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Balancing redox reactions

Half reactions can be used to balance redox reactions. We are going to use some worked examples to help explain the method.

Magnesium reduces copper (II) oxide to copper. In the process, magnesium is oxidised to magnesium ions. Write a balanced equation for this reaction.

  1. M g M g 2 +

  2. You are allowed to add hydrogen ions (H + ) and water molecules if the reaction takes place in an acid medium. If the reaction takes place in a basic medium, you can add either hydroxide ions (OH - ) or water molecules. In this case, there is one magnesium atom on the left and one on the right, so no additional atoms need to be added.

  3. Charges can be balanced by adding electrons to either side. The charge on the left of the equation is 0, but the charge on the right is +2. Therefore, two electrons must be added to the right hand side so that the charges balance. The half reaction is now:

    M g M g 2 + + 2 e -

  4. The reduction half reaction is:

    C u 2 + C u

    The atoms balance but the charges don't. Two electrons must be added to the right hand side.

    C u 2 + + 2 e - C u

  5. No multiplication is needed because there are two electrons on either side.

  6. M g + C u 2 + + 2 e - M g 2 + + C u + 2 e - (The electrons on either side cancel and you get...)

    M g + C u 2 + M g 2 + + C u

  7. In this case, it is.

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Chlorine gas oxidises Fe(II) ions to Fe(III) ions. In the process, chlorine is reduced to chloride ions. Write a balanced equation for this reaction.

  1. F e 2 + F e 3 +

  2. There is one iron atom on the left and one on the right, so no additional atoms need to be added.

  3. The charge on the left of the equation is +2, but the charge on the right is +3. Therefore, one electron must be added to the right hand side so that the charges balance. The half reaction is now:

    F e 2 + F e 3 + + e -

  4. The reduction half reaction is:

    C l 2 C l -

    The atoms don't balance, so we need to multiply the right hand side by two to fix this. Two electrons must be added to the left hand side to balance the charges.

    C l 2 + 2 e - 2 C l -

  5. We need to multiply the oxidation half reaction by two so that the number of electrons on either side are balanced. This gives:

    2 F e 2 + 2 F e 3 + + 2 e -

  6. 2 F e 2 + + C l 2 2 F e 3 + + 2 C l -

  7. The equation is balanced.

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Questions & Answers

how does cell look like
Charu Reply
What is impulse and how does it = to momentum
Robin Reply
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Jhon Reply
what is a catanation ?
Rithabele Reply
The ability of a few elements, most especially carbon, to yield chains and rings by forming covalent bonds with atoms of the same element.
Ofentse
different structural formula
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different type of structural formula
Lucas
primary,secondary and tertiary
Snowy
how does the earth revolute
Kevin Reply
What is the momentum
Given Reply
The product of the object's mass and it's velocity
Mvula
electric field def
FREENCO
what is secondary alcohol?
Raymond
is when Carbon that is bonded with OH is also bonded to 2 carbons of the chain.
Gontse
Electric field is the region of space around a charged object where other charged objects will experience an electrostatic force ( coulomb forces) without coming into contact with the object that is generating the field.
Kagiso
Electric field is the region of space around a charged object where other charged objects will experience an electrostatic force ( coulomb forces) without coming into contact with the object that is generating the field.
Kagiso
how is ester formed
Aubrey Reply
how is n ester formed
Aubrey
Alcohol reacts with a carboxylic acid
Texas
and the reaction is catalysed by sulphuric acid
Vhulenda
An ester is form when an alcohol reacts a carboxylic acid and sulphuric acid is used as a catalyst which therefore eliminates water.
Azola
an athlete with a mass of 70kg runs at a velocity of 45km . determine the athlete's momentum
Lesedi Reply
Is that a velocity or something else
msawenkosi
45km/h i guess
Texas
Change to m/s
Texas
45km/h = 12.5 m/s p=mv =70×12.5 =875 kg.m/s
Thato
what are the measures of the rates of reaction
Lesego Reply
Volume Concentration Temperature Pressure Surface Area
Thato
the principle of superposition of waves
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is this a group chat
Nobuhle Reply
Hey can y'all define newton's 2nd law
mthebzification
If a resultant force act on an object...the object will accelerate in the direction of a resultant force,the acceleration of the object is directly proportional to the net force and inversely proportional to the mass of the object
mosa
how do you calculate tension force
Bulumko
use the formula Fnet=ma if there is tension connecting two objects
Sboniso
to calculate Tension, usually calculate acceleration first Draw separate free body diagrams for each body. Apply Fnet = ma to calculate Tension
Kevin
Hi people
Paul
when a resultant force acting on an object the object will accelerate in the direction of a force at an acceleration directly proportional to the force and invesly proportional to the mass of the object.
Raymond
Hey people
Kaytee
how does temperature affect the equilibrium position
Blessing Reply
an increased temperature increases the average kinetic energy thus in turn increases the number of effective collisions........
Lwando
so...which reaction is favored between endothermic and exothermic .when temperature is increased..?
Blessing
exothermic reaction because energy is realised to the surroundings as heat and light energy ....graphical so much energy is realised as reactants to form product and because temperature is high rate of reaction is fast which means there is a successful collision
Code
INTEMENDO - INCREASE IN TEMPERATURE FAVOURS ENDOTHERMIC DETEMEXO - DECREASE IN TEMPERATURE FAVOURS EXOTHERMIC
Thato
an object will continue in a state of rest unless it is acted upon an unbalanced force
Junior Reply
Newton's Law 1
Code
First Newton's Law
Azola
Newton's first law
Surprise
newton first law
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Newton's first law
Blessing

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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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