Acids

Introduction

An acid is a chemical substance having strong hydrogen ion concentration with a strong corrosive property. An acid can turn the blue litmus into the red. Acid has the ability to neutralizing alkali and dissolving some metals.

 

Table Of Contents

1. Introduction

2. Classification of Acids

3. Chemical Properties of Acids

4. Common acid-base tests

 

Classification of Acids

There are various criteria to classify acids. They are classified based on their source, the presence of oxygen, acid strength or concentration, etc.

Classification based on source

Organic acids are extracted from organic materials such as plants and animals. For e.g. Citric acid (Citrus fruits), Acetic acid (Vinegar).

 

Mineral Acid is obtained from minerals. These are also known as inorganic acids as they don’t have carbon in them. For e.g.  H2SO4, HCl. HNO3, etc.

 

Classification based on the presence of Oxygen

 

Oxy-acids consist of oxygen in their composition, e.g. H2SO4, HNO3, etc.

 

Hydracids have hydrogen combined with other elements. They do not contain any oxygen in their composition e.g. HCl, HI, HBr, etc.

 

Classification based on the Strength of the acid

 

 Strong Acids are those which can be dissociated completely for e.g.  Sulphuric acid, nitric acid, hydrochloric acid, etc.

 

Weak Acids will not dissociate completely or dissociates negligibly in water. Example, citric acid, acetic acid, etc.

 

Classification based on the concentration

Concentrated Acids are aqueous solutions with a relatively high percentage of acid dissolved in it. E.g. concentrated hydrochloric acid, concentrated Sulphuric acid, concentrated nitric acid, etc.,

 

Diluted acid is an aqueous solution that has a relatively lesser percentage of acid dissolved in it. E.g. dilute hydrochloric acid, dilute nitric acid, etc.

 

Classification based on the basicity of the acid

 

Monobasic Acid, which has only one hydrogen ion. Therefore, these acids combine with one hydroxyl group of the base to form salt and water.  For e.g. HCl, HCOOH, HBr, etc.

Dibasic Acids share two hydroxyl groups and these are dibasic acids.

H2SO4(aq) <—> H+(aq) + HSO-4(aq)

2NaOH(aq) + H2SO4(aq) <—-> Na2SO4(aq) + 2H2O(l)

Tribasic acids can combine with three hydroxyl groups, and they produce 3 types of salts. For e.g. H3PO4

NaOH(aq) + H3PO4(aq) <—> NaH2PO4(aq) + H2O(l)

2NaOH(aq) + H3PO4(aq) <—> Na2HPO4(aq) + 2H2O(l)

 

 

 

Chemical Properties of Acids

Chemicals act differently when combined with different organic and inorganic materials, metals, salts, carbonates, etc. 3 main reactions of acids are as follows.

1. Acid+metal→salt+hydrogen gas

2. Acid+base→salt+water

3. Acid+carbonate→salt+water+carbon dioxide gas.

 

Common acid-base tests

Testing acids and Bases Reaction with Metals

  Acid+metal

1. Acids react with metals to result in a salt and hydrogen gas. Metals that are more active than acids can undergo a single displacement reaction. For example, when zinc metal combined with hydrochloric acid the products are zinc chloride and hydrogen gas.

2. Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g) Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)

3. Bases react with certain metals like zinc or aluminium, for example, to also produce hydrogen gas. Sodium hydroxide reacts with zinc and water to form sodium zincate and hydrogen gas. Zn(s)+2NaOH(aq)+2H2O(l)→Na2Zn(OH)4(aq)+H2(g).

 

Base +metal

Base + Metal →Salt + Hydrogen .i, e NaOH + Zn+ Na2ZnO2 (Sodium Zincate) + H2

 

Testing of acids, and bases (Neutralization Reactions)

A. When HCl is mixed with a base such as NaOH, the resulting products are salt (sodium chloride) and water (H2O). It can be written as HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)(14.5.1)(14.5.1)HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)

B. Here Salts are ionic compounds containing a positive ion other than H+ and a negative ion other than the hydroxide ion, OH-. Double displacement reactions of this type are called neutralization reactions.

C. When a strong acid and a strong base are combined in the proper amounts - i,e, when [H+] [H+] equals [OH− [OH−]\) - a neutral solution results in which pH = 7. The acid and bases will get neutralized with each other, and the acidic and basic properties are no longer present.

D. Salt solutions do not always have a pH of 7, however. Through a process known as hydrolysis, the ions produced when an acid and base combine may react with the water molecules to produce a solution that is slightly acidic or basic.

 

Testing  acids with carbonates

Acids react with carbonate or hydrogen carbonate to form a salt, water, and carbon dioxide gas as shown in the equation.

A. E.g. Calcium carbonate (limestone), CaCO3 and dilute hydrochloric acid react to give off carbon dioxide gas.

B. CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)

C. The carbon dioxide given off produces an effervescence. Carbon dioxide turns the limewater chalky.

 

What happens to acid and base in water?

Acids in water solution dissociates H+ ions. Let us consider the reaction between water and hydrochloric acid (HCl).  HCl in the presence of water produces H+ ion. This ion cannot exist alone and hence combines with water molecules and forms H3O+. The reaction HCL+H2O =H3o+Cl. Base -when dissolved in water produces OH- ion. Let us consider the reaction between water and sodium hydroxide NaOH. NaOH in the presence of water produces OH- ion. The reaction is NaOH(s) + H2O (l) => Na+ + OH- + H20 + HEAT.

 

The Pop Test

When a burning candle is held close to the test tube containing hydrogen gas, it burns with a ‘Pop’ sound ( see image below). This will demonstrate the presence of hydrogen gas. Equipment required includes, test tube and rack, dilute acid (egg. hydrochloric, Sulphuric or nitric) and a piece of magnesium Wooden splint, Safety glasses

 


 

STEP1: Take about 2 cm depth of diluted acid in a test tube and add a small piece of magnesium to this and wait for one minute, bubbles of Hydrogen will have come off the magnesium and there should be a high pressure on your thumb.

STEP2: Bring a lighted splint lamp to the end of the test tube carefully and immediately take your finger or thumb away. Soon you will see that there is a distinctive 'squeaky pop' sound. If you do not hear the ' pop' then try the experiment again.

STEP3: Dispose of it carefully. Separate the metal and liquid carefully. Pour the liquid down the sink and place the metal in the bin.

 

Lime water Test

Lime water is chemically composed of Calcium hydroxide Ca (OH) 2 and it is a colourless solution with alkaline structure.

Procedure:

1. Make lime water by adding a small amount of calcium hydroxide to a test tube then water. Now filter out the clear solution and transfer it to the beaker.

2. Add about 2 cm depth lime water to a test tube and blow it through a drinking straw to bubble CO2 through the lime-water solution. What will you see? You can see a thick precipitate or cloudy substance in 20-30 seconds.

3. Turn the lime water clear again. Continue to bubble CO2 through the solution until the precipitate just disappears.

The equation for the test is written as,

=Ca(OH)2 (aq) + CO2(g) = CaCO3(s) + H2O(l)

=On passing excess CO2 it leads to CaCO3(s) + H2O(l) + CO2(g)  = Ca(HCO3)2 aq Soluble in water

 

Litmus test

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The litmus test is a quick way to find whether a liquid or gaseous solution is acidic or basic (alkaline). The test is based on how the liquid can cause a change in the colour of a litmus paper. To perform this test, place a drop of liquid sample on a small strip of paper or dip a piece of litmus paper in a small specimen of the sample. The litmus test can produce a range of colours ( as shown in the picture below). A change in the colour will decide whether the solution is acidic, alkaline or neutral. It can be performed with a blue litmus paper where the blue paper changes to red, indicating acidity somewhere between the pH range of 4.5 to 8.3. (Note, however, that 8.3 is alkaline.) Red litmus paper can indicate alkalinity with a change to blue.

 

 





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