OXIDATION NUMBER at a glance

 Oxidation number (O.N) of an element is the charge on an atom of the element whether it is by itself or bonded to another atom. It indicates the number of electrons the atom has gained or lose at that moment. That is, it is the charge an element will have if electrons were transferred to or from it. It is usually zero (0) for an element in the uncombined state. It is also referred to as the oxidation state of the element

          NOTE: - The sign or charge for O.N is written before the number (–2) but it is written after the number for an ionic charge i.e O^2–

Rules for calculating oxidation number

The following rules are applied when assigning an oxidation number or calculating the oxidation number of an element thus

1.       The O.N of oxygen is always equal to -2 except in peroxides (–1)

2.   O.N of hydrogen is always equal to plus one (+1) except again in metallic hydrides (–1).

3.        The O.N for an element in the elemental (or ground) state O.N = O (zero) e.g.

          Na = O, Cl₂ = O,  O₂ = O etc.

4.   For an ion or radical the O.N is equal to the charge on it for example 

   Na+ = +1,   Cl^– = –1,     O^2–  =  -2

   CO₃^2-  = –2,  NO₃^- =-1,  SO₄^2- =-2

3        The algebraic sum of the O.N of all the atoms in a compound is equal O e.g.

    H₂O = O,  i.e O.N of H + O.N of O =

(+1 x 2) + ( -2 x1) 

          2 -2 = 0

NaOH = (+1 x1) + (-2 x 1) + (1 x 1) 

                         +1 - 2+ 1

                           +2-2=0

      

Rules for specific groups table groups

I        For group 1A elements (comprising Li,Na,K, e.t.c) their O.N = +1

II       For group 2A elements (comprising Be, Mg, Ca e.t.c) their O.N = +2 in all compounds

III     For group 3 elements (B, Al, e.t.c) their O.N = +3 especially in their binary compounds.

IV     For group 5 = -3

V     For group6 = -2 except Oxygen (O) in peroxides).

 VI   For group 7 = –1 respectively especially in their binary compounds 

 

Determining the oxidation number of an element

1   Find the O.N of the underlined elements in the following

  a). ZnCl₂        b). SO₃      c). NO₃^-         d).Ca²⁺

          Solution

  To determine the O.N of the underlined elements, we must refer to the general rules for assigning O.N to an element.

   a). ZnCl₂: The algebraic sum of the O.N of all the atoms in a compound is equal to zero, i.e. ZnCl₂  = O. 

       Since Cl is a group 7(A) element and ZnCl₂ is a binary compound then the O.N of Cl is –1, therefore, the O.N of Zn is

        (O.N of Zn) + (O.N of Cl ´ 2) = 0
                   x + (–1 ´ 2) = 0
                   x – 2 = 0
                   x = +2

 

          b)      SO₃

                    Solution

(O.N of S) + (O.N of 0 ´ 3) = 0
            x + (–2 ´ 3) = 0
             x – 6 = 0
           x = +6

                   Trioxosulphate(IV) ion

 

 

          c)        NO₃^-

                   Solution

         If we recall from the rules s for assigning O.N to an element. The O.N of a radical is equal to the charge on it, hence

                    NO₃^- =  –1
                   that is,
                   (O.N of N) + (O.N of 0 ´ 3) = –1
                   x + (–2 ´ 3) = –1
                   x – 6 = –1
                   x = +6 – 1
                    =  +5

                   Trioxonitrate (V) ion

    d)Ca²⁺ – here we refer to the rule that says the O.N of an ion is the charge on it, i.e., Ca²⁺ = +2  Calcium  = ion

 Uses of oxidation number

Oxidation number is used for in the

1   It is used in the  IUPAC (Internal Union of Pure and Applied chemistry) system of naming compounds e.g. H₂SO₄: TetraoxosulphateVI acid

2. It is used to know the oxidation state or number of an element in a compound 

 1.       Find the O.N of the following underlined elements

          a)       Na₂SO₄                                  (a = +4,  b = =2,  c = +6,  d  =  –5)

          b)      [Al(H₂O)₆]³⁺                           (a = +3,  b = –3,  c = +6,  d = –6)

          c)       K₂Cr₂O₇                                 (a = +5,  b = +4,  c = –6,  d = +6)

          d)      Mn                                            (a = +6,  b = +7,  c = +5,  d = +3)

 

2        Which species undergoes reduction in the reaction represented by the equation below?

 H₂S_(aq)+2FeCl_3(aq)→S_(s) + 2HCl +3FeCl₂

   a) Fe³⁺     (b). H₂S            (c). Cl^–      (d)  S

3. Find the oxidation numbers of the following underlined elements.

 a) K₂Cr₂O₇           

   b).KMnO₄       

  c). HNO₃

   d). S^2-       

  e). Cl^-                 

  f). Cr₂

OBJECTIVE QUESTIONS 

1. Oxidation is a reaction which involves the following except 

a. Loss of electrons

b. Increase in oxidation number 

c. Gain of oxygen 

d. addition of hydrogen

2. 

THEORY QUESTIONS 

1. State two applications of oxidation numbers 

Rates of Chemical Reactions

During a chemical reaction, reactants collide with one another to form products and the formation of these  products do not  occur at the same rates. Hence 

Rate of a chemical reaction can be defined as the number of moles of  reactants that are converted, or products that are formed per unit time.

Mathematically 

Rate =      mass in grammes 

                      time taken

For instance, if 6g of zinc metal is placed in dilute tetraoxosulphate (VI) acid and it takes 3 mins to completely react,  then the rate of chemical reaction is given by. 

Rate =      6g     = 2g/mins

                3min

That is, 2g of the zinc was converted to ZnCl2 per minute 

Factors used for Measuring Rates Of Reaction

The rate of a chemical reaction can be measured or can be  determine by any one of the following.

i.   decrease in mass of reactants.

ii.  increase in volume of a gaseous product 

iii. change in pH

iv. change in colour intensity 

v. Change in pressure

 

These are all measurable factors.

 Factors Affecting the Rate of Chemical Reactions.

The following factors will a reaction to be fast or slow.

i. Nature of reactants

ii. Temperature

iii.. Concentration 

iv. Pressure 

v.  Presence of light

vi. Surface area 

vii. Presence of a catalyst

Before we discussed how each of these factors will affect the rate of a chemical reaction. It is important that we understand the concept of collision theory.

Collision theory assumes that for a chemical reaction to occur, there must be Collision between reactants particles and these Collisions must be effective.

A collision is said to be  effective when it leads to formation of products

What this concept is actually implying is that, all collisions do not  lead to the formation of a product, only the ones that are effective. So the various factors that affect a chemical reactions are factors that actually increase the number of effective collisions.

I. Effect of Nature of reactants:- this is one of the factors that affects the rate of a reaction, since different elements/ compounds behave differently.

The reactivity of metals varies as you go down the activity series and so when metals react with acids for example the reactions are not the same.

Example by virtue of their nature (their high reactivity) sodium and potassium will react explosively with dilut acids while metals like zinc and iron will react moderately with dilute acids.

II. Effect of Temperature: - An increase in temperature will cause reactants particles to gain more kinetic energy leading to an incr1ease in effective collision and hence, an increase in the rate of reaction. An increase in temperature will also lead to an increase in the energy of the system.

III. Effect of Concentration: - An increase in the concentration of the reactants will lead to an increase in the number of reactants per unit area (the reactants becomes closer) thus, leading to overcrowding and increase in the effective collision of the reactants and hence an increase in the rate of the reaction.

IV. Effect of pressure: - An increase in pressure will lead to a decrease in volume that is, a decrease in the intermolecular space between the reactants particles leading to an increase in the effective collision and hence an increase in the rate of reaction. And vice versa 

V. Effect of presence of light: - Some reactions are photochemical, that is, affected by light. When such reactions are exposed to light the reactants, particles become more activated and collide more increasing the number of effective collisions and hence an increase in the rate of reaction.

VI. Effect of a catalyst: - A catalyst increases the rate of a reaction by creating a different pathway with a lower activation energy.

ACTIVATION ENERGY: see definition in chemistry.

  For a collision to be effective, the reactants must possess the minimum amount of energy needed to overcome the energy barrier (activation energy for the reaction) for that reaction. The higher the activation energy, the slower the rate of reaction; positive catalyst helps to lower the activation energy.

 Energy Profile Diagram 

This is a diagram / graph that shows the pathway of a chemical reaction. whether the reaction is exothermic or endothermic.

Energy profile diagram for an Exothermic reaction

Energy profile diagram for an Endothermic reaction

       Rate Curve

A rate Curve is the graph which shows the rate of a reaction. It is a graph of reaction against time.( that is, change in concentration against time, decrease in mass against time, e.t.c) 

The slope or the gradient of the curve is steep at the beginning because the reaction is fastest ( since  , it becomes less steep as the reaction progresses and slows down, then it finally becomes horizontal. 

The point at which the graph becomes horizontal indicates the end point of the reaction, when one of the reactants is completely used up

OBJECTIVE QUESTIONS 

1. 

THEORY QUESTIONS.

1.(a)(i) Define rate of a chemical reaction?

     ii). mention three factors that can affect the rate of a chemical reaction

     iii). state the collision theory

b).  A sample of carbon is burnt at a rate of 0.50gper second for 30 minutes to generate heat.

    (i). write a balanced equation for the reaction

   (ii). determine the    I. volume of carbon (IV) oxide produced at s.t.p.  II. moles of oxygen used up in the process at s.t.p. [ C= 12.0, O= 16.0, Molar volume Vm = 22.4dm³

c. State how each of the following affects the rate of chemical reactions  (i) surface area (ii) catalyst  [waec]

2.a(I) What is meant by the rate of chemical reaction? 

(ii). Explain in terms of collision theory, the effect of temperature increase on reaction rate.

(b) When hydrogen peroxide is exposed to air, it decomposes  (I) write an equation for the reaction (ii). Outline an experiment to illustrate the effect of a named catalyst on the rate of decomposition. 

(iii).         

3a(i) Sketch an energy profile diagram to show the effect of a catalyst on the reaction rate, given that is exothermic.

b.The graph below is the ratio curve for the following reaction carried out in an open vessel 

MgCO3 + 2HCl  MgCl2 +H2O + CO2 

(i)  For how long did the reaction occur 

(ii) why was there a loss in mass?

(iii) State whether the reaction rate was fastest at the beginning,  the middle  or towards the end of the reaction.  Give reason for your answer.

(iv) List three reaction conditions that can affect the slope of the curve.(waec)

4(a)In an experiment, excess 0.050mol/dm3 HCl was added to 10g of granulated zincin a beaker. Other conditions remaining constant state how the reaction rate would be affected in each case if the experiment was repeated using 

(i) 1.0mol/dm3 HCl

(ii) 8.0g of granulated zinc

(iii) 10g of zinc dust 

(iv) a higher volume of 0.5mol/dm3 HCl 

(v) a reaction vessel dipped in crushed ice 

(vi) equal volumes of water and 0.50mol/dm3 HCl.

b.