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Before we begin this section, it might be useful to think about some different types of reactions and how quickly or slowly they occur.

Thinking about reaction rates

Think about each of the following reactions:

  • rusting of metals
  • photosynthesis
  • weathering of rocks (e.g. limestone rocks being weathered by water)
  • combustion
  1. For each of the reactions above, write a chemical equation for the reaction that takes place.
  2. How fast is each of these reactions? Rank them in order from the fastest to the slowest.
  3. How did you decide which reaction was the fastest and which was the slowest?
  4. Try to think of some other examples of chemical reactions. How fast or slow is each of these reactions, compared with those listed earlier?

In a chemical reaction, the substances that are undergoing the reaction are called the reactants , while the substances that form as a result of the reaction are called the products . The reaction rate describes how quickly or slowly the reaction takes place. So how do we know whether a reaction is slow or fast? One way of knowing is to look either at how quickly the reactants are used up during the reaction or at how quickly the product forms . For example, iron and sulfur react according to the following equation:

F e + S F e S

In this reaction, we can observe the speed of the reaction by measuring how long it takes before there is no iron or sulfur left in the reaction vessel. In other words, the reactants have been used up. Alternatively, one could 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 M g ( s ) + O 2 2 M g O ( s )

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

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 (mols/second or mol.s - 1 ).

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

Average reaction rate of M g = moles Mg used reaction time ( s )


Average reaction rate of O 2 = moles O 2 used reaction time ( s )


Average reaction rate of MgO = moles MgO produced reaction time ( s )

The following reaction takes place:

4 L i + O 2 2 L i 2 O

After two minutes , 4 g of Lithium has been used up. Calculate the rate of the reaction.

  1. n = m M = 4 6 . 94 = 0 . 58 m o l s
  2. t = 2 × 60 = 120 s
  3. Rate of reaction =

    m o l e s o f L i t h i u m u s e d t i m e = 0 . 58 120 = 0 . 005

    The rate of the reaction is 0.005 mol.s - 1

Got questions? Get instant answers now!

Reaction rates

  1. A number of different reactions take place. The table below shows the number of moles of reactant that are used up in a particular time for each reaction.
    Reaction Mols used up Time Reaction rate
    1 2 30 secs
    2 5 2 mins
    3 1 1.5 mins
    4 3.2 1.5 mins
    5 5.9 30 secs
    1. Complete the table by calculating the rate of each reaction.
    2. Which is the fastest reaction?
    3. Which is the slowest reaction?
  2. Two reactions occur simultaneously in separate reaction vessels. The reactions are as follows: M g + C l 2 M g C l 2 2 N a + C l 2 2 N a C l After 1 minute, 2 g of MgCl 2 have been produced in the first reaction.
    1. How many moles of MgCl 2 are produced after 1 minute?
    2. Calculate the rate of the reaction, using the amount of product that is produced.
    3. Assuming that the second reaction also proceeds at the same rate, calculate...
      1. the number of moles of NaCl produced after 1 minute.
      2. the mass (in g) of sodium that is needed for this reaction to take place.

Questions & Answers

are UV rays dangerous?
Khathutshelo Reply
what's this?
i hv a problem with this chapter because i also have problem with Newton's laws of motion
Kearabetswe Reply
what is the wave model of atom ?
Trazy Reply
Can you state the Doppler effect in words
Nombuyiselo Reply
State Coulomb's law in words
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
Newton's first law of motion
Nnuso Reply
When an object is at rest or traveling at a constant velocity it will remain at rest unless an unbalanced force acts upon it.
yes that's true
Always know Newton's second law of motion. It appears to be in every question paper
Is the normal force always 0
Mpilo Reply
no. newtons 3rd law states that if something exerts a force into something else it will experience that same magnitude of force but in the opposite direction. so the net force is equal to the force the object applies to the surface but in the opposite direction
define the term rate of reaction in word
David Reply
I think it a chemical process in which substance act mutually on each other.
how many bones are in the human body?
it's the rate at which the reactants are able to change to products
what is wave lengh
mama Reply
is the de broglie wavelength of the particle
calcium and magnesium, which one can displace aluminum from its compound?
Markia Reply
how is aluminum ion formed?
a substance that has the lower electronegativity which will be in this case the Ca -1.0 this is because the Ca would have a lower ionization energy -needs less energy to fill its outer most shell and therefore will cause a displacement of the Al
an Al ion would form when it has lost electrons and will normally be a cation -positively charged (3+)
molecules with the same molecular formula but different chain
Malaza Reply
what will be the answer
chain isomer
what is structural isomer?
Hey if you define an isomer it will come as an organic molecule with the same number of atoms,same molecule formula ,same molecular mass but different STRUCTURAL FORMULA so what we have is only the chain ,positional and functionally isomer.Never heard of STRUCTURAL isomer
what does the newton's law says?
there are 3 newtons laws which are newton's law of universal gravitation, law of cooling, law of motion
Did the feather or leaf hit the ground first?
Shaloom Reply
both at the same time. air friction ignored
2 to 30 minut long distance race 20 km what's the avarage speed for the race
Jamilla Reply
what is meaning of covalent bonds
Lungani Reply
bonds that exists between non metal atoms. each atom contribute electron(s) which will form a bond joining the two atoms. electrons in the bond now belongs to both atoms
how do we name haloalkanes
Thobeka Reply
how do we name haloalkanes
what are examples of haloalkanes
haloalkanes are compounds that consist of the halogen group chlorine bromine flourine and so on like 2-bromobutan

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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