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An in-depth exploration of acids and bases, their real-life significance, and their chemical properties. Topics covered include the definition of acids and bases, their relationship with pH, and the use of acid-base indicators. The document also includes activities and experiments to help illustrate the concepts.
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Acids and bases have real-life significance. The human body functions properly only when delicate acid-base balances are maintained and crops grow best in soil with the proper pH. In addition, many substances used in the home are acids or bases.
PROPERTIES OF ACIDS The word acid comes from the Latin word acere , which means "sour." Well known from ancient times were vinegar, sour milk and lemon juice. Aspirin, or acetylsalicylic acid, tastes sour if you don't swallow it fast enough. Chemically, all acids are compounds that generate hydrogen ions [H+] when mixed with water. Examples of acids are hydrochloric acid (HCl) which is secreted by our stomachs to aid in the digestion of food; sulfuric acid (H 2 SO 4 ) which is found in car batteries; acetic acid (H 2 C 4 O 2 ) which is found in vinegar; phophoric acid (H 3 PO 4 ) which is found in Coca~Cola; and ascorbic acid (C 6 H 6 O 6 ) which is also known as vitamin C and found in fruits and vegetables.
PROPERTIES OF BASES Bases taste bitter (like soap or baker's chocolate) and feel slippery to the touch. Chemically, bases generate hydroxide ions [OH-] when mixed with water. Examples of bases are sodium hydroxide (NaOH) used in drain cleaning agents or magnesium hydroxide (Mg(OH) 2 found in some antacids;. Other bases contain carbonate such as calcium carbonate (CaCO 3 ) found in some antacids or sodium bicarbonate (NaHCO 3 ) also known as baking soda.
PURE WATER: IT ’ S NEUTRAL H 2 O OH-^ + H+ When water exists in its pure state, H 2 O, it undergoes a natural dissociation to a very small degree to form [H+] and [OH-] ions. Notice that when one molecule of water dissociates, one of each chemical species is produced. Thus in pure water, the concentrations of hydrogen and hydroxide are equal. They are in equilibrium with each other and that is why pure water is neutral. If an acid or a base is added to water this equilibrium is disturbed, which causes one of them to increase in concentration while the other decreases in concentration. The more concentrated the acid or base, the greater the disruption in the equilibrium.
PH SCALE The pH scale is used to measure how acidic or basic a solution is. The pH scale ranges from 0 - 14. As the pH becomes lower, the concentration of hydrogen ions becomes greater, and the solution becomes more acidic. As the pH becomes higher, the concentration of hydroxide ions becomes greater, and the solution becomes more basic. A pH from 0 - 6 is acidic, and a pH from 8 - 14 is basic. A pH of 7 is a neutral solution.
CONCENTRATION Chemists use the concentration units of MOLES PER LITER (abbreviated M), to represent a known mass of a substance dissolved in a liter of solution. The concentrations of acid solutions that are commonly used vary between 10-1^ and 10-14^ moles per liter. Because this concentration range is so large and cumbersome to communicate, chemists have developed a more convenient method of describing acid concentrations. This method is the PH SCALE. The pH scale is a logarithmic scale, which means that if an acid is diluted to one tenth of its original concentration, the pH of the solution will increase by only one exponential unit.
The concentrations of [H+] and [OH-] in solution have an inverse relationship with each other. However, there are limits to the extent these concentrations can exist in water. As it turns out, the product of their concentrations in water is always equal to 10-14. This inverse relationship of of [H+] and [OH-] is illustrated in the diagram to the right. Mathematically this relationship can be written as:
Calculating pH of an Acidic Solution The pH scale is a reverse logarithmic representation of hydrogen ion [H+] concentration, so a change in value on the pH scale represents a ten-fold difference in H+^ concentration. For example, a change in pH from 2 to 3 represents a 10-fold decrease in H+^ concentration, and a shift from 2 to 4 represents a one-hundred (10 × 10)-fold decrease in H+^ concentration. The formula for calculating pH is:
Taking the log of a number experessed in scientific notation is simply the exponent of the number. Taking the negative log is done by multiplying the exponent by -1. Therefore, if a solution that has an acid concentration of 10-1^ moles per liter, it has a pH of 1. If we dilute that solution to one tenth of its original concentration, the acid concentration will be 10-2^ moles per liter and the pH will be 2.
Calculating pH of a Basic Solution If the solution which is being tested is basic, then the hydroxide concentration [0H-] is greater than the hydrogen concentration [H+]. For example, if the hydroxide concentration is 10-2^ moles per liter, then the hydrogen concentration can be determined as follows: 10 -14^ = [10-2] x [H+] [H+] = 10-
Using the equation above for determining the pH of a solution, the pH is: pH = - log [10-12] or pH = 12
The pH is always determined from the hydrogen concentration. For acids, the hydrogen concentration is high and the pH is below 6. For bases, the hydrogen concentration is low and the pH is 8 or above.
Acid-base Indicators Acid – Base indicators are chemicals that produce different colors when put into solutions of different acidity or basicity. For example, litmus is a chemical obtained from lichens. In an acid solution (pH less than 7) blue litmus turns red. In a basic solution (pH greater than 7) red litmus turns blue. When cabbage is chopped up and soaked in water, the resulting cabbage juice displays different colors when put in solutions of different acidity. When cabbage juice is added to a solution the pH of the solution can be estimated depending upon the color which the solution changes. In this laboratory we will use “Universal Indicator” that will react to indicate a range of pH values.
high
high
[ H+] low
[OH-] low
1 7 14
Located around the room are several demonstrations which help illustrate the properties of acids, bases, and indicators. Try each one and provide an explanation of how each demonstrates these concepts.
STATION #1: DISAPPEARING INK
Moisten an area of the paper towel with the disappearing ink (do not soak it). Hold up the towel, stretching it between your hands, and blow on it. The carbon dioxide in your breath will cause the disappearing ink to change from blue to colorless. Once the ink has vanished, make it reappear by spraying a small amount of sodium hydroxide solution on the disappearing ink.
Under what conditions (acidic or basic) is the ink blue? Clear?
Blue – Basic Clear - Acidic
How does blowing on the towel change the pH?
Decreasing pH.
Answer this question after completing Station 4: pHacts About Rainwater. What chemical is responsible for lowering the pH of the Disappearing Ink when blowing on it?
Carbon dioxide (CO 2 ) reacts with the water to form carbonic acid (H 2 CO 3 )
STATION #2: NATURAL INDICATORS
Chemicals that change color due to changes in pH are indicators. They indicate the acidity or basicity of their environment. Add a small portion of cabbage juice indicator to 2 wells of a clean 6-well plate. What is the color of the neutral solution? Purple
Add a small amount of vinegar to one of the wells. What is the color change under acidic conditions. Pink
Now add some baking soda solution to the other well and note the color change under basic conditions. Green
Sample pH Sample pH
Carbonated Water 4 Ammonia 11
Lemon Juice 4 Tap Water 9
7-Up 4 Drain Cleaner 12
Vinegar 4 Baking Soda Solution 9
Distilled Water 7 Milk
Soap Solution 9 Poland Springs
Tea 5
Many of the products you eat or use contain natural acids and bases as flavorings or active ingredients. Look at the list of ingredients on the packages and identify the acids or bases.
Item Ingredient Acid or Base Insect Sting Medication Ammonium hydroxide^ Base Coca~Cola Phosphoric Acid Acid
Jello Citric Acid, Fumaric Acid Acid
Antacid Calcium Carbonate Base
Drain Cleaner Sodium Hydroxide Base
Sour Candy Citric Acid, Fumaric Acid Acid
Provide an explanation how the medicine reduces the itching caused by insect bites?
The insect injects an acid when it bites which causes the itching sensation. The base in the medicine neutralizes the acid to alleviate the itching.
STATION #4: THE PHACTS ABOUT RAINWATER
As rain precipitates from clouds if falls through a mixture of atmospheric nitrogen (N 2 ), oxygen (O 2 ), Argon (Ar), carbon dioxide CO 2 ), and other chemicals. The first three components of air do not react with water; however, rain in the "natural" state will be slightly acidic because carbon dioxide from the air reacts with water to form carbonic acid (H 2 CO 3 ). Even though this is a weak acid, it generates enough hydrogen ion in natural water to drop the pH to about 5.6, well below the neutral value of 7.
Test for the production of carbonic acid by putting 50 mL of distilled water into the flask and add 10 drops of universal indicator. Using a clean drinking straw, exhale into the water and note the color change of the indicator.
What color is the indicator when neutral? Yellow/Green When acidic? Red
Write a chemical equation to show the reaction of water and carbon dioxide to form carbonic acid. CO 2 + H 2 O H 2 CO 3