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Strong Acids and Bases

Strong Acids and Bases. Note. It is important that you don't confuse the words strong and weak with the terms concentrated and dilute . As you will see, the strength of an acid is related to the proportion of it that has reacted with water to produce ions.

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Strong Acids and Bases

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  1. Strong Acids and Bases

  2. Note • It is important that you don't confuse the words strong and weak with the terms concentrated and dilute. • As you will see, the strength of an acid is related to the proportion of it that has reacted with water to produce ions. • The concentration tells you about how much of the original acid is dissolved in the solution. • It is possible to have a concentrated solution of a weak acid, or a dilute solution of a strong acid.

  3. Acids • When an acid dissolves in water, a proton (hydrogen ion) is transferred to a water molecule to produce a hydroxonium ion and a negative ion depending on what acid you are starting from. • In the general case . . . HA + H2O H3O+ + A- • The strength of an acid is defined by the equilibrium position of the dissociation reaction shown above

  4. Strong Acids • The reaction is reversible, but with a strong acid the equilibrium lies far to the right. This means the original acid is virtually 100% dissociated(ionized) • For example, when hydrogen chloride dissolves in water to make hydrochloric acid, so little of the reverse reaction happens that we can write: HCl + H2O H3O+ +Cl-

  5. Strong Acids • At any one time, virtually 100% of the hydrogen chloride will have reacted to produce hydroxonium ions and chloride ions. Hydrogen chloride is described as a strong acid. • A strong acid is one which is virtually 100% ionized in solution. • Other common strong acids include: HCl - hydrochloric acid, HNO3- nitric acid H2SO4- sulfuric acid, HBr- hydrobromic acid, HI- hydroiodic acid, and HClO4- perchloric acid

  6. Strong Acid There are four ways to describe a strong acid: • Ka is large • Position of the dissociation equilibrium lies far to the right • The equilibrium concentration of H+ approximately equal to the original concentration of HA ([H+]≈[HA]) • The Strength of conjugate base of a strong acid is a much weaker base than H2O

  7. Working out the pH of a strong acid • With strong acids this is easy. • Hydrochloric acid is a strong acid - virtually 100% ionized. Each mole of HCl reacts with the water to give 1 mole of hydrogen ions and 1 mole of chloride ions • That means that if the concentration of the acid is 0.001M then the concentration of hydrogen ions is also 0.001M • So finding the pH is easy, just use your equation: pH=-log[H+]

  8. Example-Strong Acids Calculate the pH of an aqueous solution containing 2.5x10-5M of HCl.

  9. Solution So first write the dissociation expression with HCl and water HCl + H2O H3O+ + Cl- Second, since we know that all the HCl will be converted to products, because HCl is a strong acid, we know that the concentrations of H3O+ and Cl- will be the same as the original acid. All we need to do now is plug the value into the pH equation Recall from yesterday pH=-log[H3O+] Doing so we get pH=-log(2.5x10-5)=4.6

  10. Strong Bases • Much the same as strong acids, strong bases dissociate 100% into the cation and OH- (hydroxide ion). The hydroxides of the Group IA and Group IIA metals usually are considered to be strong bases, they include: Group IA- LiOH, NaOH,KOH, RbOH and CsOH(only NaOH and KOH are common because the others are expensive to work with) Group IIA- Ca(OH)2, Ba(OH)2, Sr(OH)2 For these soultions 2 moles of hydroxide ion is produced for every 1 mole of metal hydroxide dissolved in aqueous solution.

  11. Strong Bases Much the same as acids, there are four ways to describe a strong base: • Kb is very large • Position of the dissociation equilibrium lies far to the right • Equilibrium concentration of OH- compared t original strong base is approximately equal • Strength of conjugate acid is much weaker acid than H2O

  12. Working out the pH of a strong base • With strong bases this is easy as well. • Sodium hydroxide is a strong base - virtually 100% ionized. Each mole of NaOH reacts with the water to give 1 mole of hydroxide ions and 1 mole of sodium ions • That means that if the concentration of the base is 0.001M then the concentration of hydroxide ions is also 0.001M • So finding the pH is easy, use the equation pH+pOH=14, find the pOH and subtract from 14 to get the pH.(You could also use the Kw expression to find the [H+] then find the pH, either way is acceptable)

  13. Example Calculate the pH of a 5.0x10-2M KOH

  14. Solution So first write the dissociation expression with KOH and water KOH K+ + OH- Second, since we know that all the KOH will be converted to products, because KOH is a strong base, we know that the concentrations of K+ and OH- will be the same as the original base All we need to do now is find pOH and use the realtionship between pOH and pH Recall from yesterday pOH=-log[OH-] Doing so we get pOH=-log(5.0x10-2)=1.30 Next subtract from 14 to get pH pH=14-pOH=12.7 H2O

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