Properties of Acids and Bases & Theories defining acids and bases

The acids and bases represent a large part of human life , Acids are used in many chemical industries like Fertilizers , Medicines , Plastic & Car batteries , Bases also have many uses in chemical industries like Soap , Detergents , Dyes & Medicines .

For example , The vinegar that is used in some food and cleaning processes is an acidic solution that was early discovered .

Natural and industrial products and the acids and bases entering in their composition or preparation 

Acidic plants ( lemon – oranges – tomatoes ) contain Citric acid and Ascorbic acid , Dairy products ( milk – cheese – yoghurt ) contain Lactic acid , Soft drinks contain Carbonic acid and Phosphoric acid .

Soap contains Sodium hydroxide , Baking soda contains Sodium bicarbonate , Washing soda contains Hydrated sodium carbonate .

Properties of Acids

Acids have sour taste , They change the color of litmus dye or litmus paper into red , Acids react with bases producing salt and water .

Na OH (aq ) + HNO3 ( l ) → NaNO3 (aq ) + H2 O ( l)

Acids react with active metals producing salt of acid and hydrogen gas H2 evolves .

Zn ( s ) + 2 HCL (aq ) → ZnCl2 (aq ) + H2 ( g ) ↑

Acids react with Carbonate or bicarbonate salts causing effervescence and producing carbon dioxide gas CO2 which makes limewater turbid . 

Na2 CO3 ( s ) + H2 SO4 ( aq ) → Na2 SO4(aq ) + H2 O ( l ) + CO2 ( g ) ↑

Properties of Bases

Bases have bitter taste and slippy feel , Bases change the colour of litmus dye or litmus paper into blue , Bases react with acids producing salt and water .

Acids and bases

Acids and bases

Theories defining acids and bases

The external appearance of the acid and base leads to an experimental definition for each , The experimental definition is based on observation only and doesn’t describe or explain the unseen properties .

The more overall definitions of acid and base come from the studies and experiments that were placed in the form of theories , Acid-base theories are Arrhenius’s theory , Bronsted- Lowry’s theory and Lewis’s theory .

Arrhenius’s theory in 1887

Arrhenius observed that the aqueous solutions of acids and bases conduct the electricity , This observation proves that acids and bases are ionized in the water .

Dissolving of hydrogen chloride in the water : When hydrogen chloride gas dissolves in the water , It is ionized into hydrogen ions ( H+ ) and chloride ions ( Cl)  , Dissolvig of acid in the water produces hydrogen ions .

Dissolving of sodium hydroxide in the water : When sodium hydroxide dissolves in the water , It is dissociated to sodium ions ( Na+ ) and hydroxide ions ( OH ) , Dissolving of base in the water produces hydroxide ions .

Arrhenius revealed ( proved ) his theory to define acids and bases , Arrhenius acid is the substance that dissolves in the water and ionized or dissociated giving one or more hydrogen ions ( H+ ) .

Arrhenius base is the substance that dissolves in the water and ionized or dissociated giving one or more hydroxide ions ( OH ) .

According to Arrhenius’s theory we observe that Arrhenius acid must contain a source of hydrogen ions ( H+ ) , Arrhenius base  must contain a source of hydroxide ions ( OH ) .

So , Acids works on increasing the concentration of ( H+ ) ions in the aqueous solutions , Base works on increasing the concentration of ( OH ) ions in the aqueous solutions .

Explaining neutralization reaction according to Arrhenius’s theory

Neutralization reaction is the reaction between an acid and a base to produce salt and water .

HCl ( aq ) + NaOH ( aq ) → NaCl ( aq ) + H2 O ( l )

The ionic equation of this reaction according to Arrhenius’s theory is :

 H+(aq) + Cl(aq) + Na+ (aq) + OH( aq ) → Na+ (aq) + Cl(aq) + H2 O ( l )

Na+ (aq) and Cl(aq) ions are present in both sides of the equation without changing , So , they can be neglected ( cancelled ) from the both sides of the chemical equation , So , the neutralization reaction can be represented by the following equation :

 H+(aq) + OH( aq ) → H2 O ( l )

Observations on ( Deffects of ) Arrhenius’s theory 

Carbon dioxide CO2 doesn’t contain a source of ( H+ ) ions , but it dissolves in water giving an aqueous acidic solution .

Ammonia NH3 doesn’t contain a source of ( OH ) ions , but it dissolves in water giving an aqueous basic solution .

These observations do not agree with Arrhenius’s theory .

The Bronsted- Lowry’s theory in 1923

The Danish Scientist Bronsted and the British one Lowry proposed a new acid-base theory .

Bronsted-Lowry acid is the substance that give up the proton ( H+ ) , Proton donor .

Bronsted-Lowry base is the substance that accepts the proton ( H+ ) , Proton acceptor .

Application

Bronsted-Lowry theory describes the behavior of ammonia as a base .

H2 O ( l ) + NH3 ( g ) → NH+4 (aq) + OH (aq)

When ammonia gas dissolves in the water , H2 O acts as an acid because it gives up a proton ( H+ ) to ammonia molecule ( NH3 ) .

As a result of this transfer , OH formed and it is called Conjugate baseConjugate base is the substance which is formed when the acid loses a proton ( H+ ) .

, NH3 acts as a base because it accepts the proton ( H+ ) from the water molecule .

As a result of this transfer , NH+4 formed and it is called Conjugate acid , Conjugate acid is the substance which is formed when the base accepts a proton ( H+ ) .

Lewis’s theory in 1923

Lewis’s proposed an acid-base theory which depends on sharing of a lone pair of electrons between acid and base , Lewis acid is the substance that accepts an electron pair or more , Lewis base is the substance that donates an electron pair or more .

Ammonia is considered as a base , although it does not contain hydroxide group in its structure , Because according to Bronsted – Lowry’s theory , Ammonia accepts a proton from another substance ( as water ) during the reaction and according to Lewis’s theory it donates a lone pair of electrons to another substance ( as water ) during the reaction .

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