Acids and basesAcids and baseskey theory: Bronsted-Lowry theory of acids and bases:Acid-base reactions are ones in which protons are given and takenAcidsareprotondonorsBases are proton acceptors
1 Acids and bases Acids and bases key theory: Brønsted-Lowry theory of acids and bases: Acid-base reactions are ones in which protons are given and taken. Acids are proton donors Bases are proton acceptors
Acids and basesConjugate pairsEvery acid has what is called a conjugate base. This is particleleft when the acid has given away its proton. In the same way,every base has a conjugate acid, which is the particle made whenthe proton has arrived. So the conjugate base of HClis CI andthe conjugate acid of ammonia is NH t.Familiar conjugate pairsAcidBaseH,0OH-H,O+H0H,SO4HSO4SO,2-HSO4CH,COOHCH,COOHNO3NO3H,CO3HCO3CO,2-HCO32NH4+NH3
2 Acids and bases Conjugate pairs Every acid has what is called a conjugate base. This is particle left when the acid has given away its proton. In the same way, every base has a conjugate acid, which is the particle made when the proton has arrived. So the conjugate base of HCl is Cl- and the conjugate acid of ammonia is NH4 + . Familiar conjugate pairs Acid Base H2O OHH3O+ H2O H2SO4 HSO4 - HSO4 - SO4 2- CH3COOH CH3COOHNO3 NO3 - H2CO3 HCO3 - HCO3 - CO3 2- NH4 + NH3
Acids and basesK.=5.55×108H,O+ + ClHCI + H,OCONJUGATECONJUGATEBASEACIDACIDBASEK_=3.12×10-8NH + + OHH,O+NHTo be a base, a substanceTo be able to give protons,must have a lone pair whichan acid must have acan become a sigma pair ashydrogen atom which canit accepts a protonbe lost:e.g. :NH3e.g. HCl; H,SO43
3 Acids and bases To be able to give protons, an acid must have a hydrogen atom which can be lost: e.g. HCl; H2SO4 To be a base, a substance must have a lone pair which can become a sigma pair as it accepts a proton e.g. :NH3
Acids and basesK.=5.55×108H,O+ + CHCI + H,O→CONJUGATECONJUGATEBASEACIDACIDBASEK,=3.12×10-8NH + + OHH,O +NHAmpholytesThe water in the first reaction above is acting as a base. One of itslonepairs accepts aproton.In the second reaction the water is being an acid, giving one of itsprotons to ammonia.Substances like water, with hydrogens that can be lost and lonepairs that can be protonated, are called ampholytes. They can beeither acids or bases depending on what they react with.You can see some other common ampholytes in the list on the topright, because they appear in both the acid and base columns.Even ammonia can be an ampholyte in the right conditions.having NH+ as its conjugate acid and NH, as its conjugate base
4 Acids and bases The water in the first reaction above is acting as a base. One of its lone pairs accepts a proton. In the second reaction the water is being an acid, giving one of its protons to ammonia. Substances like water, with hydrogens that can be lost and lone pairs that can be protonated, are called ampholytes. They can be either acids or bases depending on what they react with. You can see some other common ampholytes in the list on the top right, because they appear in both the acid and base columns. Even ammonia can be an ampholyte in the right conditions, having NH4 + as its conjugate acid and NH2 - as its conjugate base. Ampholytes
Acids and basesSTRONGANDWEAKACIDSANDBASESThe equilibrium constants for the two reactions above are verydifferent.The first one, 5.5 X 108, tells us that the reaction lies well over to theright. That means that there are practically no hydrogen chloridemolecules left because they have almost all become ions. Thissolution would conduct strongly and so the hydrochloric acid isdescribed as a strong acid.In the second reaction the constant is 3.12 X 10-8. This tells us thatabout one in a thousand ammonia molecules have reacted withwater. There are hardly any ions in this solution and so it conductsweakly. Ammonia is called a weak baseSo in water strong acids and bases exist mainly as ions, weak acidsand bases exist mainly as molecules.5
5 Acids and bases STRONG AND WEAK ACIDS AND BASES The equilibrium constants for the two reactions above are very different. The first one, 5.5×108 , tells us that the reaction lies well over to the right. That means that there are practically no hydrogen chloride molecules left because they have almost all become ions. This solution would conduct strongly and so the hydrochloric acid is described as a strong acid. In the second reaction the constant is 3.12×10-8 . This tells us that about one in a thousand ammonia molecules have reacted with water. There are hardly any ions in this solution and so it conducts weakly. Ammonia is called a weak base. So in water strong acids and bases exist mainly as ions, weak acids and bases exist mainly as molecules