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Redox
Oxidation-Reduction Reactions
| Question | Answer |
|---|---|
| What is oxidation? | 1) Gain of oxygen 2) Loss of hydrogen 3) Loss of electrons |
| What is reduction? | 1) Loss of oxygen 2) Gain of hydrogen 3) Gain of electrons |
| Define oxidation in terms of electron transfer. | OIL Oxidation is LOSS OF ELECTRONS by an element in its free state or in a compound. |
| Define reduction in terms of electron transfer. | RIG Reduction is GAIN OF ELECTRONS by an element in its free state or in a compound. |
| What is a redox reaction? | It is a chemical reaction in which one reactant is reduced and the other is oxidised. |
| Define oxidation number( oxidation state) of an element. | This is the theoretical charge that an atom of an element would have if all the compound containing it were comprised of ions. |
| Define oxidation in terms of oxidation number (state) | The INCREASE in oxidation number of an element in its free state or in a compound. |
| Define reduction in terms of oxidation number (state). | The DECREASE in oxidation number of an element in its free state or in a compound. |
| What is oxidising agent? | 1) The reactant which CAUSES OXIDATION of the OTHER reactant while ITSELF IS REDUCED. |
| What is the reducing agent? | 1) The reactant which CAUSES REDUCTION of the OTHER reactant while ITSELF IS OXIDISED. |
| What is the oxidation number of any atom in a neutral substance that has only one kind of atom (e.g. O2, Fe, P4, S8) ? | Zero (0) |
| What is the oxidation number of simple ions (e.g. Cl-, K+)? | The charge on the ion. |
| What is the oxidation number of hydrogen when it is combined with a non-metal (e.g. CH4, NH3, H2O, and HCl) ? | +1 |
| What is the oxidation number of hydrogen when it is combined with a metal (e.g. LiH, NaH, CaH2, and LiAlH4) ? | -1 |
| What is the usual oxidation of number of oxygen in a compound? | -2 Except when in peroxides (H2O2) where it is -1. |
| The sum of the oxidation numbers in a polyatomic ion is equal to | the charge on the ion |
| What is the oxidation number of fluorine? | -1 |
| How is rust formed? | When iron and its alloy, steel, come into contact with oxygen and moisture the iron is oxidised by the oxygen to form hydrated iron (III) oxide (Fe2O3 . xH20) (rust). |
| Why do certain fruits and vegetables turn brown when cut and exposed to the air? | Enzymes in the plant cells on the surface of the fruit/vegetable begin to oxidise certain chemicals in the cells to brown compounds known as melanins. |
| What is the action of sodium sulfite and sulfur dioxide used in certain foods for preservation? | They are reducing agents and thus prevent oxidation e.g. preventing wine from being oxidized to vinegar(sour). |
| What is the chemical principle behind the breathalyzer test? | Any ethanol vapours in the driverβs breath will reduce the orange dichromate((VI) ion (πΆπ2 π7^(2β) to the green chromium (II) ion πΆπ^(3+) |
| How does bleach work ? | Bleach operates by oxidising the coloured chemicals resulting in them becoming colourless. ClO- (aq) + coloured dye -> Cl- (aq) + colourless dye |
| What is the oxidation state of sulfur in SO2 ? (NB. You have to work it out not guess) | +4 |
| What is the oxidation state of manganese in MnO4^- ? | +7 |
| What is the oxidation state of chromium in K2Cr2O7 ? | +6 |
| What is the oxidation state of nitrogen in the nitrate ion (NO3^-)? | +5 |
| List examples of common oxidising agents. | Bromine, chlorine, concentrated sulfuric acid, nitric acid, oxygen, potassium manganate, potassium dichromate, hydrogen peroxide |
| List examples of common reducing agents. | Carbon, carbon monoxide, hydrogen, hydrogen sulphide, metals, potassium iodide, sulphur dioxide, ammonia. |
| What is the colour change of potassium manganate when it is added to a reducing agent? Give reason for the change. | Manganate(VII) (MnO4^-) ion is reduced to manganese (II) Mn2+ ion. Purple to pale pink (or colourless) |
| What is the colour change of potassium dichromate when it is added to a reducing agent? Give reason for the change. | Dichromate(VI) (Cr2O7^2-) ion is reduced to chromium (III) ion(Cr3+) Orange to green |
| What is the colour change when iron (III) sulfate is in contact with a reducing agent? Give reason for the change. | Yellow-brown to pale green The yellow-brown Fe3+ ions are reduced to pale green Fe2+ ions. |
| What is the colour change when iodine is in contact with an oxidising agent? Give reason for the change. | Colourless to brown I- is oxidised to I2 |
| What is the colour change when iron (II) sulfate solution is in contact with an oxidising agent? Give reason for the change. | Pale green to yellow-brown Fe2+ is oxidised to Fe3+ |
| How can the presence of a reducing agent be confirmed? | Test with a strong oxidising agent with visible colour change. |
| How can the presence of an oxidising agent be confirmed? | Test with a strong reducing agent with visible colour change. |
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CSEC Chemistry 11th Grade
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