - (aq) H 2 O (l) The H + from the acid and the OH - from the base react to form water:

Strong Acids A strong acid is one that ionizes (dissociates) 100% in aqueous solution. HNO 3 H 2 O H + (aq) + NO 3 - (aq) 100% Acids can be recognized because the formula begins with “H”. Common strong acids include: HCl hydrochloric acid HBr hydrobromic acid HI hydroiodic acid H 2 SO 4 sulfuric acid HNO 3 nitric acid

Weak Acids Weak acids only partially ionize (dissociate) in aqueous solution. They only produce a small percentage of hydrogen ions (H + ). HF H 2 O H + (aq) + F - (aq) not 100% A double arrow indicates an equilibrium, so the reaction occurs both ways, to the right and to the left. The arrow to the left is longer because there is more of the non ionized acid present than of the ions at any given time.

Weak Acids Common weak acids include: HC 2 H 3 O 2 acetic acid H 2 CO 3 carbonic acid HC 2 H 3 O 2 H 2 O H + (aq) + C 2 H 3 O 2 - (aq) In the case of acetic acid, only about 1% of the acid molecules ionize in aqueous solution.

Strong Bases Strong bases are those that completely (100%) ionize (dissociate) in aqueous solution. NaOH H 2 O Na + (aq) + OH - (aq) 100% Common strong bases include: NaOH sodium hydroxide, common name lye, which is used to make soap. KOH potassium hydroxide Also the other Group IA and Group IIA hydroxides except for Be(OH) 2 and Mg(OH) 2.

Weak Bases Weak bases only partially ionize (dissociate) in aqueous solution. Mg(OH) 2 Mg 2+ (aq) + 2OH - (aq) not 100% NH 3(aq) + H 2 O NH 4 + (aq) + OH - (aq) not 100%

The pH Scale The strength of acids and bases is measured using the pH scale. A neutral substance, like water, which is neither an acid nor a base, has a pH of 7. Acidic solutions (acids dissolved in water) have a pH lower than 7. Basic solutions (bases dissolved in water) have a pH higher than 7.

Salts When an acid reacts with a base a salt (ionic compound, except bases) is formed. Water is also formed: HCl + NaOH NaCl + H 2 O acid base salt water 2HNO 3 + Ca(OH) 2 Ca(NO 3 ) 2 + 2H 2 O acid base salt water Neutralization always occurs 100% between an acid and a base even if they are weak.

Importance of Salts Salts are important in the diet, since we need them as electrolytes and as a source of certain elements. These are minerals. Plants also require the elements that can be provided by certain salts and are added to plants as fertilizers. For example, K, N (supplied as nitrates, NO 3 - ), and P (supplied as phosphates, PO 4 3- ). Not all salts are soluble in water. Insoluble salts is what gives rise to hard water that ends up forming rings in your bathtub because when mixed with soap insoluble salts are formed.

Electrolytes Electrolytes are substances or solutions which conduct electricity. Often these will be aqueous solutions. In order to conduct electricity there has to be a flow of electrons through the solution. In order for there to be a flow of electrons through a solution there have to be ions in the solution. The more ions there are the stronger the electrolyte will be.

Only ionic compounds Or acids dissolved in water will produce ions in solution and electricity can be then conducted through the Solution.

Ionic Compounds dissolved in Water NaCl (s) H 2 O Na + (aq) + Cl - (aq) CaCO 3(s) H 2 O CaCO 3(s)

Non Electrolytes All covalent compounds except for acids and ammonia are non electrolytes, since no ions are present in solution. Water is a covalent compound, so it is a non electrolyte, so are all alcohols and most other substances which are liquids or gases at room temperature. Acids are unique, since they are covalent compounds but they ionize when dissolved in water. All acids are soluble in water.

Practice Exercises p Applying the Concepts: # 20, 21, 22, 23, 24, 25, 26, 27 New Book: p # 3, 9, 12, 13, 14, 15, 16, 18, 32, 33, 37, 38, 39, 40, 41, 42, 43, 44, 46, 49.

Review for Chapter 11 (cont.) Water is polar and is able to dissolve many ionic compounds. It also dissolves most acids and many bases. Ionization of strong and weak acids in aqueous solution (100% for strong acids, equilibrium for weak acids). (Strong acids are sulfuric, nitric, hydrochloric, hydrobromic, and hydroiodic acids. The rest are weak acids.) Ionization of strong and weak bases in aqueous solution.(100% for strong bases, equilibrium for weak bases). (Strong bases are the group IA and IIA (alkali metals and alkaline earth metals, except Mg(OH) 2 and Be(OH) 2 ) combined with hydroxide ion. Properties of acids and bases. How ammonia, NH 3 is a base because it reacts with water to form NH 4 + and OH - in aqueous solution. Neutralization reactions: Acid + Base react to produce salt (ionic compound) plus water. The pH scale: is neutral, like water. Less than7 is acidic, higher than 7 is basic. The importance of salts: They provide electrolytes to living organisms, including plants, animals and humans and they are a source of some essential elements.