O-Level Chemistry: Redox Reactions (Reduction & Oxidation) – Hydrogen Atoms

In the previous post, you have learnt that OXIDATION is the gain of  oxygen atoms and REDUCTION is the loss of oxygen atoms.

Today, we shall discuss them based on the gain or loss of hydrogen atoms.

OXIDATION is also the loss of hydrogen atoms and REDUCTION is also the gain of hydrogen atoms.

E.g. Reaction of hydrogen sulfide and chlorine to form hydrogen chloride and sulfur

H2S(g) + Cl2(g) –> 2HCl(g) + S(s)

In the above chemical reaction, hydrogen sulfide has lost hydrogen, hence it has been OXIDISED.

Chlorine, on the other hand, has gained hydrogen and is therefore REDUCED.

Hydrogen sulfide is considered the REDUCING AGENT as it has caused the the reduction of chlorine.

Chlorine, on the other hand, has caused the oxidation of hydrogen sulfide and is therefore considered to be the OXIDISING AGENT.

In the above chemical reaction, we say it is a REDOX Reaction since both OXIDATION and REDUCTION occurs simultaneously.


An OXIDISING AGENT helps the other substance to get OXIDISED, and itself gets REDUCED.

A REDUCING AGENT helps the other substance to get REDUCED, and itself gets OXIDISED.

Hope you find the above discussion on REDOX Reactions (based on Hydrogen atoms) useful. Feel free to share it with your friends that are taking O-Level Chemistry (or IGCSE, IB and IP Chemistry).

Hmm…before i end this post, how about trying out a question that test you on the above fundamentals?

Quick Check 1:

Copper (II) oxide reacts with hydrogen based on the following equation:

CuO(s) + H2(g) –> Cu(g) + H2O(l)

State the substance that

(i) has been oxidised

(ii) has been reduced

(iii) is the oxidizing agent

(iii) is the reducing agent

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Many people talked about teaching Chemistry, I simply LOVE it. I am a passionate Chemistry Coach based in Singapore, Southeast-Asia and aspire to be one of the most dynamic, powerful and humorous speakers in Asia. My 18+ years of coaching experience has equipped me to understand the true reasons why students are not able to perform well in Chemistry, and allow me to structure my teaching methodology to cater to different levels of learners. If you have found this post useful, please share it with your friends. I would really appreciate it! Sean Chua

12 Responses to O-Level Chemistry: Redox Reactions (Reduction & Oxidation) – Hydrogen Atoms

  1. Hi Moses,

    There are 4 ways to describe if something has been oxidised or reduced.

    A) Gain/Loss of Oxygen Atoms
    B) Gain/Loss of Hydrogen Atoms
    C) Gain/Loss of Electrons
    D) Increase/Decrease of Oxidation States (or Oxidation Numbers)

    Now, for a chemical reaction to be a Redox Reaction, you must have both Oxidation Reaction & Reduction Reaction occurring simultaneously.

    Hope the above information helps.

    Sean Chua

  2. Hi Yan,

    You must first study the concepts related in Organic Chemistry. The covalent compound CH3COOH that you have indicated is a Carboxylic Acid, which is a covalent compound. For Carboxylic Acid, they will always have a functional group -COOH and you should learn how to draw the full structural formula for it.

    For other carboxylic acids, the structure will be similar to CH3COOH. e.g. C2H5COOH, C3H7COOH, etc.

    Hope the information helps.
    Sean Chua

  3. Could you please tell me where I can find the answer key for Marshall Cavendish GCE ‘O’ Level “Chemistry Matters” Textbook

  4. yh sean, my examz are cummn up and i really want to make a good effort t pass with good grades….so plz try to help me by posting videos on electrolysis…..
    with all the 4 steps

  5. (i) has been oxidised
    (ii) has been reduced
    (iii) is the oxidizing agent
    (iii) is the reducing agent

  6. hey hello ! sorry to cut in just like that. i would like to ask, how do you draw the covalent compound of CH3COOH, in other words, vinegar? :O thanks! i mean, yeah simple diagrams i should be able to draw. But what about complex compound like this? what is the way to go about it? Thank you! (:

  7. Which is the best way of defining Redox reaction?

    In the basis of loss and gain of hydrogen,oxygen or the increase/reduction of oxdation states/number?

    Help me please!

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