Acids and Bases Help (page 3)
Introduction to Acids and Bases
The most important thing to remember about acids is that they have very low pH values and burn skin, clothes, shoes, and almost everything they touch. They do not burn as a fire does with a flame, but by reacting strongly with the atoms of the substance and changing them permanently. Acids and bases cause chemical burns. Students are warned in their first laboratory safety class about strong acids and strong bases. Besides, (1) don’t eat or drink anything in the lab; (2) don’t run your fingers through the burner flame, and (3) don’t blow yourselves up; always remember to (4) handle strong acids and bases very carefully .
Hydrochloric acid, HCl, is one of the first acids a beginning chemistry student learns. This strong acid reacts with most metals and forms hydrogen ions. Many experiments use HCl as a reactant. It is easy to spot the sloppy chemistry student, just look at his or her clothes and shoes. If they have a lot of tiny holes in their clothes, chances are good they don’t have moths, but have splashed an acid or base on themselves while pouring. This is why chemists wear laboratory coats and eye goggles for protection. In this chapter, we will find out more about acids and bases and why they are so fierce.
Thousands of years ago, people learned that liquids were able to transform into different forms; grape juice fermented to wine and then if left too long to vinegar. In fact, the Greek word for vinegar is acetum , from which we get acetic acid, the main acid found in vinegar. Acidus is the Latin word for sour, a common characteristic of acids.
An acid is any solution that releases hydrogen (H) ions when added to water and has a pH of less than 7.0. A base is any solution that releases hydroxide (OH) ions in water and has a pH of greater than 7.0.
How do we figure out if a solution is acidic or basic? In 1909, a Danish chemist, P. L. Sorensen, calculated the negative logarithm of the hydrogen ion concentration. He wrote this as 1 × 10 – n with the pH equal to n . The name of the pH scale came from the French words for the “power of hydrogen” or “ pouvoir hydrogene .” The pH scale measures the amount of acidity in a solution.
pH scale measures the acidity of a liquid by measuring the concentration of hydrogen ions. Neutral pH is equal to 7.0 on the pH scale.
The most often used piece of equipment in any laboratory beside the beaker or Bunsen burner is the pH meter. The pH meter has a special sensor that is inserted into a liquid sample to be tested. By passing a current through the sample and measuring the resistance and change in current, the amount of hydrogen ions (positively charged ions) can be determined. Figure 14.1 shows a typical pH meter.
Acids And Bases
Nearly 120 years ago, a Swedish graduate student in chemistry, Svante Arrhenius, came up with a few “rules of thumb” for deciding whether or not a solution is an acid:
1. In dilute solutions, acids taste sour. ( Never taste a lab sample!)
2. Litmus paper changes from blue to red.
3. Acids react with metals like iron, magnesium, and zinc and release hydrogen gas.
4. When combined with bases, the products are water and salt.
To find out if a solution is basic, Arrhenius wrote that:
1. In dilute solutions, bases taste bitter. (But you are never going to taste a lab sample, right?!)
2. Litmus paper changes from red to blue.
3. Bases react with acids to form water and salt.
4. Bases feel soapy or slippery between the fingers and on the skin.
When an acid completely ionizes in water and gives up all its protons to water to form a hydronium ion, H 3 O + , it is considered a strong acid. Table 14.1 lists some common acids from strongest to weakest.
Table 14.1 Common acids from strongest to weakest should be handled with matching caution.
Since acids are formed from the ionization of samples and the release of H + ions, acids that contain carbon held together by covalent bonds are generally much weaker.
Sulfuric acid is the most widely produced acid in the United States. It is used to make fertilizers (70%) from ores containing phosphate rock. The remaining 30% is used in processing industrial metals, oil refining, car batteries, and in general cleaners.
Boric acid is a weak acid used in eye washes and to clean glass. Boron is also a natural insecticide, since many insects cannot metabolize the boron molecule. Boric acid when mixed with equal parts white flour and powdered sugar makes a tempting insect bait for cockroaches. It causes internal blockage and death, while being safe for animals and children.
Bases ionize almost completely in water. A base that does not ionize in water and is nearly insoluble in water may be thought of as a weak base, but it is really the amount of ionization that matters. Ca(OH) 2 is not particularly soluble, but the little that does dissolve is completely ionized, so it is called a strong base.
Sodium hydroxide (NaOH) is the strongest base that most chemistry students will use in the laboratory. The metal sodium (Na + ) forms a tight bond with the (OH – ) ion. It is used as a strong cleaner, to stabilize soil in road construction, and in the making of soap.
Magnesium hydroxide Mg(OH) 2 acts as a mild reactant by gently neutralizing stomach acids. When spicy or acidic foods, like tomatoes, cause problems, an antacid containing magnesium hydroxide neutralizes the acid and brings speedy relief. Table 14.2 lists a few common bases.
Brönsted-lowry Acids And Bases
In 1923, two chemists, Johannes Brönsted and Thomas Lowry, described acids and bases in the scientific literature. They were studying how the transfer of hydrogen ions (protons) took place between reacting molecules. A Brönsted-Lowry acid donates a proton in a reaction, while a base is on the receiving end of the proton transfer. In the Brönsted-Lowry definition of acids and bases, ions as well as larger more complex molecules are included. The pairs of (H 2 O, OH − ) and (NH 3 , ) are called Brönsted–Lowry conjugate acid–base pairs .
When an acid reacts and loses a hydrogen ion, it forms a base. In the above reaction, the H 2 O acts like an acid, loses a hydrogen ion (H + ) and becomes a base (OH − ). The base, NH 3 , gains a hydrogen ion (H + ) and becomes more acidic .
A conjugate base of a sample or ion occurs when an H + ion is lost.
A conjugate base of a sample or ion occurs when an H + ion is added.
Some acids and bases are chameleons and can accept protons in one reaction and then turn around and donate them in a subsequent reaction. A lot of physiological reactions work like this with water or blood serving as the go-between solution. Ions or molecules, which can swing either way, depending on the environment and company present, are called amphiprotic. They can either lose or add a proton (H + ) in a reaction.
An ion or molecule is amphoteric when it can serve as either an acid or a base in a reaction, but has no protons (H + ).
To bring the Brönsted–Lowry acid and base idea all together, remember the following:
1. A base accepts protons.
2. An acid provides protons in a reaction.
3. Acidic and basic reactions don’t just occur with protons (H + ).
4. Ions as well as molecules can be acidic or basic.
5. Some reactants can swing either way, providing or accepting protons.
Strengths of Acids and Bases
As shown in Table 14.3 , acids and bases have different strengths and weaknesses. Some mild acids like water (remember, it provides a proton) can be used to wash your hands of dirt. You couldn’t use sulfuric acid for the same purpose.
Table 14.3 Many foods and household solutions are strongly acidic or basic.
The strengths of acids can be found by how well they ionize. If two acids react with a certain solution, generally one will be ionized more than the other. Just as people are all individuals, elements have strengths and weaknesses too. Reactions usually go in the direction of the weaker acid or weaker base. A stronger acid will be transformed into products that include a weaker acid. The same is usually true of reactions with bases.
One exception happens in the presence of water. Water bends the rules. In water, strong acids seem to have the same strengths. They ionize well and seem to “even up” the differences.
You may see the words conjugate acids and conjugate bases . A conjugate acid is the part of an acid-base reaction that donates the proton. A conjugate base is the part of the joined compound that can accept a proton.
Strong acids have weak conjugate bases. Strong bases have the weakest conjugate acids.
The strength of acids and bases is based on the amount of hydronium ions (H 3 O + ) and their concentration in a solution. The pH meter records this concentration with strong acids ranging from 2.0 to 5.0, neutral pH being a pH of 7.0, and strong bases ranging in values from 8.5 to 10.0. The lower the pH number, the stronger the acid; the higher the pH number, the stronger the base.
There are two clues to figuring out the strength of an acid:
1. Check the polarity of the H + bond. The more polar the bond, the more easily the H + proton is removed and the stronger, more interactive the acid.
2. Check the size of the atom bonded to the H + .
Commonly, you will find that the larger the atom, the weaker the bond and the stronger the acid. When hydrogen bonds to chlorine, or bromine, the larger molecule with greater orbitals has more interactivity. When comparing the acids HF, HCl, HBr, and HI, the acidity increases as the size of the atoms increases. So they can be arranged as HI > HBr > HCl > HF > H 2 O.
The strength of bases can be found in much the same way. Stronger bases ionize almost completely in water, while weak bases do not. This happens because less ionized bases do not have free orbitals to allow the acceptance of additional protons. (Everybody is comfortable and stable and resists change.) A stronger base is able to accept H + more easily than a weaker one because it goes into the ion state more easily. The following bases can be arranged by strength OH > NH 3 > HCO 3 > C 2 H 3 O 2 > NO 3 > HSO 4 .
Practice problems for these concepts can be found at – Acids and Bases Practice Test
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