Slow and Fast Reactions

Different chemical reactions proceed with different speeds. Rusting of iron and burning of fuel both are oxidation reactions. The first one is a typical slow reaction while the second one is a fast reaction. The reaction between dry hydrogen and dry chlorine gas in absence of light is very slow reaction but it becomes explosive when a trace of sodium vapour is introduced. The same reaction can occur at different speed depending on the conditions. For example, hydrogen burns rapidly to form water in excess of air but it burns very slowly in presence of limited air.