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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

The phenomenon that describes the change in the frequency
Neo Reply
Doppler effect
sinelizwi
does the rate of reaction calculated by reactant equal to product
mudodzwa Reply
formula of heat reaction
Anelisiwe Reply
final energy-initial energy of substances
Chantel
what is the difference between ethene and ethane
Joy Reply
Ethane is a saturated organic molecule while ethene is unsaturated Ethane has 2 carbon atoms and 6 hydrogen atoms, while ethene has 2 Carbons and 4 hydrogens Ethane has single bond between the 2 Carbon atoms while ethene has double bond
Rito
How do you know that the reaction is in acid medium or basic medium
Miss Reply
What formular is used when source and Observer are moving towards to each other
Esihle Reply
put a + on top n - on the bottom
Sanele
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Ansiwe Reply
doppler effect is the perceived change in frequency of a sound caused by either the source or the listener
Qaqamba
moving relative to each other
Qaqamba
A Catalyst is a chemical substance that speeds up a chemical reaction.
Sheron Reply
It does this by lowering the activation energy.
Sheron
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Nambile Reply
reaction that releases energy
Beauty
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Joseph
Catalyst is to speed up the reaction
CELO
catalyst is a substance added into a solution to speed up the process
Joseph
As it speeds up the reaction by lowering the activation energy, it does not take part in the reactants or products formed
Rito
reaction that release energy
Qaqamba
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Given
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mudodzwa
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Nambile Reply
reaction that gives off heat to the surroundings
Thabile
formula for calculating the heat of reaction
Anelisiwe
I need help in electrostatic
Muthuphei Reply
in exactly in electrostatic
Beauty
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Beauty
factors affecting the rate of reaction
thabo Reply
hi
Xiluva
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Mashweme
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The first thing you can do is to understand ohm's law and the difference between a series and parallel connection concerning current, resistance and potential difference
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Hi Xiluva,not sure if this will be helpful but to understand electricity better I suggest you download Siyavula physical science books there's both a learner book and a teacher's guide I was struggling too they really did help you might be able to do some exercises after reading siyavula material
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temperature surface area concentration addition of a catalyst
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zama Reply
ep=mgh
mudodzwa
Ep=mgh
Maximurm
What is polymer
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chemical compound with molecules bonded together in long ,repeating chains
Bukani
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Monica Reply
A force is a pull or push and it can be applied in rocket launchers moving cars & stoping (breaking them).
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A force is that Agency which changes the object's state of rest or motion. A push or pull are the effects of force.
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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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