strong & weak acids

• ACIDS:


  Solutions that release HYDROGEN IONS (H⁺) in water.

  • They do this because they IONISE when dissolved in water.
  • Meaning the molecules of the acid BREAK UP into their IONS.

  
• STRONG ACIDS:


  • Acids that FULLY IONISE in water, meaning ALL of the acid molecules break up into IONS.
  • This means ALL their available H+ ions are released into the solution.
  • The reactants turn completely into products.
  • This complete ionisation leads to a HIGHER CONCENTRATION OF H⁺ IONS, which generally results in a LOWER pH.

  
• Hydrochloric Acid:

  • HCl -> $H^+$$\ \rightarrow\ Cl^-$

  Nitric Acid:

  • $HNO_3\ \rightarrow\ H^+$$\ +$$\ NO_3^-$

  Sulfuric acid:

  • $H_2SO_4\ \rightarrow\ 2H^+$$\ +$$\ SO_4^{2-}$
• WEAK ACIDS:


  • Acids that PARTIALLY IONISE in water, meaning only SOME of the acid molecules break up into their ions.
  • The incomplete release of H⁺ ions results in a LOWER CONCENTRATION OF H+ IONS.
  • Which generally results in a HIGHER pH compared to strong acids of the same concentration.
• • The MORE H+ IONS there's in a give volume of solution the MORE ACIDIC the solution is.
  • Meaning the HIGHER the H+ CONCENTRATION of an acid, the lower the pH.
• If you INCREASE the H⁺ ION CONCENTRATION by a factor of 10 TIMES, the pH will DECREASE by 1.
• A solution of hydrochloric acid was diluted, until its concentration decreased by a factor 100. What's the change in pH?

  pH will increase by 2
• The CONCENTRATION of an acid:


  Tells the NUMBER OF ACID MOLECULES in a given volume.

  • More concentrated solutions contain more acid per unit of volume.
• The STRENGTH of an acid:


  The acid's degree of ionisation in water (how easily it breaks up into its ions).
• High strength of an acid:

  • Strong acids fully ionise in solution

  Low strength of an acid:

  • Weak acids partially ionise in solution
• High concentration of an acid:

  • Concentrated acids have a high amount of acid in a given volume.

  Low concentration of an acid:

  • Diluted acids have a low amount of acid in a given volume.
•  Concentration & strength are INDEPENDENT of each other, meaning you can have:

  • A STRONG, CONCENTRATED Acid
  • A STRONG, DILUTE Acid
  • A WEAK, CONCENTRATED Acid
  • A WEAK, DILUTE Acid
• If you INCREASE the H⁺ ION CONCENTRATION by a factor of 10 TIMES, the pH will DECREASE by 1:

  A) +3

  B) +2

  C) +1

  D) -1

  E) -2

  F) -3

  
• • 0-3 is strong acid, 4-6 is weak acid
  • react them with a reactive metal
  • a strong acid would have a faster reaction or a greater volume of gas produced in a given time.
  • weak acids are only partially ionised in aqueous solution.
  • strong acids are completely ionised in aqueous solution / greater concentrations of H+ ions
  • aqueous solutions of acids at the same concentration

  
• • pH depends on H+ ion concentration
  • the higher the concentration of H+ ions, the lower the pH

  Strength:

  • The stronger an acid, the greater the ionisation in aqueous solution
  • so the stronger the acid, the lower the pH

  concentration:

  • The higher the concentration of an acid, the more acid in the same volume of solution and the lower the pH.

  
• a dilute solution of a strong acid.

  
• • strong because completely ionised in aqueous solution.
  • dilute because small amount of acid per unit volume.

  
• A hydrogen ion is a hydrogen atom that’s lost one electron, so it’s a proton.
• Weak acids:


  • Carbonic acid
  • Citric acid
  • Ethanoic acid (CH3COOH)
• When acid molecules are added to water & split apart, they 'ionise' or 'dissociate'.
• CH3COOH ⇌ H+ + CH3COO- 

  • The double arrow shows the reaction is reversible.
• Reversible reaction:


  Where the reactants able to form the products & the products can react to reform reactants.

  • The reaction can go forwards & backwards.

  
• The dissociation of weak acids is a reversible reaction.

  • So the products can react together to reform the acid.
• For a weak acid, the position of equilibrium lie to the left.

  • (more molecules of undissociated acid, then molecules of dissociated acid).
  • Since only a few of the acid particles actually dissociate.
• Carbonic acid is described as a weak acid:

  • It will form a solution with a pH of less than 7
  • It does not fully ionise to release hydrogen ions
• Hydrochloric acid with 2 mol/dm3 would mean 2 moles of HCl molecules per dm3 of solution.
• As concentration of hydrogen ions get higher, the pH gets lower.

  • a low pH mean a high concentration of H+ ions.
• Each decrease of one on the pH scale = the concentration of hydrogen ions increasing by the factor of 10.
• With hydrochloric acid, it can get a low pH at most concentrations, since each particle dissociates fully, so there’s loads of hydrogen ions being released.
• A weak acid like carbonic acid has to be really concentrated to get a low pH.

  • Since so few of the acid particles would actually ionise & release their hydrogen ions.
• At any given concentration, a strong acid will have a lower pH than a weak acid.

  • Because a higher proportion of the strong acid molecules will dissociate to release their hydrogen ions.