Acids bases and salts - Chemistry Form 4
Facts about acids and bases
Perhaps no two classes of compounds are more important in chemistry than acids and bases.For thousands of years people have known that vinegar, lemon juice and many other foods taste sour. However, it was not until a few hundred years ago that it was discovered why these things taste sour - because they are all acids. The term acid, in fact, comes from the Latin term acere, which means "sour"
Acids taste sour, are corrosive to metals, change litmus (a dye extracted from lichens) red, and become less acidic when mixed with bases.
Bases feel slippery, change litmus blue, and become less basic when mixed with acids.
In Form 1 we learnt about acids as substances that have sour taste, ability to change colour of indicator in a certain way from alkalis e.g. blue Litmus paper changes red, ability to neutralize alkalis to form salt and water and also power to act upon metals to produce hydrogen gas and metal salt.
Bases have characteristic properties opposite to those of acids e.g. neutralize acids . Solutions of soluble bases turn litmus paper to blue and react with some metal ions such as Copper, Zinc, Lead and aluminium ions.
Therefore acids are substances which dissociate in water to give hydrogen ions (H+) while bases are substances which dissociate in water to give hydroxide ions (OH-) (according to Arrhenius theory).
Acids and bases play a central role in chemistry
Acid-base chemistry is important to us on a practical level as well, outside of laboratory chemical reactions. Our bodily functions, ranging from transport of ions across nerve cell membranes to the digestion of food in the stomach, are all bases on the principles of acid base reactions.
The temperature and chemical balances in our bodies is maintained by acid base reactions.
In this lesson we will discuss acids and bases.
An acid is a substance that releases hydrogen ions in solution as the only positive ions. A base is a substances that releases hydroxide ions in solution as the only negative ions. The table below shows some common bases and their formulae and solubility.
Bronsted Lowry Theory defines an acid as a proton donor. Acids show normal properties only if water is present since the properties are due to hydrogen ions and not the molecules of the acids.
A base is a proton acceptor as shown by the following equations.
Hydroxide ions (proton acceptor) are from Sodium hydroxide while the hydrogen ions (proton) are from the Sulphuric (VI) acid ( H2SO4)
Acids can therefore be said to be proton donors whereas bases are proton acceptors.
In the above equation ammonia gas acts as the base because it accepts hydrogen ion (proton) from water in the forward reaction to form Ammonium ion In the backward reaction Ammonium ion acts as the acid because it donates hydrogen (proton) to the hydroxide ion to form water and Ammonia gas.
A salt is an ionic substance formed when a cation from a base combines with an anion from an acid.
NaOH(aq) + HCl (aq) ➙ NaCl (aq) + H2O(l)
Sodium ions (Na+) from the base combine with Chloride ions (Cl-) to form the salt (Sodium Chloride)
In this lesson we will discuss Strong and Weak acids and bases.
From the previous lesson, an acid is a proton (H+) donor while a base is a proton (H+) acceptor.
Those acids that dissociate completely in water releasing (H+) ions are called strong acids.
Weak acids are those that dissociate to a small extent in water releasing few hydrogen ions (H+).
The table below shows some weak acids and their formula.
A strong acid contains more hydrogen ions in solution than a weak acid for the same concentration at the same condition.
HCl (aq) ➙ H+(aq) + Cl -
(aq) + CH3COO-
A strong base is one that dissociates completely in water to form many hydroxide ions in solution.
NaOH(aq) w Na+ (aq) + OH- (aq)
A weak base is one that dissociates in water to a small extent to form few hydroxide ions in solution.
Determining the ph of different solution.
Click to play the following video and observe how to determine the pH of different solutions.
From the experiment we can say that hydrochloric acid is strong because the pH of the solution is found to be in the range of 1- 3 while ethanoic acid is a weak acid since the pH range is between 4 - 6.
Sodium hydroxide is a strong base since the pH range is between 11 - 14 while Ammonia solution is a weak base as the pH range is 8 - 9. The following is a pH chart.
In this lesson we will discuss Amphoteric oxides and Hydroxides.
Some oxides also have both both acidic and basic properties and are known as amphoteric oxides. Click to play the following video showing the reaction of metal oxides and Hydroxides with dilute acid and alkalis.
Click to play the following video showing the reaction of metal oxides and Hydroxides with dilute acid and alkalis
Oxides and hydroxides of Zinc, Lead and Aluminium react with acids to form salt and water as shown in the chemical equations below.
Oxides and Hydroxides of Zinc, Lead and Aluminium also react with strong alkalis like Sodium Hydroxide to form salt and water. This makes them behave as acidic oxides since they neutralize alkalis.The following equations show the reactions that take place
The salts formed are complex and different from those formed when thes oxide react with acids. The hydroxide also react with the strong alkalis to form complex salts.
These Oxides and Hydroxides that react with both acids and bases are said to be Amphoteric.
In this lesson, we will discuss the effect of solvents on substances.
The following video is carried out to determine properties of Hydrochloric acid in water and methyl benzene.
The results shows that aqueous solution of Hydrogen chloride behaves as an acid but the solution in methylbenzene has no acidic properties. When ammonia gas is used in place of Hydrogen Chloride gas then its solution in water behaves as a base but the solution in methylbenzene has no acidic properties.
This is because OH- ions are formed in water but not in methylbenzene because the gas remains in molecular nature. This is due to the fact that H+ ions are formed in water but not in methylbenzene as illustrated in the following diagram.
Let us now discuss a group of other ions known as complex ions.
The following table shows the names and the formulae of complex cations formed with excess ammonia solution.
Cu 2+ ions and Zn 2+ form precipitate with few drops of aqueous ammonia that dissolve in excess ammonia to form deep blue and colourless solution respectively as shown in the following equations.
To investigate the reactions of cations in aqueous solution of Sodium hydroxide. Click to play the video and observe what happens carefully.
The cations of the salt solution react with Sodium hydroxide solution to form insoluble metal hydroxide which are observed as precipitates. The alkali provide the hydroxide ion to the cations.
The following are the ionic equations of the precipitates.
The table below summarises the names and the formulae of tht complex anions.
In excess of Sodium hydroxide solution the hydroxides of Zinc, Aluminium and Lead ions dissolve to form complex
solutions. This is due to the formation of complex compounds as shown in the following equations.
In form three we also learnt that the hydroxides of Cu2+ ions and
Zn 2+ ions dissolve in excess ammonia solution to form deep blue solution and a colorless solution respectively.
The following table shows the names and formulae of complex cations.
The following terms will be used in this subtopic. Solution, solute, solvent and saturated solution. A solution is a uniform mixture of a solute and solvent.
A solute is a liquid or a solid substance that can dissolve in a solvent.
A Solvent is a liquid that can be used to dissolve a solute to make a solution.
A saturated solution is a solution which cannot dissolve any more of the same solute at a particular temperature.
In this lesson we will discuss Solubility of salts and Solubility curves.
The following video clip was carried out to determine the solubility of potassium nitrate. Click to play the video and observe what happens carefully.
The following sample results were obtained when Potassium nitrate solution was cooled to 25oC.
What is the solubility of potassium nitrate at the temperature when it is cooled down in the evaporating dish.
The following graph shows the solubility curves of different salts. Study it carefully and note the effect of temperature on different salts.
From the graph increase in temperature affects the solubility of different salts. For most salts such as Potassium nitrate and potassium sulphate solubility increases with increase in temperature. For other salts like sodium chloride solubility decreases with increase in temperature. However for some salts their solubility increases more rapidly for example Potassium nitrate.
The experiment may be repeated at various temperature to get values that may be used to draw a solubility curve for potassium nitrate or any other salt.
The solubility of a solid or salt is the maximum mass in grams of a solute that dissolves in 100g of water with excess of the solute being present at a particular temperature.
The solubility of Potassium nitrate as shown in the experiment is affected by increase in temperature but the solubility of a salt like sodium chloride is less affected by increase in temperature.
Solubility curve is a graph of solubility of a pure salt against various temperatures.
At low temperatures Sodium chloride is more soluble than Potassium nitrate and Potassium sulphate. As temperature increases Potassium nitrate becomes soluble.
Sodium Chloride is the least affected by increase in temperature.
Solubility of salts has several uses. One of the main applications is in the extraction of salt from sea water. The following video clip shows how salt is extracted from sea water. Click to play and observe what the processes that take place.
In this lesson we will try to understand what causes water hardness and find out ways of controlling it.
To investigate hardness of water. Click to Observe how different solutions form lather with soap.
To investigate hardness of water in Boiled solutions.From the animation given note how later is formed in these solutions.
Distilled water and rain water produced lather very easily with soap.
Tap water produced lather but not as much as that of distilled water.
Calcium hydrogen carbonate produced lather less readily compared to tap water and distilled water.
Distilled water contains no dissolved solid particles. It lather easily with soap. Rain water lathers almost the same as distilled water. The water that readily foams lather with soap is known as soft water.
Water containing Calcium hydrogen carbonate produced lather less readily compared to tap water and distilled water. It contains hydrogen carbonate ions that decompose on heating/ boiling to form soft-water and therefore forms more lather with soap.
Calcium chloride and magnesium sulphate on the other hand does not form lather easily with soap when cold and after boiling. They contain Chloride and Sulphate ions of Calcium and Magnesium that do not decompose because they are stable even on heating.
From the above explanation it shows that there are two types of water hardness:
i. Temporary water hardness is due to dissolved calcium hydrogen carbonate or magnesium hydrogen carbonate that can be removed by boiling.
Ca (HCO3) (s)
(s) + H2O
(l) + CO2(g)
ii. Permanent hardness is due to dissolved calcium sulphate and chloride and magnesium Sulphate and Calcium chloride and magnesium sulphate. This hardness cannot be removed by boiling but can be done through chemical treatment or distillation.
The main constituent of common soap is a soluble sodium salt known as sodium
octadecanoate. When soap dissolves in water is forms octadecanoate ions which combines with Calcium and Magnesium ions from hard water to form an insoluble salt called scum as shown in the following equation.
Sodium octadecanoate + calcium chloride ➙ calcium octadecanoate (scum) + sodium chloride.
The following flowchart shows the advantages of hard water.
The following are the advantages and disadvantages of hard water.
Washing soda/ Sodium carbonate is also used to remove temporary hardness. This softens hard water as shown below.
Ca(HCO3)2 (aq) + Na2CO3 (aq) ➙ CaCO3(s) + 2NaHCO3(aq)
The sodium ions from sodium hydrogen carbonate do not cause water hardness like calcium and magnesium ions. The following illustration shows how this works.
Temporary hardness can be removed by:
i. Boiling where the hydrogen carbonate ion is composed of carbonate ion of either Mg2+ or Ca2+ which are insoluble hence removed from water making it soft. This is shown in the following equations.
Ca(HCO3)2 (aq) ➙ CaCO3(s) + H2O (l) + CO2 (g)
Mg (HCO3)2 (aq) ➙ MgCO3(s) + H2O (l) + CO2 (g)
ii. Use of calcium hydroxide or sodium carbonate. The method of
boiling is expensive on large scale. Calculated amounts of either
calcium hydroxide or sodium carbonate are used to cause precipitation of
Mg2+ or Ca2+ions as carbonates .The equations below show how
Ca2+ and Mg2+ are removed.
Mg (HCO3)2 (aq) ➙ MgCO3(s) + H2O (l) + CO2 (g)
Ca(HCO3)2 (aq) ➙ CaCO3(s) + H2O (l) + CO2 (g)
Ion- exchange method involves use of a resin made from an insoluble complex of a sodium salt called sodium permutit or sodium aluminum silicate. The permutit releases sodium ions which exchange either calcium or magnesium ions in hard water as shown below.
When all the sodium ions in permutit column have been exchanged with calcium or magnesium ion, it is discharged since it contains calcium and magnesium ions.
The permutit can be regenerated by passing concentrated sodium chloride or brine through it to initiate another ion exchange process to recharge it and make it ready for use again.
Permanent hardness can be removed by
i) Addition of sodium carbonate to precipitate calcium and magnesium ions as insoluble carbonate. Thus
MgSO4(aq) + Na2CO3(s) ➙ MgCO3(s) + Na2SO4(aq)
ii) Distillation method removes both temporary an permanent hardness since it removes all dissolve substances but it is expensive on large-scale. It also distils water from analytical work or for batteries.
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