Chemistry 101

[hallenrm]

Originally Posted by Chemnut

What is a strong acid? What is a weak acid? Why are these strong or weak or otherwise?

Many a Scientist has been stumped by a 6 year old kid that asked the question, Why?.

An acid is said to be stronger than another whe it produces more Hydrogen ions in an aqueous solution of same molarity. So a weak acid is one that is weaker in its capacity to produce hydrogen ions and thus its aqueous solution has a higher pH.

In the series that you have presented, I would offhand say that Water is the weakest acid.

[Mercedes Benzene]

Hallenrm is right.
Acids are said to be "strong" or "weak" depending on how fully the substance dissociates in solution.
Strong acids completely, or almost completely dissociate in solution.
In your series, the correct order (weakest to strongest) should be:

H2O, H3PO4, HF, H2SO4, HCl

I would double check on the H3PO4 and the HF though. They may need to be switched.

[learnin to learn]

what about acids and bases? which one is stronger?

[Mercedes Benzene]

You can't say that an acid or base is stronger.
They are completely different things!

The pH scale is a measure of the Hydrogen ion concentration in a given substance.
The pH scale ranges from 1 to 14.
The number 7 is neutral.
As the number increases past 7, the substance in more basic (or alkaline).
As the number decreases below 7, the substance is more acidic.
a pH of 1 would be extremely acidic.
a pH of 14 would be extremely basic.

now pH is simply the negative logarithm of whatever the concentration may be.

For instance, lets say a substance has a hydrogen ion concentration of .08
The pH can be determined thusly:
-log10(.08) = ~1.09
This would be a very strong acid.

On the other hand, if the ion concentration was 1.06x10^-13, when plugged into the equation:
-log10(1.06x10^-13) = ~12.97
That would be a fairly strong base.

[Chemnut]

In order of increasing acidity in water,

H20 > HF > H3PO4 > HCl > H2SO4

The acid dissociation constants, Ka, are 1 E-07, 6.8 E-04, 6.9 E-03, Large, Larger respectively. The larger the Ka, the stronger the acid.

The acid dissociation constant is a measure of the degree of ionization or how many ions are formed or how many ions dissolve in water.

pH is a measure of the molar concentration of Hydrogen ion in solution (hydronium ion). The pH scale is commonly displayed as 1 to 14. pH paper (generally a single indicator) and pH sticks (often 4 or more indicators) use indicators or substances that change color with changes in the H+ or the OH- concentration. These indicators typically change colors between pH 1 and 14.

The rule for pH is pH + pOH =14. It is possible to have pH < 0 or > 14.

I know of 3 definitions of acids. Each should be kept in mind when discussing acids.

Arrhenius—An acid is a substance that, when dissolved in water, increases the concentration of the hydrogen ion (H+ ).

Bronsted-Lowry—A substance that can donate a proton (H+).

Lewis—A species or substance that forms a covalent bond (sharing of electrons) by accepting an electron pair.

So, Why is HF a weak acid? If you look at the strengths of the other halide acids, they increase as you go up the periodic chart. HCl is a stronger acid than HBr which is stronger than HI.

Learning, is sometimes like asking a fireman holding a 4 inch hose for a drink of water.

[KickAssClown]

Ok why do we need to know that? What purpose does it serve to know this information? What can I do with this?

Perhaps an example of a Acid or Base in use would be prudent. I have heard of testing and adjusting the PH of the ground that you use for planting.

What are some other chemistry synergies, where will this play an important role in future lessons?

[Jay-qu]

Yes plants will be best grown in a certain pH soil, another example is swimming pools and other water sources, the pH can tell you about whats going on in the water.

[Chemnut]

The pH of your blood must be within the 6.8 to 7.8 range to sustain life.
The pH of your bile(stomach) acid (hydrochloric acid) ranges between 1 and 3.
The pH of Pepsi, Coke, and other colas (read the ingredients on the can for all the acids in the soda), and lemon juice(citric acid) is around 2.2. The pH of most soda pop (carbonic acid) is about 4. The pH of beer is 4-4.5. The pH of milk (contains lactic acid) is 6.4.

In order to ship or transport liquid products as non hazardous, the pH of the solution must be > 2 and < 10. Whether it is shiping new products or determining is waste is hazardous or not or government agencies monitoring compliance to a rule or the general public or watchdogs monitoring government agencies or powerplant emissions or monitoring of commercial industry, many people are employed by this regulation.

Potable water supplies are also regulated and pH is monitored.

The more sour the taste, the more acidic it is, the lower the pH. Many food, candy and juice companies regulate the acidity of the product to maintain freshness and taste. Most of the acids in foods, are weak acids. They are often used as preservatives.

Vegetation grows in pH range about 5.5 to 8.5.

A few common household items that are acidic are: bleach (hypochlorous acid), toilet bowl cleaners, drain cleaners, vinegar (acetic acid). All weak acids.

A few common household items that are basic are: Ammonia, many cleaners, baking soda (sodium bicarbonate), some oven cleaners and soaps that contain Lye (sodium hydroxide a strong base).

Safety professionals have the job of ensuring worker safety during the manufacturing and use of chemicals including acids and bases.

If this post gives you a headache, you can take Aspirin (acetylsalicylic acid).
If it turns your stomach, you may take a weak base to sooth it such as Tums (Calcium Carbonate) or Mylanta or Rolaids etc.

[KickAssClown]

So what happens when I mix an extreame with a weak?

Also don't acids and bases cause state changes to things they are exposed to?

Check: If I take an equal base and mix it with an equal acid then they neutralize?

[Turtle]

Originally Posted by Chemnut

A few common household items that are acidic are: bleach (hypochlorous acid), toilet bowl cleaners, drain cleaners, vinegar (acetic acid). All weak acids.

A few common household items that are basic are: Ammonia, many cleaners, baking soda (sodium bicarbonate), some oven cleaners and soaps that contain Lye (sodium hydroxide a strong base).

Wonderful post Chem! On topic, moderate length, informative, & with a twist of humor. My only quibble (and a minor one at that ) is that most drain cleaners contain sodium hydroxide & so are base not acid. (Drano brand crystal drain opener for example.) Not only does sodium hydroxide break down hair & grease, it won't harm the pipes.