Explain how the enthalpy change of neutralisation can be used to determine the relative strength of an unknown acid.

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Aim: To explain how the enthalpy change of neutralisation can be used to determine the relative strength of an unknown acid.

Provided Information:

Relative Formula Mass: 135

Appearance: White crystalline solid

Properties: Very soluble in water

Monoprotic (monobasic): An acid that has only one acidic hydrogen atom in its molecules; common examples are hydrochloric acid (HCl) and nitric acid (HNO3).

Prediction and Scientific Background: Standard Molar Enthalpy of Neutralisation (/\Hn,) is the enthalpy change per mole of water formed in the neutralisation of an acid by an alkali, (298K and 1 atm).

  • Unknown Acid (aq) + NaOH (aq)  Salt (aq) + Water (l)
  • In aqueous solution, strong acids and bases are completely dissociated and Hneut is approximately equal to –57.9 kJ mol–1. This neutralization process corresponds to the reaction: H+ (aq) + OH (aq) → H2O (l)
  • For weak acids, this enthalpy change is less exothermic because some input of energy is required to dissociate the acid. Therefore the Hneut of weak acids and bases is more positive than -57kJmol-1.


Hydrochloric acid (strong acid): As HCl is a strong acid; it is a good proton donor, with near to complete dissociation in water. HCl (aq) → H+ (aq)  + Cl(aq). Hence a strong acid completely dissociates into ions.

Ethanoic acid (weak acid): As CH3COOH is a strong acid; it is a poor proton donor. The dissociation in the water is equilibrium, with the equilibrium position well to the left-hand side of the equation.

CH3COOH (aq)  H+ (aq) + CH3COO(aq). For every 250 molecules of ethanoic acid, only 1 molecule dissociates and only a small proportion of the potential acidic power is released as H+. A weak acid only partially dissociates into ions.

Strong 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:

These reactions are all reversible, but in some cases, the acid is so good at giving away hydrogen ions that we can think of the reaction as being one-way. The acid is virtually 100% ionised. For example, when hydrogen chloride dissolves in water to make hydrochloric acid, so little of the reverse reaction happens that we can write:

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. Thus, a strong acid is one which is virtually 100% ionised in solution. Other common strong acids include sulphuric acid and nitric acid.

You may find the equation for the ionisation written in a simplified form:

This shows the hydrogen chloride dissolved in the water splitting to give hydrogen ions in solution and chloride ions in solution. This version is often used in this work just to make things look easier. If you use it, remember that the water is actually involved, and that when you write H+(aq) what you really mean is a hydroxonium ion, H3O+.

Weak Acids: A weak acid is one, which does not ionise fully when it is dissolved in water. Ethanoic acid is a typical weak acid. It reacts with water to produce hydroxonium ions and ethanoate ions, but the back reaction is more successful than the forward one. The ions react very easily to reform the acid and the water.

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At any one time, only about 1% of the ethanoic acid molecules have converted into ions. The rest remain as simple ethanoic acid molecules. Most organic acids are weak. Hydrogen fluoride (dissolving in water to produce hydrofluoric acid) is also a weak inorganic acid.

Apparatus:

  • 3 – 100cm3 beakers
  • 50cm3 burette
  • Coffee cup calorimeter: This is a simple, inexpensive device used in many general chemistry labs. It is made of two nested and capped cups made of Styrofoam; making it a very good insulator. When a reaction occurs in such a ...

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