Mr. Harwood
Updated: May 2015
Course: SNC2D1
Unit: Chemistry
Lesson : ACids and Bases
Lesson:
Pure water, H2O, does not conduct electricity. It only conducts electricity if an ionic compound dissolves in it. This is because ions are charged atoms (or groups of atoms) and electricity is the movement of changes. Covalent compounds like sugar, do not make water conduct electricity. Tap water has enough ions in it to conduct electricity.
The types of compounds that make ions are acids, bases, and salts.
ACIDS
Acid formulas normally begin with H. (exception H2S) They produce H+ ions when they dissolve in water.
à Binary acids:
HF, HCl, HBr hydrofluoric acid, hydrochloric acid, hydrobromic acid.
à Oxyacids:
HNO3, H2CO3, H2SO4, H3PO4 (no “hydro” in names) HClO3 = chloric acid
à Organic acids: These are weaker acids and are often in our food. Their names don’t begin with H
Most organic acid structures look like the diagram at right. The “R” changes for the different acids. The H that is on the OH will detach and form H+ ions (which is why this is an acid).
acetic acid (vinegar): CH3COOH,
lactic acid C3H6O3 or C2H4OHCOOH
citric acid: C6H8O7,
tannic acid: C76 H52 O46
Properties of Acids
- taste sour
- corrosive: react with metal to produce H2 gas
- burn flesh
- react with carbonates to make CO2
BASES
Formulas normally end in OH and they make OH- ions when dissolved in water. Hydroxides which are insoluble are not bases.
Examples: NaOH, Ca(OH)2
Lye: NaOH – drain cleaner, oven cleaner
Lime, slaked lime: Ca(OH)2 – used in industry a lot in cement, in iron making (blast furnaces)
Used to be used to line fields.
Other compounds which act like bases (and may be weak bases) are ammonia NH3, bleach NaClO, sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3).
Many bleaches, soaps, toothpastes and cleaning agents
Properties of bases
- some taste bitter
- dissolve flesh
- strong base + fat à soap
Neutralization reactions:
Because acids make H+ and bases OH-, then can combine to make HOH, which is water.
eg. HNO3 + Ca(OH)2 à Ca(NO3)2 + H2O
We’ve already looked at these when we discussed double displacement reactions.
è Play “All about that base” music video
pH scale
How do we tell how strong an acid or base is?
The pH scale measures this:
0 –––––––––––––––––––––––––––––––– 7 ––––––––––––––––––––––––––––– 14
strong acid weak acid neutral weak base strong base
(water)
Yes, it is possible to go outside this range, but it’s very rare.
“For example, commercially available concentrated HCl solution (37% by mass) has pH ≈ -1.1, while saturated NaOH solution has pH ≈15.0 (22). Hot springs near Ebeko volcano, with naturally occurring HCl and H2SO4, have estimated pH values as low as –1.7 (23, 24). Waters from the Richmond Mine at Iron Mountain, CA, have pH = -3.6”
- from http://jchemed.chem.wisc.edu/HS/Journal/Issues/2006/OctACS/abs1465.html
Lim, Kieran F. (). J. Chem. Educ. 2006 83 1465.
Most foods we eat are slightly acidic
oranges pH = 3.5 eggs pH = 7.5 stomach acid: pH = 1-3
INDICATORS
How do we determine pH? How do we tell if something is an acid or base?
à use a pH meter
à use an indicator: a chemical which changes colour with pH.
There are paper strips called litmus paper (litmus is a dye extracted from lichens)
bases turn litmus blue, acids turn it red/pink
There are many liquid indicators. e.g. universal indicator. Others have names that sound like they’re from Harry Potter (bromthymol blue, alizarin yellow, congo red, …)
Universal Indicator
Universal indicator is a pH indicator composed of a blend of several compounds that exhibits several smooth colour changes over a pH value range from 1-14 to indicate the acidity or basicity of solutions.
The colours that indicate the pH of a solution, after adding a universal indicator are shown below.
pH range / Description / Colour0-3 / Strong acid / Red
3-6 / Acid / Orange/Yellow
7 / Neutral / Green
8-11 / Base / Blue
11-14 / Strong Base / Purple
Some common indicator colour changes:
Homework:
Find 5 substances that require sulfuric acid in their manufacturing process.
do p326 #2,5,7,8, 9,10a, 11 (which textbook??)