Properties of Acids

Physical Properties of Acids


‍1. Acids have a sour taste

‍2. Acids dissolve in water to form solutions which conduct electricity.

‍3. Acids such as sulfuric acid dissociates in water to produce hydrogen ions and sulfate ions.

‍4. These mobile ions are responsible for the electrical conductivity of acids.

5. Acids turn blue litmus paper red.

‍6. Acids have pH values of below 7.

‍Chemical Properties of Acids

1.Reaction of Acids with Metals (Displacement Reaction)

acidsbaseeqn.jpg

  • ‍Acids react with reactive metals to form hydrogen gas and a salt.
  • ‍The reactivity of the metals depend on its position in the reactivity series.

‍Group I elements are very reactive. They react explosively with acids. See the video clip below.

Hydrogen gas was given off during the reaction.

Test for Hydrogen Gas

Magnesium ribbon reacts wtih hydrochloric acid
Observation:
  • Effervescence of colourless and odourless gas, which extinguished burning splint with a "pop" sound, was produced.


2.Reaction of Acids with Carbonate

  • Acids react with all carbonates to form a salt, carbon dioxide and water.
Calcium carbonate (marble) reacts with hydrochloric acid to produce calcium chloride, water and carbon dioxide.
Observation:
  • A colourless and odourless gas, which form a white precipitate in limewater is produced
Note:
  • **Calcium carbonate is also known as marble or limestone.
    • Limewater is calcium hydroxide.
    • Calcium hydroxide (limewater) reacts with carbon dioxide to form calcium carbonate (white precipitate).

Test for carbon dioxide (Note the source of carbon dioxide in this link come from the heating of copper(II) carbonate)


Carbon dioxide test

3.Reaction of Acids with Bases (Neutralisation Reaction)



  • This is a neutralisation reaction.
  • Bases are metal oxides and metal hydroxides.
  • Examples of bases are aluminium oxide, copper(II) oxide, sodium hydroxide, magnesium oxide, magnesium hydroxide etc.
  • Acids react with metal oxides or metal hydroxides to form salt and water.

Properties of Alkalis

Physical Properties of Alkalis


1. Alkalis have a bitter taste and soapy feel.
2. Alkalis turn red litmus paper blue.
3. Alkalis conduct electricity.
Alkalis such as sodium hydroxide dissociates in water to produce hydroxide ions and sodium ions.
These mobile ions are responsible for the electrical conductivity of alkalis.
4. pH above 7.

Chemical Properties of Alkalis

‍1. Neutralisation


  • All alkalis can react with acids to form a salt and water only.

Alkali + Acids --> Salt + Water
NaOH + HCl --> NaCl + H2O

2. Reaction of Alkalis and Ammonium Salts


  • Alkalis react with ammonium salts, with heating, will produce ammonia gas.
  • The general equation for this reaction is shown below:

alkali + ammonium salt --> ammonia + water + salt
Sodium hydroxide + ammonium chloride à sodium chloride + water + ammonia

Test for ammonia gas

‍If a colourless, pungent gas that turns moist red litmus paper blue is produced, gas is ammonia.

‍3. Precipitation Reaction


  • ‍Alkalis can react with a solution of one metal salt to give metal hydroxide (which is insoluble) and another metal salt (which is soluble).
Alkali (hydroxide of metal A) + salt (of metal B) --> hydroxide metal B + salt (of metal A)
‍The metal hydroxide will appear as a precipitate if it is insoluble in water.

‍Sodium hydroxide reacts with copper(II) sulfate solution to give a blue precipitate of copper(II) hydroxide.
‍Sodium hydroxide + copper(II) sulfate à sodium sulfate + copper(II) hydroxide

NOTE:
  • The reaction only work if the metal hydroxide produced is insoluble in water.
  • All Metal hydroxides are insoluble except LiOH, NaOH, KOH, and Ba(OH)2


pH and Indicators


The pH Scale

  • The pH scale is a set of numbers from 1 – 14 used to indicate whether a solution is acidic, neutral or alkaline.
  • Acids have pH values less than 7.
  • Alkalis have pH values more than 7.
  • A neutral solution (eg. salt solution or water) has a pH value of exactly 7.
  • The pH of some common substances are shown in the pH comparison table below:

The pH and Concentration of hydrogen ions


  • The pH of a solution is calculated mathematically based on the number of hydrogen ions or hydroxide ions present in a solution.
  • Acids with a smaller pH value have a higher concentration of hydrogen ions.
  • Alkaline solutions with a larger pH value have a higher concentration of hydroxide ions.

  • Neutral solutions have equal number of hydrogen ions and hydroxide ions, i.e. neutralization giving predominantly water molecules.
  • Thus pH is used to compare the strength of acids and alkalis. (Refer to 1.4 for strength of acid and 2.4 for strength of alkali).

Indicators


  • Indicators are substances that change colour when an acidic or alkaline solution is added to them.
  • The pH value of a solution can be determined by using Universal Indicator. It contains a mixture of dyes and gives different colours in solutions of different pH.
  • Colours of the Universal Indicator in substances of varying acidity and alkalinity:

Very acidic - Red
Acidic - Orange/Yellow
Neutral - Green
Basic/base/alkali - Blue
Very basic/base/alkali – Purple

  • Table below shows the colour changes of other common indicators.

Indicator
Colour in acidic solution
Colour in alkaline solution
pH range at which indicator change colour
Methyl Orange
Red
Yellow
3-5
Phenolphthalein
Colourless
Pink
8-10
Litmus
Red
Blue
5-8

On-line resources:
1. Animation on pH http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/acidbasepH/ph_meter.html
2.Animation on titrationhttp://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/stoichiometry/acid_base.html

Links:
1. Acids and Bases : http://www.visionlearning.com/library/module_viewer.php?mid=58&l=&c3=
2 pH:http://www.purchon.com/chemistry/ph.htm
3. Virtual Chemistry Lab – The pH Factor : http://www.miamisci.org/ph/