Redox reactions are a chemical reactions involving simultaneously (serentak) oxidation and reduction processes
Oxidation & reduction :
a addition (gain) or loss (elimination) of oxygen or hydrogen
accepting(receives) or donating of electrons
change in oxidation number
A Redox Reactions :
Oxidation is the process of gaining oxygen & elimination of hydrogen
reduction is the process of losing oxygen & addition of hydrogen
oxidising agent is the substance which experiences reduction & receives electrons .
reducing agent is the substance which experiences oxidation & donates electrons .
PbO – oxidising agent (experiences reduction Mg – reducing agent ( experiences oxidation) Mg + PbO MgO + Pb Gain of oxygen ---- oxidation Loss of oxygen ---- reduction
Cl 2 – oxidising agent ( undergoes reduction ) --- chlorine oxidises hydrogen sulphide to sulphur H 2 S – reducing agent ( undergoes oxidation) --- hydrogen sulphide reduces chlorine to hydrogen chloride Redox reaction H 2 S + Cl 2 2HCl + S Loss of hydrogen --- oxidation Gain of hydrogen --- reduction
is a process that occurs when two metals come into contact(bersentuhan) with an electrolyte .
the more electropositive metal will donate (release) electrons & is corroded
If the iron comes into contact with metal that is more electropositive ,it will not corrode .
the corrosion of iron can be accelerated by the presence of electrolytes such as acid & salt solution.
Example : Electrochemical Corrosion of Metals More easily corroded Difficult to be corroded Tendency for corrosion increases
Example : The effect of rusting when iron comes into contact with other metals ( Mg, Cu , Zn , Sn) Hypothesis : Iron is protected from rusting when it comes in contact with more electropositive metals, but rusts when it contact with less electropositive metals .
Potassium hexacyanoferrate(III) solution ---- to detect the presence of iron(II) ions .
Iron nail is corroded , the dark blue colour will be seen in the solid agar .
Phenolphthalein will turn pink in colour if OH - ions are present.
The gas bubbles formed are hydrogen gas .
The iron nail in test tube B is not corroded , while zinc is corroded because Zn is more electropositive than iron .
In test tube A ,the iron nail is corroded because Iron more electropositive than copper .
B A rusting
PREVENTION THE RUSTING OF IRON Coating a layer of metal such as Al or Sn on food tins Applying paint,oil or grease on surface such as engine Wrapping the iron with a layer of plastic . Ex: hangers Applying a coat of Al such as car bumpers or water pipes Iron sheet used as house roofs Are galvanised with a layer of zinc Iron is alloyed with other metals such as chromium or nickel to produce stainless steel Huge iron construction structure such as bridges protected from corrosion by using sacrificial metals(logam korban) such as Mg & Zn
Reactivity Series (R.S) of Metals A : Metals with Oxygen
Metal is heated in oxygen to produce
metal oxide .A more reactive metal will displace
a less reactive metal from its oxide.
Mg ----- burn very rapidly & vigorously with a very bright flame
------ metal oxide colour : white powder ( Hot & cold )
Zinc ---- burns rapidly , glows brightly
----- metal oxide colour : yellow when hot & white when it is cold .
Iron ----- burns rapidly, glows less brightly than Zn
----- metal oxide colour brown when hot & cold
Cu ----- very slow reaction
----- metal oxide colour black ( hot & cold )
Pb ----- burns slowly
----- metal oxide colour : brown when hot & yellow when colour
K Na Ca Mg Al C Zn Fe Sn Pb Cu Ag Au Reactivity decreases Example : The reaction between Lead(II) oxide with Carbon Observation : burn brightly : produces a grey solids Inference : Carbon is more reactive than Lead Equation : PbO + C Pb + CO 2 If carbon is more reactive than metal X , a flame or glows(baraan) can be seen . If carbon is less reactive than metal Y ,the flame or glows will not be seen when carbon react with metal oxide Y is heated . The position of Carbon in the R. S.