We already know about Acid and Bases from previous classes. Here it is brief refresher
An acid is a substance that prodices (H⁺ ion) when dissolved in water. Acids taste sour, can turn blue litmus paper red, and react with bases to form salts and water.
A base, on the other hand, is a substance which produces (OH- ion) when dissolved in water. Bases taste bitter, can turn red litmus paper blue, and also react with acids to form salts and water.
Acids and bases can be identified using indicators like red litmus, blue litmus, phenolphthalein, and methyl orange.
Olfactory indicators like onion, vanilla, and clove change their odour in acidic or basic media
Now in this chapter we will look at the chemical properties of acids and bases
Reaction of Acid with Metals
When an acid reacts with a metal, a salt and hydrogen gas are produced. This is a general reaction that can be represented by the following equation:
Acid + Metal -> Salt + Hydrogen gas
In this reaction, the acid reacts with the metal to form a compound called a salt, and hydrogen gas is released.
For example, when hydrochloric acid (HCl) reacts with zinc (Zn), zinc chloride ($ZnCl_2$) and hydrogen gas ($H_2$) are produced:
$2HCl(aq) + Zn(s) -> ZnCl_2(aq) + H_2(g)$
In the above reaction, the hydrochloric acid (HCl) reacts with zinc (Zn) to produce zinc chloride (ZnCl₂), a type of salt, and hydrogen gas (H₂).
It is important to note that not all metals react with acids to produce hydrogen gas. For instance, copper does not react with dilute acids to produce hydrogen because it is less reactive than hydrogen.
Metals which can displace hydrogen from dilute acid are known as active metals
Example Na, K,Zn,Fe,Ca,Mg
Reaction of Base with Metals
Bases react with some metaks to librate hydrogen. This generally happen with Strong Bases line NaOH,KOH
$Zn(s) + 2NaOH(dilute) -> Na_2ZnO_2 (aq) + H_2 (g)$
In this reaction Sodium zincate is formed
Another example will be
$Sn(s) + 2NaOH(dilute) -> Na_2SnO_2 (aq) + H_2 (g)$
Reaction of Acids with Metal carbonates and metal hydrogen carbonates
Metal carbonates and metal hydrogencarbonates react with acids to produce a salt, carbon dioxide, and water. This is a general reaction that can be represented by the following equations:
For metal carbonates:
Acid + Metal Carbonate -> Salt + Carbon Dioxide + Water
For metal hydrogencarbonates:
Acid + Metal Hydrogencarbonate -> Salt + Carbon Dioxide + Water
For example, when hydrochloric acid (HCl) reacts with sodium carbonate ($Na_2CO_3$), it forms sodium chloride (NaCl), carbon dioxide ($CO_2$), and water ($H_2O$):
$2HCl(aq) + Na_2CO_3(s) -> 2NaCl(aq) + CO_2(g) + H_2O(l)$
Similarly, when hydrochloric acid (HCl) reacts with sodium hydrogencarbonate (also known as sodium bicarbonate, ($NaHCO_3$), it forms sodium chloride (NaCl), carbon dioxide ($CO_2$), and water (H₂O):
$HCl(aq) + NaHCO_3(s) -> NaCl(aq) + CO_2(g) + H_2O(l)$
In both reactions, the acid reacts with the carbonate or hydrogencarbonate to form a salt (sodium chloride in these examples), and carbon dioxide gas and water are released.
The release of carbon dioxide gas can be observed as effervescence or fizzing, and this is a common test for the presence of carbonate or hydrogencarbonate ions in a sample.
One more example will be
$H_2SO_4(aq) + Na_2CO_3(s) -> 2Na_2SO_4(aq) + CO_2(g) + H_2O(l)$
$H_2SO_4(aq) + 2NaHCO_3(s) -> Na_2SO_4(aq) + CO_2(g) + H_2O(l)
Reaction of Acids with Metallic Oxides
Metallic oxides react with acids to form a salt and water. This type of reaction is known as a neutralization reaction because the metallic oxide (a base) and the acid neutralize each other. The general equation for this reaction is:
Acid + Metallic Oxide → Salt + Water
For example, consider the reaction between hydrochloric acid (HCl) and copper(II) oxide (CuO):
$2HCl(aq) + CuO(s) -> CuCl_2(aq) + H_2O(l)$
In this reaction, hydrochloric acid (HCl) reacts with copper(II) oxide (CuO) to form copper(II) chloride (CuCl₂), which is a salt, and water (H₂O).
Another example is the reaction between sulfuric acid ($H_2SO_4$) and iron(III) oxide ($Fe_2O_3$):
$6H_2SO_4(aq) + Fe_2O_3(s) -> 2Fe_2(SO_4)_3(aq) + 3H_3O(l)$
In this reaction, sulfuric acid ($H_2SO_4$) reacts with iron(III) oxide ($Fe_2O_3$) to form iron(III) sulfate ($Fe_2(SO_4)_3$), which is a salt, and water ($H_2O$).
Reaction of Acids with Bases
The reaction between an acid and a base is known as a neutralization reaction. In a neutralization reaction, the acid and base react to form a salt and water. The general equation for this reaction is:
Acid + Base → Salt + Water
Example
$HCl_{(aq)} + NaOH_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)}$
In this reaction, hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to form sodium chloride (NaCl), which is a salt, and water (H2O).
Another example
$H_2SO_4 (aq) + 2KOH(aq) \rightarrow K_2SO_4(aq) + 2H_2O$
In this reaction, sulfuric acid (H2SO4) reacts with potassium hydroxide (KOH) to form potassium sulfate (K2SO4), which is a salt, and water (1₂O).
Neutralization reactions are exothermic, meaning they release heat. This is why the solution often feels warm to the touch after the reaction has occurred. The pH of the resulting solution will be closer to 7, which is neutral on the pH scale, hence the name "neutralization" reaction.
Reaction of Metallic Sulphites and Hydrogen sulphites
Metallic Sulphites and Hydrogen sulphites react with acids to produce a salt, suplhure dioxide, and water.
$CaSO_3(s) + H_2SO_4 -> CaSO_4(aq) + H_2) + SO_2(g)$
$NaHSO_3(s) + HCL -> NaCL(aq) + H_2) + SO_2(g)$
Reaction of Metallic Sulphides and Hydrogen sulphides
Metallic Sulphides and Hydrogen sulphides react with acids to produce a salt, hydrogen Sulphides, and water.
$FeS + H_2SO_4 -> FeSO_4 (aq) + H_2S(g)$
$KHS + HCl -> KCL(aq) + H_2S(g)$
Reaction of Non Metallic Oxide with Bases
Non-metallic oxides react with bases to form a salt and water. This is similar to the reaction of acids with bases, and is also considered a type of neutralization reaction. The general equation for this reaction is:
Base + Non-metallic Oxide -> Salt + Water
For example, consider the reaction between sodium hydroxide (NaOH), a base, and carbon dioxide ($CO_2$), a non-metallic oxide:
$2NaOH(aq) + CO_2(q) -> Na_2CO_3(aq) + H_2O$
In this reaction, sodium hydroxide (NaOH) reacts with carbon dioxide ($CO_2$) to form sodium carbonate ($Na_2CO_3$), which is a salt, and water ($H_2O$).
Another example is the reaction between calcium hydroxide ($Ca(OH)_2$), a base, and sulfur dioxide ($SO_2$), a non-metallic oxide:
$Ca(OH)_2(aq) + SO_2(g) -> CaSO_3(aq) + H_2O$
In this reaction, calcium hydroxide ($Ca(OH)_2$) reacts with sulfur dioxide ($SO_2$) to form calcium sulfite ($CaSO_3$), which is a salt, and water ($H_2O$).
What do all acids and all bases have in common
A common thing about all the acids is that they all produces $H^+$ when dissolves in water. Basically they dissociate in aqueous solutions to produce hydrogen ions
Example:
$HCL -> H^+ + Cl^-$
contain H+ ion as cation and anion such as Cl–
Similarly
$HNO_3 -> H^+ + NO_3^-$
$H_2SO_4 -> 2H^+ + SO_4^{2-}$
A common thing about all the bases is that they all produces $OH^-$ when dissolves in water. Basically they dissociate in aqueous solutions to produce hydroxide ions
Example:
$NaOH -> Na^+ + OH^-$
Bases contain Na+ ion as cation and anion such as OH–
Similarly
$Mg(OH)_2 -> Mg^{2+} + 2OH^-$
$Ca_2(OH)+2 -> 2Ca^+ + 2OH^-$
Acid and Bases in Water
The acidic nature of acid is because of the H+ ions only and acid can produce H+ ions when dissolves in water only. If the acid is dry, then it will not show the acidic nature.
So,
$HCl(aq) -> H^+ + Cl^-$
Also H+ cannot exist alone, so they combine with water to forms hydronium ions
$H^+ + H_2O -> H_3O^+$
Here basically water helps in disociating the acids into ions
Similarly The basic nature of base is because of the OH- ions only and base can produce OH- ions when dissolves in water only. If the base is dry, then it will not show the basic nature.
So,
$NaOH(aq) -> Na^+(aq) + OH^-(aq)$
Bases which are soluble in water are called Alkalis. All bases do not dissolve in water
Now we know that acid produces H+ ions in water and bases produces OH- in water, so this explain the formation of H_2) during the neutralisation reaction
$HCL + NaOH -> NaCl + H_2O$
$H^+ + OH^- -> H_2O$
How strong are acids and bases
Strong Acids and Bases
Strong acids and bases are those that completely ionize or dissociate in water. In other words, they donate or accept protons completely. Examples of strong acids include hydrochloric acid (HCl), nitric acid ($HNO_3$), and sulfuric acid ($H_2SO_4$). Strong bases include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide ($Ca(OH)_2$).
Weak Acids and Bases
Weak acids and bases are those that do not completely ionize in water. They only partially donate or accept protons. Examples of weak acids include acetic acid ($CH_3COOH$) and citric acid ($C_6H_8O_7$). Weak bases include ammonia ($NH_3$) and methylamine ($CH_3NH_2$).
pH Scale
The pH scale is a measure of the acidity or alkalinity of a solution. It ranges from 0 to 14 and gives the measure of hydrogen ion concentration in a solution.
A neutral solution has a pH of exactly 7.
An acidic solution has a pH less than 7.
A basic solution has a pH more than 7.
The term "pH" stands for 'potenz' in German, meaning power. On the pH scale, we can measure pH generally from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution.
The higher the hydronium ion concentration, the lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH– ion concentration in the solution, that is, increase in the strength of alkali.
Importance of pH in Everyday Life
pH in Animals and Plants
Living beings carry out their metabolic activities within an optimal pH range. For example, our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change.
When pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water. The survival of aquatic life in such rivers becomes difficult.
pH in Our Digestive System
Our stomach produces hydrochloric acid which helps in the digestion of food without harming the stomach. During indigestion, the stomach produces too much acid and this causes pain and irritation.
To get rid of this pain, people use bases called antacids. These antacids neutralise the excess acid. Magnesium hydroxide (Milk of magnesia), a mild base, is often used for this purpose.
pH Change as the Cause of Tooth Decay
Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel, made up of calcium hydroxyapatite, is the hardest substance in the body. It does not dissolve in water, but is corroded when the pH in the mouth is below 5.5. Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.
Self Defence by Animals and Plants through Chemical Warfare
Bee-sting leaves an acid which causes pain and irritation. Use of a mild base like baking soda on the stung area gives relief. Stinging hair of nettle leaves inject methanoic acid causing burning pain.
Salts
Salts are ionic compounds that result from the neutralization reaction of an acid and a base.
They are composed of cations (positive ions) other than H+ and anions (negative ions) other than OH- and can be formed from various acids and bases.
$H_2SO_4 + 2 NaOH -> Na_2SO_4 + 2H_2O$
Families of Salts
Salts having the same positive or negative radicals are said to belong to a family.
For example, NaCl (sodium chloride) and $Na_2SO_4$ (sodium sulfate) belong to the family of sodium salts.
Similarly, NaCl and KCl (potassium chloride) belong to the family of chloride salts
Potassium Sulpate($K_2SO_4$) and Sodium Sulphae($Na_2SO_4$) belongs to sulphate family
Naming of Salts
Salts obtained hydrochloric acid are called Chlorides e.g NaCl, KCl
Salts obtained Sulphuric acid are called sulphates e.g $Na_2SO_4$, $CuSO_4$
Salts obtained carbonic acid are called carbonates e.g $Na_2CO_3$, $CaCO_3$
Salts obtained Nitric acid are called Nitrates e.g $KNO_3$, $NaNO_3$
pH of Salts
The pH of a salt solution can be acidic, basic, or neutral.
Salts that form from a strong acid and a strong base are neutral (pH = 7).
Salts that form from a strong acid and a weak base are acidic (pH < 7).
Salts that form from a strong base and a weak acid are basic (pH > 7).
Common Salt (Sodium Chloride)
Common salt, also known as sodium chloride (NaCl)
It is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms.
It is essential for life
It is used in various ways in our daily life, such as in our diet, in medicine, and in chemical industries.
Chemicals from Common Salt
Common salt is an important raw material for various materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder, and many more.
Sodium Hydroxide
When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.
$2NaCl(aq) + 2H_2O(l) -> 2NaOH(aq) + Cl_2(g) + H_2(g)$
Bleaching Powder
Bleaching powder is produced by the action of chlorine on dry slaked lime [$Ca(OH)_2$]. Bleaching powder is represented as CaOCl2, though the actual composition is quite complex.
$Ca(OH)_2+ Cl_2 -> CaOCl_2 + H_2O$
Baking Soda
Baking soda, or sodium bicarbonate (NaHCO3), is commonly used in the kitchen for making various dishes. It is produced using sodium chloride as one of the raw materials.
$NaCl + H_2O + CO_2 + NH_3 -> NH_4 Cl + NaHCO_3$
Washing Soda
Washing soda, or sodium carbonate (Na2CO3), is another chemical that can be obtained from sodium chloride. It is used in laundry as a water softener.
$Na_2 CO_3 +10H_20 -> Na_2 CO_3.10H_20$
Gypsum
Gypsum is a soft sulphate mineral composed of calcium sulphate dihydrate, with the chemical formula CaSO4.2H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard chalk and wallboard.
Plaster of Paris
Plaster of Paris is a white powdery substance that is used for various purposes like making toys, materials for decoration, and for making surfaces smooth. It is also used by doctors as a plaster for supporting fractured bones in the right position.
Plaster of Paris is obtained by heating gypsum at 373 K. When gypsum is heated to such a temperature, it loses water molecules and becomes calcium sulfate hemihydrate (CaSO4.1/2H2O). This is called Plaster of Paris. The reaction can be represented as:
$CaSO_4.2H_2O --> CaSO_4.1/2H_2O + 1.5H_2O$
Note that only half a water molecule is shown to be attached as water of crystallisation. It is written in this form because two formula units of CaSO4 share one molecule of water.
When Plaster of Paris is mixed with water, it changes back to gypsum, giving a hard solid mass. This property of Plaster of Paris is used in construction, arts, medicine, and many other fields.
