5. Oxidation-Reduction reactions Flashcards
Oxidation-reduction reactions all involve what?
The transfer of electrons.
In this, the modern system, redox reactions occur when one species loses electrons and the other gains electrons. For example, when black copper oxide is heated with grey magnesium, it produces white magnesium oxide and orange-pink copper metal:
Which ions do not lose or gain electrons, so they are not participants in the redox reaction?
Spectator ions.
They do not change. Many chemical reactions occur in solution (dissolved in water), so the ions are separate, and we can imagine the spectator ions simply watching (spectating) while the reaction occurs around them.
What are some examples of redox reactions?
Oxidation-reduction, or redox reactions, are among the most common and important chemical processes in everyday life.
The processes of respiration through which we obtain energy from our food are oxidation-reduction reactions, as are photosynthesis reactions in plants. The decay of organic matter also involves redox reactions.
When fire burns something, that’s a redox reaction. Explosions are very fast redox reactions. The corrosion process by which rust forms is a much slower redox reaction.
What is a good mnemonic to remember oxidation reactions?
Oxidation and reduction occur simultaneously.
A mnemonic is OIL RIG: Oxidation Is Loss of Electrons, Reduction Is Gain of Electrons.
What are a measure of the ‘degree’ to which an element has been oxidised or reduced?
Oxidation numbers, also known as oxidation states,
What is this known as?
The reactivity series of metals.
A more reactive metal can do what to a less reactive metal from a compound?
Displace it.
More reactive metals can more easily lose electrons to form stable ions. Metals can displace other metals that are below them on the reactivity series (that is, less reactive), but not those above. The further apart two metals are in the series, the more vigorously they will react.
This can occur in reactions with metal salts or metal oxides. For example, when iron metal is placed in a blue copper sulfate solution, it will displace the copper ions in the solution, and orange-pink copper metal will precipitate on the iron.
How metals are extracted from their ores varies according to their place on the reactivity series.
Metals higher on the series are more difficult to separate from their what?
Ores.
On the other hand, the metals at the bottom of the series (copper, mercury, silver and gold) are easy to mine because they occur as native elements, i.e. they are sufficiently unreactive that they are commonly found in pure form.
Metals more reactive than carbon can be extracted by what?
Electrolysis.
This uses electricity to induce a non-spontaneous redox reaction. The ore is melted or dissolved, and an electrical current is run through it. The non-metal anion (negative ion) is attracted to the positive electrode and oxidised there. The metal cation (positive ion) is attracted to the negative electrode and reduced to form the metal.
Carbon lies between which 2 elements on the reactivity series?
Aluminium and zinc.
Oxidation numbers are a powerful tool to determine what’s happening in redox reactions.
When the oxidation number of an element changes during a reaction, oxidation or reduction has occurred. How the oxidation numbers change show which element has been oxidised, and which reduced.
Oxidation involves a what to the oxidation number?
An increase in oxidation number, while reduction involves a decrease in oxidation number. The two always occur together, so one element is oxidised and another is reduced.
Elements in their elemental form have an ON of what?
0.
The ON of a monatomic ion is equal to the charge on that ion.
In polyatomic ions, the sum of the ONs of each element is the charge on the ion.
In molecules, the sum of the ONs of each element is 0.
The ON of hydrogen in a compound is +1, except in metal hydrides ((such as NaH), where it is −1.
The ON of oxygen in a compound is −2, except in hydrogen peroxide (H2O2), where it is −1.
Most common reductants are what?
Metals.
Metal atoms have few valence electrons, so they tend to lose electrons to form metal cations. In redox reactions, these electrons go to the oxidant, reducing it. The metal atom is oxidised to a metal cation.
Other common reductants include carbon, and hydrogen gas.
Many common oxidants are more effective in what conditions?
Acidic.
Redox equations are usually represented as ionic equations, where what are not shown?
Spectator ions.
