Redox Flashcards
Define Oxidation. What happens to the oxidation number?
A gain of oxygen and loss of electrons when a chemical species gains oxygen or loses hydrogen. The oxidation number of the element is increased
Define Reduction. What happens to the oxidation number?
Reduction is gain of electrons, loss of oxygen or gain of hydrogen. The oxidation number of the element is decreased
Why do oxidation and reduction occur simultaneously?
Because any loss of electrons by one substance must be accompanied by a gain in electrons by something else, oxidation and reduction always occur together. Involves transfer of electrons
Where does the term redox come from?
The term redox comes from the two reactions that occur, reduction and oxidation
Explain the redox of potassium chloride
Potassium is oxidated while chloride is reduced
Explain an ionic half equation
An ionic half equation shows the transfer of electrons. When oxidation occurs, the electrons are written on the right side while when reduction occurs, the electrons are written on the left side
What is an oxidation state
The numbers, associated with a positive or negative sign that indicate how many electrons an atom can donate or accept
What substances have OS of 0?
Any substance that is formed from one or more atoms of the same element has the OS=0 (eg. Br2, H2, O2, i2). Any molecule regardless of the number or type of atoms has the overall OS equal to 0. This is typically because the OS of each of the elements within the molecules cancel each other.
OS of halogens
F, Cl, Br, i… have OS= -1
OS of oxygen in peroxides and in F2O
Oxygen is –2, except in peroxides where it is in –1 and in F2O where it is +2
OS of hydrogen and hydrites
Hydrogen is +1, except in hydrides like NaH where it is –1
OS of all metals in group 1
+1, Na, K…
OS of all metals in group 2
+2, eg. Mg
OS of elements in group 3 (boron group)
+3, Al, Ga, Ti
How do you find the OS?
To find the o, calculate the electron configuration
Do metals have positive or negative OS and why?
Positive os because they share electrons to reach a stable configuration
Do non metals have positive or negative OS and why?
Always form negative os because they gain electrons, to reach a stable configuration
What does the roman numeral after the element’s name represent?
When naming, the charge on the ion is shown by using a Roman numeral after the element’s name
For example, iron can form different ions
Iron ions with a 2+ charge, Fe2+, are called iron(II) ions
Iron ions with a 3+ charge, Fe3+, are called iron(III) ions
The Roman numeral is the oxidation number of the element
Name an example of a redox reaction
The reaction between zinc oxide and carbon is an example of a redox reaction
Zinc oxide is reduced because it has lost oxygen
Carbon is oxidised because it has gained oxygen
What does +/- sign show in oxidation numbers?
Positive oxidation number = loss of electrons
Negative oxidation number = gain of electrons
OS of fluorine
Fluorine is always –1
What is the sum of oxidation numbers of an ion equal to?
The sum of the oxidation numbers in an ion is equal to the charge on the ion
SO4 2–
S = +6
Four O atoms = 4 x (–2) = –8
Sum of oxidation numbers = 6 – 8 = –2
Which element is given a negative oxidation number in a compound or ion?
In either a compound or an ion, the more electronegative element is given the negative oxidation number F2O
Two F atoms = 2 x (–1) = –2
O = +2
What do the tests for redox reactions involve? What are two common examples?
The tests for redox reactions involve the observation of a colour change in the solution being analysed
Two common examples are acidified potassium manganate(VII), and potassium iodide
What is potassium manganate used to test? Explain and describe the test
Potassium manganate(VII), KMnO4, is an oxidising agent which is often used to test for the presence of reducing agents
When acidified potassium manganate(VII) is added to a reducing agent its colour changes from purple to colourless
What is potassium iodide used to test? Explain and describe the test
Potassium iodide, KI, is a reducing agent which is often used to test for the presence of oxidising agents
When added to an acidified solution of an oxidising agent such as aqueous chlorine or hydrogen peroxide (H2O2), the solution turns a red-brown colour due to the formation of iodine, I2
The potassium iodide is oxidised as it loses electrons
The hydrogen peroxide is reduced
Therefore, potassium iodide is acting as a reducing agent
What is an an oxidising agent?
An oxidising agent is a substance that oxidises another substance, and becomes reduced in the process
An oxidising agent gains electrons as another substance loses electrons
Common examples include hydrogen peroxide, fluorine and chlorine
What is a reducing agent?
A reducing agent is a substance that reduces another substance, and becomes oxidised in the process
A reducing agent loses electrons as another substance gains electrons
Common examples include carbon and hydrogen
The process of reduction is very important in the chemical industry as a means of extracting metals from their ores
