Strength of acid and alkali

Today's Science lesson we covered about acids(produces H+ ions in water) and bases(produces OH- ions in water), and were given a worksheet to do.  There was a question about the meaning of strong and weak acids or alkalis. We were told to answer that strong acids or alkalis form more H+ or OH- ions when dissolved in water. However, I think that this answer is rather vague and seek for a definition or measurement of "strong". I found out that one way to determine this could be through  obtaining the enthalpy change of the substance. This is the amount of heat energy given out or taken in during a chemical reaction. When the acid or alkali molecules dissolve in water, they break up into different ions. A acid is usually made up of a non-metal(usually halides or halogens) and hydrogen ions, while an alkali is made up of a metal and hydroxide ions. For example, when hydrochloric acid dissolves in water, H+ and Cl- ions are formed. During this process, heat is needed to break the ionic bonds between the elements, and more heat would thus be used to break down the molecules completely into more H+ or OH- ions.Thus, it can be deduced that the greater the enthalpy change of an acid or alkali, the stronger it is. For example, the enthalpy change of a strong acid, hydrochloric acid is -74.84 kJ, while that of acetic acid is- 1.51kJ.  Ammonia, a weak alkali has a change of -30.50kJ, while potassium hydroxide's is -57.61kJ. The heat of reaction is either positive or negative, depending on whether it is exothermic or endothermic. I am glad that my own questioning has allowed me to learn out of the box, and I hope to continue doing so in future.


Blueprint for this topic:

Strong Acids= More H+ ions , Weak Acids= Less H+ ions
Strong Alkali=More OH- ions,  Strong Alkali=Less OH- ions
Acid+Metal=Salt+Hydrogen
Acid+Carbonate=Salt+Water+Carbon dioxide
Neutralisation: Acid+Base=Salt+Water
Base+Acid=Salt+Water
Base+Ammonium Salt=Salt+Ammonia+Water


Basic Information: