WATER, ph, ACIDS, BASES, AND BUFFERS

Transcription

1 COURSE READINESS ASSESSMENT FOR PHYSIOLOGY WATER, ph, ACIDS, BASES, AND BUFFERS

2 Sections in this module I. Water is a polar molecule II. Properties of water III. ph IV. Acids and bases V. Buffers

3 I. Water is a polar molecule

4 Water is a polar molecule Chemical formula: H 2 O It is a polar molecule: Atoms are held together by polar covalent bonds. This gives the oxygen a slight negative charge (δ - ) and each hydrogen a slight positive charge (δ + ). Hydrogen atoms are attached to the oxygen at an angle H O δ + δ - H δ + Maksim/ Wikimedia Commons / CC-BY-SA-3.0 / GFDL

5 Hydrogen bonds Polar molecules can form them The slight charges of a polar molecule will attract opposite slight charges of other polar molecules A single hydrogen bond is weak, but many hydrogen bonds together can be strong! Michal Maňas/ Wikimedia Commons / CC-BY-SA-3.0 / GFDL

6 II. Properties of water

7 Some important properties of water Water is an essential reactant in chemical reactions that build and break down biological molecules Water has special properties because it is a polar molecule and can form hydrogen bonds: Water molecules are cohesive and adhesive Water has a high heat capacity Water is an excellent solvent

8 Water: Cohesion and adhesion Water molecules are cohesive: Hydrogen bonds holds water molecules to other water molecules. This cohesion is exhibited by the surface tension of liquid water At the surface of liquid water, the water molecules make hydrogen bonds with each other but not with molecules in the air This hydrogen bonding between water molecules forms a skin at the surface of liquid water. A drop of water forms a bead because of hydrogen bonding. Time3000/ Wikimedia Commons / CC-BY-SA-3.0 / GFDL

9 Water molecules are adhesive: Hydrogen bonds can form between water and other polar molecules. Example: The water molecules in a glass are not only attracted to each other, they are also attracted to the molecules of the glass The adhesion of water allows liquid water to travel through a dry paper towel by capillary action Surface tension and capillary action help to pull water up the length of a plant or tree, from the roots to its leaves!

10 Water: High heat capacity Hydrogen bonding is also responsible for water s high heat capacity. This means that the temperature of water is slow to raise or lower. Example: For liquid water to boil, the water molecules must be moving fast enough to break the hydrogen bonds holding them together so they can evaporate. It takes a lot of heat to get water molecules to move that quickly! Life can exist on most of the earth because of water s high heat capacity.

11 Water: The solvent of life The slight charges of a water molecule are attracted to other the charges of other atoms or molecules. This makes water an excellent solvent for: Ions, such as Na +, K +, Ca 2+, Cl -. Thus, salts dissolve in water. Polar molecules, such as the ones required for life: carbohydrates,nucleic acids, and proteins. Polar molecules are said to be water soluble or hydrophilic because their slight charges allow them to interact with water.

12 Example: Table salt (NaCl) dissociates, or breaks apart to form ions, in water. NaCl Na + + Cl - The slight charges of water are attracted to the positive charge of Na + and the negative charge of Cl -, breaking the ionic bond between them. Nonpolar molecules lack charges, so they are insoluble in water, or hydrophobic. An aqueous solution is a solution where water is the solvent

13 Ionic compounds: Dissociation Other molecules can dissociate in water to form ions. Some examples: NaNO 3 Na + + NO 3 - sodium nitrate sodium ion nitrate H 2 SO 4 2 H + + SO 4 2- sulfuric acid hydrogen ion sulfate NaHCO 3 Na + + H + + CO 3 2- sodium bicarbonate (sodium hydrogen carbonate) carbonate

14 Ionic compounds like these dissociate to form cations and anions A cation is a positively charged ion H + and Na + are cations An anion is a negatively charged ion Cl -, NO 3-, SO 2-4, and CO 2-3 are anions

15 III. ph

16 Dissociation of water In liquid water, a very small number of water molecules (1 in every 10 7 molecules) dissociate to form ions. The hydrogen of a water molecule is pulled off because of its attraction to the oxygen of another water molecule: H 2 O + H 2 O H 3 O + + OH - hydronium ion hydroxide ion This can be simplified and rewritten as: H 2 O H + + OH - hydrogen ion (proton)

17 ph We can measure the concentration of hydrogen ions or protons (H + ) in an aqueous solution. This measurement is a property called ph. Square brackets [ ] mean concentration of Therefore, the ph of a solution relates to [H + ] of that solution

18 ph scale ph values range from 0 to 14. The values are positive and do not have to be whole numbers. A ph of 7 is a neutral ph. Distilled water has a ph of 7. An acid is a solution with a ph less than 7. Lower ph values are more acidic. A base is a solution with a ph greater than 7. Higher ph values are more basic or alkaline. Edward Stevens/ Wikimedia Commons / CC-BY-SA-3.0 / GFDL

19 ph and [H + ] The ph of a solution is inversely related to its [H + ] Basically, the higher a solution s [H + ], the lower its ph. The lower the [H + ], the higher the ph. Example: Solution A has a ph of 2. Solution B has a ph of 6. Since ph is inversely related to [H + ], Solution A has a higher [H + ] than Solution B. You can also say that Solution A is more acidic than Solution B, or that Solution A is a stronger acid than Solution B.

20 The ph of a solution is calculated as follows: ph = - log [H + ] Note that log is the base-10 logarithm. expressed as a molarity (Units: M,or molar ). [H + ] is usually Example: Solution A has a [H + ] = 1 x 10-8 M. (1 x 10-8 = ) Thus, ph = - log (1 x 10-8 ) = 8 The logarithm isolates the exponent of 10, which is -8. The negative sign in the equation turns this into a positive value.

21 IV. Acids and bases

22 Definitions of acid and base The Bronsted-Lowry definitions are standard definitions: An acid is a H + or proton donor A base is a H + or proton acceptor Example: In the chemical reaction HCl + NH 3 Cl - + NH 4 + HCl releases (donates) a H + so it is the acid NH 3 accepts the H + and so it is the base

23 Neutralization reactions These chemical reactions mix an acid with a base and produce a salt and water. They essentially cancel out the acidic and basic phs. Example: Mixing HCl (an acid) and NaOH (a base) HCl + NaOH NaCl + H 2 O Your stomach contains HCl (ph = 1-2) in it. If your stomach is too acidic, you can ingest sodium bicarbonate (baking soda, ph = 8.5) to neutralize some of the stomach acid. Products like baking soda are called antacids ( anti-acids ).

24 V. Buffers

25 What is a buffer? A buffer is a solution that is a mixture of two components: A weak acid A weak base A buffer maintains the ph of a solution, resisting ph changes: The weak acid donates H + if a base is added to the solution. This prevents a ph increase. The weak base accepts H + if an acid is added to the solution. This prevents a ph decrease.

26 Example: Blood must stay within the ph range of , preferably Death can result if ph becomes too acidic (at ph 6.8) or too alkaline (at ph 7.8). However, CO 2 is constantly released by the cells, making the blood more basic. Certain nutrients and lactic acid can make the blood more acidic.

27 There is a buffer in blood that consists of carbonic acid (weak acid) and bicarbonate (weak base): H 2 CO 3 H + + HCO 3 - If the blood is too acidic, the excess H + will be absorbed by the bicarbonate (HCO 3- ), raising the ph. If the blood is too basic (alkaline), then H + will be released by carbonic acid (H 2 CO 3 ), lowering the ph.

28 Summary of Water, ph, Acids, Bases, and Buffers Water is a polar molecule Polar molecules like water are able to form hydrogen bonds Water has special properties: It is cohesive and adhesive, it has a high heat capacity, and it is an excellent solvent Water can dissociate to form ions. Aqueous solutions can be described according to their [H+]. This is a solution s ph. An acid has a ph less than 7 and is a proton donor A base has a ph greater than 7 and is a proton acceptor A buffer is a mixture of weak acid and weak base that maintains the ph of a solution, resisting ph changes