Everything Science Grade 12

3.1 CHAPTER 3. REACTION RATES

Fe + S→ FeS

In this reaction, we canobserve the speed of the reaction by measuringhow long it
takes before there is noiron or sulphur left in the reaction vessel. In other words, the
reactants have been used up. Alternatively, onecould see how quickly the iron sulfide
product forms. Since iron sulfide looks very different from either of its reactants, this is
easy to do.

In another example:

2 Mg (s) + O 2 → 2 MgO (s)

In this case, the reactionrate depends on the speed at which the reactants (solid mag-
nesium and oxygen gas) are used up, or the speed at which the product (magnesium
oxide) is formed.

DEFINITION: Reaction rate

The rate of a reaction describes how quickly reactants are used up or

how quickly products are formed during a chemical reaction. The units

used are: moles per second (mol/second or mol.s−^1 ).

The average rate of a reaction is expressed as the number of moles ofreactant used
up, divided by the totalreaction time, or as thenumber of moles of product formed,
divided by the reactiontime. Using the magnesium reaction shown earlier:

Average reaction rate ofMg =

moles Mg used

reaction time (s)

or

Average reaction rate ofO 2 =

moles O 2 used

reaction time (s)

or

Average reaction rate ofMgO =

moles MgO produced

reaction time (s)