C4.2.6 - Strong and weak acids

• Strong Vs Weak acids

  Strong acids dissociate fully in water to produce the maximum number of H+ ions.

  Weak acids, such as ethanoic acid (CH3COOH), do not fully dissociate. In fact, about only one per cent of ethanoic acid molecules split up at any one time.

  This means that the pH values of strong acids are lower than that of weak acids, which explains why the rate of reaction of strong acids with substances (such as metals, metal carbonates etc) is higher than that of weak acids.

  
• pH calculations

  As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.
• Dilute Vs Concentrated Acid

  A dilute acid has the acid molecules mixed with a large amount of water, so that there is only a low concentration of H+ ions. Concentrated acids have little to no water molecules mixed with the acid molecules, meaning the concentration of H+ ions is high.

  
• Describe neutrality and relative acidity in terms of the effect on hydrogen ion concentration and the numerical value of pH

  At pH 7, the substance or solution is at neutral and means that the concentration of H+ and OH- ion is the same. If pH < 7, the solution is acidic. There are more H+ than OH- in an acidic solution. The pH scale does not have an upper nor lower bound. In very general terms a solution is said to be neutral when the numbers of hydrogen ions and hydroxyl ions are equal. This neutral point is temperature dependent and occurs at pH 7.0 at 25°C.