Acids and Bases

Acids The word “acid” can refer to a wide range of chemicals that have certain properties in common. Some Properties of Acids: ◦ Corrosive  (can burn skin and damage materials, especially at higher concentrations) ◦ Sour tasting  Although we NEVER taste a chemical in the laboratory we do eat a lot of common acids in everyday life  vinegar is “acetic acid” and lemon juice is “citric acid” ◦ Lower the pH of solutions (more on this later)

Types of Acids We can divide these chemicals into a few major groups: 1. Binary Acids ◦ composed of hydrogen + another element  example: HCl, HBr, HF, HI ◦ naming rule:  “hydro ________ -ic acid”  HCl = “hydrochloric acid”  HF = “hydrofluoric acid”  HBr = “hydrobromic acid”  HI = “hydroiodic acid”

Types of Acids II. Oxyacids ◦ made of hydrogen + polyatomic ion that contains oxygen ◦ Ex – H 2 SO 4, H 3 PO 4, H 2 CO 3 ◦ naming rule: “________-ic acid” using name of non-oxygen element in the ion. ◦ H 2 SO 4 = “sulfuric acid” ◦ H 3 PO 4 = “phosphoric acid” ◦ H 2 CO 3 = “carbonic acid” Note – both types of acids have H+ in them (all start with H); this is not true of all acids in the world but will be true in this class!

Bases The word “base” can refer to a wide range of chemicals that have certain properties in common. Some Properties of Bases: ◦ Corrosive  (can burn skin and damage materials, especially at higher concentrations) ◦ Bitter tasting  the compounds that make coffee and tea bitter are bases!  Interestingly, most toxins found in nature are basic, hence our extreme distaste for anything bitter (don’t want to eat poisonous plants!)  This is why washing someone’s mouth out with soap (a base) used to be a dreaded punishment. ◦ Feel slippery to the touch  Again, we never touch a chemical in the lab but bases are very “slimy” feeling if you get them on your skin  Soap is a basic chemical – notice how it feels? ◦ Raise the pH of solutions (more on this later) Common bases: baking soda, ammonia, drain cleaner, soap

Recognizing Bases Not obvious which chemicals are bases nor are there naming rules for them It’s up to the chemist to recognize a basic chemical from! In this class, we will mostly deal with bases that contain the hydroxide ion ◦ Ex – sodium hydroxide (NaOH) is a strong base (we will use this a lot in the lab) Another word for “basic” is “alkaline” – so the opposite of an “acidic” solution is an “alkaline” solution.

Acid/Base Chemical Definitions Acid: any substance that increases the concentration of H + in a solution. Ex – HCl dissociates in water to become H + and Cl -. HCl H+H+ Cl - It is the H+ (“hydrogen ion”) that makes the solution “acidic” – the Cl - is unimportant This is why many (but not all) acids contain H – HCl, HF, HBr, H 2 SO 4 : all dissociate to make H+ in water, the other part of the compound is not that important.

Acid/Base Chemical Definitions Base: any chemical that increases the concentration of OH - in solution. Ex – NaOH dissociates in water to make Na+ and OH -. NaOH Na+OH - It is the OH - that makes the solution “basic” – the Na+ is unimportant This is why many (but not all) bases contain OH: these dissociate to make OH - in water, the other part of the compound is not that important.

Neutralization Acids and bases neutralize each other when they are mixed. We can now figure out why this happens! Acids produce H+. Bases produce OH-. What happens when H+ and OH- meet? H + + OH - → HOH Wait a second – HOH looks familiar. It can also be written H 2 O! H + + OH - → H 2 O So an acid mixed with a base produces water (which is neutral!) Sometimes bases are defined as chemicals that “accept H+” - in other words a base reduces the acidity of the solution by taking out the H+.

H2OH2O H+ OH -

Neutralization Reactions So imagine what happens if we mix equally concentrated solutions of HCl (strong acid) and NaOH (strong base). What do we end up with? HCl + NaOH = double displacement! HCl + NaOH → NaCl + HOH (H 2 O!) So you mix two nasty, corrosive solutions and end up with harmless salt water!

“Strong” vs. “Weak” acids/bases Acid + water,  you expect it to break apart and make H+ in the solution. (the same is true of bases for OH-) However, some acids/bases don’t break apart very well ◦ when put in water, maybe only half of the particles break apart – the rest don’t make H+/OH- because they stay together. These are called: “weak acids/bases”. A strong acid or base dissociates completely in water; not all particles of a weak acid or base will dissociate.

So if you put 100 molecules of HCl (a STRONG acid) into water, all 100 will break apart, making 100 H+ in the solution. It’s like the HCl is “100% effective” at acidifying the solution. 100 HCl 100 H+ 100 Cl-

If you put 100 HC 3 COOH (acetic acid, a WEAK acid) molecules into water, only 1 of them will actually break apart. 100 HC 3 COOH 1H+ 99 HC 3 COOH You put in all that acid but only got 1 H+ out of it! It‘s like the chemical is only 1% effective at acidifying the solution. 1 C 3 COOH -

“Weak” Not Same as “Safe” It is possible to make extremely concentrated, dangerous solutions using weak acids – you just have to put more of the weak acid into the water to accomplish this! You would have to add 100X as much acetic acid as hydrochloric acid to a solution to get the same amount of acidity, but it could be done!

Measuring Acidity/Alkalinity Measure how acidic/basic a solution is using the “pH scale” Most common substances are in the pH range of 0-14 with pure water defining “neutral” at pH = 7. numbers >7 mean basic, <7 mean acidic.

Measuring pH Using either a digital pH meter or by using pH indicators – chemicals that change color at different pH levels. Ex – pH paper Ex – phenolphthalein indicator turns pink in basic solutions.

Calculating pH pH = - log [H + ] Remember, [H + ] means “the concentration of H + ” Note – pH has no units (it’s just a number) So what is the pH of a solution where: ◦ [H + ] = 3 x ?  pH = - log [3 x ] = 3.5 ◦ [H + ] = 2.5 x  pH = 8.6 ◦ [H + ] =5 x  pH = 5.3

Calculating pH In real life we don’t usually know the H+ concentration, we have to do a little more work. Ex – What is the pH of a 5.3 x M solution of HCl? ◦ First, write the equation for dissociation of the substance:  HCl → H+ + Cl -  Since each HCl breaks up to give you one H+, the concentration of H+ is the same as the concentration of HCl.  So [H + ] = 5.3 x molar  pH = -log [5.3 x ]  pH = 1.3

Calculating pOH What if we are dealing with a base? Those don’t make H+! Since bases produce OH -, we calculate the pOH instead. pOH = -log [OH - ]

pOH What is the pOH if [OH - ] = 2 x ? ◦ pOH = - log [2 x ] = 2.7 What is the pOH of a 2.0 x M solution of NaOH? ◦ NaOH → Na + + OH - ◦ The ratio of NaOH:OH- is 1:1 so the [NaOH] = [OH-] [OH-] = 2.0 x 10 -4, so pOH = - log [2.0 x ] = 3.7 What is the pOH of a 2.0 x M solution of Ca(OH) 2 ◦ Be careful! Look at the equation: ◦ Ca(OH) 2 → Ca OH- ◦ Since each Ca(OH) 2 gives you 2 OH-, the concentration of OH is 2x that of the Ca(OH) 2 !  [OH-] = 2 (2x10 -4 M) = 4 x  pOH = - log [4 x ] = 3.4

Putting it all together We can use the pOH to find the pH because for any solution, ◦ pH + pOH = 14 ◦ Or in other words, pH = 14 - pOH What is the pH of a 5.0 x M solution of NaOH? ◦ NaOH → Na+ + OH- ◦ So [OH-] = 5 x M ◦ pOH = - log [5 x M] = 4.3 ◦ pH = 14 – 4.3 = 9.7

Worksheet Time!

Titration You have a solution of acid but don’t know how strong it is. How could you figure out its concentration? ◦ Do a “titration”! Titration: the technique of using a known acid/base to figure out the concentration of an unknown acid/base.

Titration Unknown amount of HCl Start adding NaOH of known concentration. The NaOH will begin neutralizing the acid and raising the pH. Add an acid-base indicator chemical (phenolphthalein) that will turn pink when the solution becomes basic. As soon as the NaOH has neutralized all of the HCl, the solution will become basic and turn pink Since NaOH + HCl → NaCl + H 2 O you know that one NaOH neutralizes one HCl. That means that the amount of NaOH you added is the same as the amount of HCl that was in the beaker – So you can figure out the concentration of HCl!

Time to do this in the lab!