What Chemistry Is All About

Chapter 5 Acids, Bases & Body Chemistry

Chemistry in Society

Review Terms: homogeneous mixtures, solutions, solvent, and solute Chemical reactions are very important in chemistry-one very common one is called acidbase reactions. In acid base reactions most of the reactions are in water or aqueous solutions: where water is the solvent. Therefore aqueous solutions are water solutions. Solute particle may be ions or molecules  Ionic solutions (Electrolytes)-conduct electricity  Molecular solutions (Non-electrolytes)-do not conduct electricity

Ionic dissociation-the separation of ions of a solute when the substance is dissolved. e.g. sodium chloride dissolving in water

NaCl  Na+(aq) + Cl-(aq)

Ionization-when a molecular solute dissolves in water to produce ions. E.g. HCl is composed of covalent diatomic molecules. Therefore produce ions when dissolved in water. HCl H+(aq) + Cl-(aq) Note: The proton (H+) in aqueous solution does not exist independently, but becomes attaché to the negative end of a water molecule. The proton in water is said to be hydrated and referred to as the hydronium ion, H3O+. Diagram of hydronium ion

Questions (1) What is an acid? A base? (2) What would happen if I put a flower into an acidic solution? a basic solution? (3) How can you tell if a solution is acidic or basic? (4) How do acidic solutions vary? basic solutions? (5) Are acids poisonous? bases?

Acids and Bases are of great significance in everyday life        

Baking soda and powder are weak bases vital to cooking Lye (NaOH) is a strong base often used in oven cleaners Lime ( CaO) is a base used to decrease the acidity in soil Antacids contain bases to neutralize excess acidity in the stomach Acid skin tends to produce pimples Radiators corrode when antifreeze acidifies Organic acids, which are commonly found in foods and animals.

Acids and bases are a way of classifying compounds based upon what happens to them when you place them in water.

What is and acid? Background Water will naturally "break up" into H+ and OH- in a process known as dissociation. When water dissociates the hydrogen atom breaks its bond with oxygen and leaves behind its electron. The hydrogen atom is now positively charged and properly called a hydrogen ion. The remaining hydrogen is still connected to the oxygen, which now has an extra electron, giving this pair a negative charge. The OH- molecule is properly called a hydroxide ion. The amount of dissociated water molecules in relation to all the water molecules is very small, and since the overall amounts of H+ and OH- are equal, they cancel each other out.

If, for some reason, the H+ and OH- are not balanced, an acid or base is formed. The extent to which a compound is acidic or basic is measured by the pH scale. On the scale, numbers range from 0 (most acidic) to 14 (most basic). Pure water has a pH of 7, which is neutral. It is important to note that this scale is logarithmic. Thus, a pH of 2 is not twice as acidic as a 4, but rather 100 times as acidic. That same pH of 2 is not three times as acidic as a pH of 6, but rather 10,000 times as acidic. pH = - log[H+]. Defined as the negative log (exponent of ten) of the proton concentration expressed as [H+]/H3O+]. Thus, when pH has low values, the concentration of hydrogen ions is high.

ACIDS Aqueous solutions that have more H+ than OH-. Acids are substances that, when dissolved in water, split into two ions, one of which is an H+ ion (The H indicates that the ion is hydrogen, and the + indicates that it is positively charged, meaning that there is no electron.). A well known acid is HCl (hydrochloric acid), which splits into two ions when placed in water: H+ and Cl-. Two definitions can be used for this class: 1. Arrhenius Definition: a substance that releases hydrogen (H+)/ hydronium ion (H3O+) when dissolved in water. E.g. HCl, H2SO4, HNO3, H3PO4, CH3COOH 2. Brønsted Lowry Definition: A proton donor (H+) NH4+ + H2O 

NH3 + H3O+ OR

NH4+ 

NH3 + H+

Properties of Acids Acids are characterized by  

 

Their sour taste. Many of the foods you eat, such as oranges, green apples, and rhubarb, taste sour due to the acids which they form. Their ability to react with (dissolve) active metals e.g Fe, Sn, Zn, Mg to produce hydrogen gas. Zn + 2HCl ZnCl2 + H2(g). Many cleaners have acids in them. It is important that you read the warning labels on your household cleaners as we do not advise that you use acidic solutions to clean water pipes in your home. Their ability to turn indicator dye litmus from blue to red. Their ability to neutralize bases.

Acids can either be strong or weak

Strong acids- are completely ionized in water (can be harmful to human tissue)

e.g. Weak acids- are not completely ionized in water.

e.g. Nonmetal Oxides-Acid anhydrides React with water to form acids:

SO3 + H2O

H2SO4

OR CO2 + H2O

H2CO3

BASES Bases are aqueous solutions that have more OH- than H+. When a base is dissolved in water, it splits into ions as well. For a base, one of them is OH- (often called a hydroxide ion). For example, NaOH splits into Na+ and OH- when placed in water, so it is a base. Two definitions can be used for this class: 1. Arrhenius Definition: A substance that releases hydroxide (OH-) ions when dissolved in water 2. Brønsted Lowry Definition: A proton acceptor Properties of Bases Bases are characterized by •

• • •

Bitter taste. When you squeeze lemon on your fish before eating it, what you are actually doing is neutralizing the basic, bitter taste with an acid, thus making it more palatable. You may have also experienced this acid-base neutralization reaction if you have ever drunk orange juice after brushing your teeth. Bases are generally slippery. The scales of fish are coated with a base that makes them slippery. Soaps are also bases, and naturally are quite slippery. Turn litmus blue Neutralize bases

Be advised that bases can be just as damaging as acids, and in some cases even more damaging (the slipperiness makes it hard to wash out of your eyes should it get into them). One advantage to using bases as cleaners is that they do not react with metals.

The cleaners used to unclog sinks (e.g. Draino, Liquid Plumber) are strong bases that readily dissolve hair and grease, but leave the pipes unscathed.

Bases can either be strong or weak Strong bases- completely dissociated in water NaOH

Na+ + OH-

Weak bases- remain mostly in molecular form when dissolved in water e.g.ammonia. NH3 + H2O  NH4+ + OHCommon Acids & Bases:

Metal Oxides-Basic Anhydrides React with water to form bases: CaO2 + H2O

Ca(OH)2 +

Neutralization When acids react with bases the properties of both species disappear. If you put both an acid and a base into the same container of water, they tend to cancel out the effects of one another. For example, if both HCl and NaOH are placed in water, the Na+ and Cl- ions combine to form NaCl (table salt), and the H+ and OH- ions combine to form H2O (water). NaOH + HCl  NaCl + H2O

Salt: A compound composed of the positive ion of a base and the negative ion of an acid. Indicators-Measuring the pH There are substances, which have the property of changing their color when they come in contact with an acidic or basic environment. These substances are called pH indicators. A dye whose colour depends on its pH e.g. litmus. Indicators change colour within a certain pH range, and the range of pH value within which the colour change take place depends on the particular indicator. Indicators measure [H+]/ [OH-] without affecting it.

Indicator thymol blue methyl orange methyl red chlorophenol red bromothymol blue phenol Red thymol Blue Phenolphthalein Alizarin yellow

Color change interval (pH) 1.2 - 2.8 3.1 - 4.4 4.4 - 6.2 5.4 - 6.8 6.2 -7.6 6.4 - 8.0 8.0 - 9.6 8.0 - 10.0 10.0 -12.0

Acid

Base

red red red yellow yellow yellow yellow colorless yellow

yellow yellow yellow red blue red blue red green

Antacids Antacids work by neutralizing acid and coating the stomach  Basic compounds used to dissolve the amount of acid (HCl) in the stomach.  Antacids are used to relieve the uncomfortable symptoms of acid indigestion, heartburn, gas, and sour stomach.  Side effects may include: Chalky taste, constipation, diarrhea, increased thirst, stomach cramps 

Buffers Buffers are compounds that tend to neutralize the pH of a solution by combining with either H+ ions or OH- ions to keep the solution neutral. Buffers play a very important role in most organisms, as many organisms cannot live at pHs that are too acidic or too basic. This is because certain reactions, which occur in organisms, are hindered by the effects of an excess of charged ions in the environment.

Buffers resist changes to pH on addition of small amounts of acid or base. Common buffer mixtures are composed of both a weak acid and its salt eg. ethanoic acid and sodium ethanoate or a weak base and its salt eg. ammonia and ammonium sulphate. In the buffer made from ethanoic acid and sodium ethanoate, two equilibria exist;

CH3CO2H (aq) + H2O (l)

H3O+ (aq)+ CH3CO2-(aq)

CH3CO2Na CH3CO2 -+ Na+ Adding a small amount of acid to equilibrium 1, will make it shift to the left hand side, to reduce the hydrogen ion concentration and therefore the pH will not change. This will also reduce the concentration of CH3CO2 - ions, but there is a large concentration of these present from equation 2. Adding a small amount of OH- ions will remove hydrogen ions to make water, and cause the equilibrium to shift to the right. The equilibrium in the second equation lies far to the right. This keeps the concentration of CH3CO2 - constant. The Carbonic-Acid-Bicarbonate Buffer in the Blood By far the most important buffer for maintaining acid-base balance in the blood is the carbonic-acid-bicarbonate buffer. The simultaneous equilibrium reactions of interest are

Calculating the pH of a Buffer The following formula is used: pH = pKa -log[acid] /[salt] How Buffers Work: A Quantitative View The kidneys and the lungs work together to help maintain a blood pH of 7.4 by affecting the components of the buffers in the blood. Therefore, to understand how these organs help control the pH of the blood, we must first discuss how buffers work in solution. Acid-base buffers confer resistance to a change in the pH of a solution when hydrogen ions (protons) or hydroxide ions are added or removed. An acid-base buffer typically consists of a weak acid, and its conjugate base (salt). Buffers work because the concentrations of the weak acid and its salt are large compared to the amount of protons or hydroxide ions added or removed. When protons are added to the solution from an external source, some of the base component of the buffer is converted to the weak-acid component (thus using up most of the protons added); when hydroxide ions are added to the solution (or, equivalently, protons are removed from the solution; protons are

dissociated from some of the weak-acid molecules of the buffer, converting them to the base of the buffer (and thus replenishing most of the protons removed). However, the change in acid and base concentrations is small relative to the amounts of these species present in solution. Hence, the ratio of acid to base changes only slightly. Thus, the effect on the pH of the solution is small, within certain limitations on the amount of H+ or OHadded or removed. The Carbonic-Acid-Bicarbonate Buffer in the Blood By far the most important buffer for maintaining acid-base balance in the blood is the carbonic-acid-bicarbonate buffer. Many people today are interested in exercise as a way of improving their health and physical abilities. But there is also concern that too much exercise, or exercise that is not appropriate for certain individuals, may actually do more harm than good. Exercise has many short-term (acute) and long-term effects that the body must be capable of handling for the exercise to be beneficial. When we exercise, our heart rate, systolic blood pressure, and cardiac output (the amount of blood pumped per heart beat) all increase. Blood flow to the heart, the muscles, and the skin increase. The body's metabolism becomes more active, producing CO2 and H+ in the muscles. We breathe faster and deeper to supply the oxygen required by this increased metabolism. Eventually, with strenuous exercise, our body's metabolism exceeds the oxygen supply and begins to use alternate biochemical processes that do not require oxygen. These processes generate lactic acid, which enters the blood stream. As we develop a long-term habit of exercise, our cardiac output and lung capacity increase, even when we are at rest, so that we can exercise longer and harder than before. Over time, the amount of muscle in the body increases, and fat is burned as its energy is needed to help fuel the body's increased metabolism.

Acidosis is a condition caused by removal of bicarbonate or an increase in carbonic acid in blood. The net result is a disturbance in the carbonic acid-bicarbonate equilibrium to produce an excess [H+] in blood causing lower blood pH. Metabolic acidosis can occur as a result of diabetes, starvation and high fat diet all of which leads to the production of ketones in the blood. Ketones bind & remove bicarbonate. If not controlled it can be fatal. Alkalosis occurs when [bicarbonate] increases forcing the equilibrium to remove protons from blood causing blood pH to rise. So pH becomes alkaline leading to vomiting, nausea, and headache. Temporary metabolic alkalosis occurs when there is an intake of sodium bicarbonate e.g. if large a mounts are taken for acid in the stomach. Respiratory alkalosis can be induced by hyperventilation i.e. excessive exhalation of carbon dioxide from lungs too quickly causing too great a loss of H+ from the large reservoir. Anything that causes sustained rapid breathing can induce temporary alkalosis, e.g. hysteria (pop concert), hot baths, training. Athletes such as marathon runners learn to control breathing so as to minimize alkalosis. Sprinters and swimmers who understand biochemistry tune their bodies for maximum effort. Strenuous bursts of muscle activity produce high levels of lactic acid as glucose is broken down for energy. Lactic acid can lower the pH of blood and cause muscle cramp/fatigue. To counteract this, athletes will prepare by rapid deep breathing for 30-40 seconds before the race to hyperventilate and introduce temporary alkaline conditions that will help to neutralize the acidity arising from lactic acid.

Other important buffers in the body are the H3PO4, amino acid buffer systems

Acid rain: refer to handout