Acids, Bases and the pH scaleDP CHEMISTRY

Rob Slider

ACIDS

Acids are compounds that:

have a low pH (below 7) taste sour turn blue litmus paper redreact with bases to neutralize them

and produce saltsrelease H2 gas in reactions with

active metalsaqueous solutions conduct electricity furnish H+

burn the skin if strongdissolve carbonates

These are some common acids. What others can you think of? Make a list in your notes.

BASESBases are compounds that:

have a high pH (above 7) taste bitter turn red litmus paper bluereact with acids to neutralize them

and produce saltsare slippery feelingaqueous solutions conduct electricityburn skin if strongreact with fats to form soap furnish OH-

What other bases/alkalis do you know?

Common ACIDSSome common laboratory acidsHCl hydrochloric acid HClO4

perchloric acidHBr hydrobromic acid HNO3

nitric acidHI hydroiodic acid H2SO4

sulfuric acid

Five of these acids are classified as monoprotic acids. They only have one hydrogen which they are able to donate. Sulfuric acid is classified as a diprotic acid because it has two acidic hydrogens that it can donate. Similarly, an acid which has three donatable hydrogens would be classified as triprotic.

Common BASESSome common laboratory basesNaOH sodium hydroxide Ba(OH)2 calcium hydroxideKOH potassium hydroxide Na2CO3 sodium carbonateNH3 ammonia NaHCO3 sodium hydrogen

carbonate

Bases are often found in everyday products such as many cleaning products (sodium hydroxide), antacid products (magnesium hydroxide )and fertilisers (ammonia). It is a common misconception that bases are not as dangerous as acids. In fact, many bases can be as much or more corrosive than many acids.

Note that an alkali is a base that is soluble in water

Equilibrium constant of water Kw

Note that water slightly dissociates in equilibrium with hydrogen and hydroxide ions:

H2O H+ + OH-

So, the equilibrium constant is:

Kw = [H+][OH-] Experimentally, it has been determined in a neutral solution:

[H+] = [OH-] = 10-7 (a very small amount!)

So,Kw = (10-7)(10-7) = 10-

14

This shows how far to the left this equilibrium is

Neutralisation

This value (10-14) is constant for water so,

addition of H+ means a decrease in [OH-] an increase in OH- means a decrease in [H+]

From the previous slide,Kw = [H+][OH-]

Kw = (10-7)(10-7) = 10-

14

pH pH is a measure of the acidity of a solution. More accurately stated it is a measure of the hydrogen

ion concentration on a logarithmic scale. pH stands for ‘potential of hydrogen’

So, in a neutral solution, the concentration of hydrogen is 10-7 M:

pH = -log[H+] = -log(10-7)pH = 7

Mathematically this is ,

pH = - log [H3O+]or

pH = - log [H+]

The inverse of the above expression is

[H+] = 10-pH

The logarithmic scaleThe log scale is based on multiples of ten where the log is the value of exponent when the number is written as an exponent.

Number Representation and Logarithms

 Number  Exponent Notation

 Log of the Number

 1000  103  3 100  102  2 10  101  1 1  100  0 0.1  10-1  -1 0.01  10-2  -2 0.001  10-3  -3 0.0001  10-4  -4

To calculate pH, just use the log button on your calculatorYou try:

If an acid has an H+ concentration of 0.0001 M, find the pH.Solution:First convert the number to exponential notation, find the log, then solve the pH equation.H+ = 0.0001M = 10-4; log of 10-4 = -4;pH = - log [ H+] = - log (10-4) = - (-4) = +4 = pH

pH scaleThe pH scale, (0 - 14), is the full set of pH numbers which indicate the concentration of H+ and OH- ions in water.

pH Scale Principle: H+ ion concentration and pH relate inversely.OH- ion concentration and pH relate directly.

The following statements may be made about the pH scale numbers.

Complete the last two

a. Increasing pH means the H+ ions are decreasing.b. Decreasing pH means H+ ions are increasing.c. Increasing pH means OH- ions ared. Decreasing pH means OH- ions are

pH exercises

Condition AnswerMost H+ ions: pH = 4; or pH = 5. Most OH- ions: pH = 10; or pH = 13. Least H+ ions: pH = 12; or pH = 13. Least OH- ions: pH = 8; or pH = 9. If acid was added to a solution of pH 4, the pH would increase or decrease? If acid was removed from a solution of pH 3, the pH would inc. or dec.? If base were added to a solution of pH 9, the pH would inc. or dec.? If base were added to a solution of pH 2, the pH would inc. or dec.?  If acid were added to a solution of pH 13, the pH would inc. or dec.? If base were removed from a solution of pH 12, the pH would inc. or dec.?

pH exercises (solutions)

Condition AnswerMost H+ ions: pH = 4; or pH = 5. pH = 4 Most OH- ions: pH = 10; or pH = 13. pH = 13 Least H+ ions: pH = 12; or pH = 13. pH = 13 Least OH- ions: pH = 8; or pH = 9. pH = 8 If acid was added to a solution of pH 4, the pH would increase or decrease? Decrease If acid was removed from a solution of pH 3, the pH would inc. or dec.? Increase If base were added to a solution of pH 9, the pH would inc. or dec.? Increase If base were added to a solution of pH 2, the pH would inc. or dec.? Increase  If acid were added to a solution of pH 13, the pH would inc. or dec.? Decrease If base were removed from a solution of pH 12, the pH would inc. or dec.? Decrease

pH of some common substances

Notice that an increase in 1 pH value means a ten fold increase in hydrogen ion concentration

pH Indicators Many substances change colour as

they are exposed to different pH levels. These can be used to “indicate” the pH of substances when the colour ranges are known

Some natural products such as litmus, cabbage, grapes and tea are natural indicators while others such as phenolphthalein and methyl orange are synthetic

pH indicators are themselves acids or bases as they donate or accept protons

Notice the variety of ranges where different indicators change colours. Some have more than one change.

pH IndicatorsSome specific examples:1. Litmus red5 8blue2. Phenolphthalein colourless8.3 10red3. Methyl orange red3.1

4.4yellow4. Bromothymol blue yellow6 7.6blue

Note: colour changes within these ranges are gradual

A pH problem - indicators

Indicator ColourLitmus BluePhenolphthalein ColourlessMethyl orange YellowBromothymol blue Blue

An unknown solution produces the colours above. What is the pH range of this solution?

8-8.3

pH meters

A pH meter is used to accurately measure pH and takes the ‘guesswork’ out of judging colours using indicators.

Basically, they work by very precisely measuring the voltage. This voltage is then converted to a pH value by the meter.