2.1.5 - Redox Flashcards
Define oxidation number
Based on a set of rules that apply to atoms. It can be thought of as number of electrons involved in bonding to a different element.
How can you use oxidation numbers?
Helps when writing formulae and in balancing electrons as a check that all electrons have been accounted for.
What is the rule for elements?
The oxidation is always zero for elements!
What is the oxidation number of H2?
0
What is the oxidation number of S8?
0
What is the oxidation number of Na?
0
What is the rule for compounds and ions?
Each atom in a compound has an oxidation number. An oxidation number has a sign which is placed before the number.
Give two examples of compounds with the oxidation number -2
- H2O
- CaO
Give two examples of compounds with the oxidation number +1
- NH3
- H2S
Give an example of a compound with the oxidation number -1
- HF
Give two examples of compounds with the oxidation number +1
- NaCl
- K2O
Give two examples of compounds with the oxidation number +2
- MgCl2
- CaO
Give three examples of compounds with the oxidation number -1
- HCl
- KBr
- CaI2
Give two examples of ‘H in Hydrides’ with the oxidation number -1
- NaH
- CaH2
Give an example of ‘O in Peroxides’ with the oxidation number +1
H2O2
Give an example of ‘O bonded to F’ with the oxidation number +2
F2O
Define the oxidation number in an ion.
The oxidation number of an ion is numerically the same as the ionic charge but the sign comes before the number.
The sum of the oxidation numbers = total charge
How do you work out the oxidation number for Sulfur in H2SO4?
- Step 1:
- Assign any oxidation numbers from the rules
- H2SO4 (Total hydrogens +1x2=+2) and (Total oxygens -2x4=-8)
- Step 2:
- What is the sum of the oxidation numbers?
- Sum of oxidation numbers = total charge = 0
- Step 3:
- Work out the unknown oxidation numbers
- Sum of oxidation numbers = (+2) + (x) + (-8) = 0
- (-6) + (x) = 0
- x = 6
Redox in terms of electrons
Oxidation
Is
Loss
Reduction
Is
Gain
Redox Reactions of Acids
Acids produce salts in neutralisation reactions. Dilute acids also undergo redox reactions with some metals to produce salts and hydrogen gas.
What are polyatomic ions?
Polyatomic ions contain oxygen for example: NO2(-) (-ite) and NO3(-) (-ate). Although still in common usage using -ite and -ate in naming this is old fashioned, today we use oxidation numbers shown as Roman Numerals.
Polyatomic ion NO2(-)
Ion - NO2(-)
Common Name - Nitrite
Oxidation Number of Nitrogen - +3
Modern Name - Nitrate (III)
Polyatomic ion NO3(-)
Ion - NO3(-)
Common Name - Nitrate
Oxidation Number of Nitrogen - +5
Modern Name - Nitrate (V)
What does redox stand for?
Redox stands for reduction-oxidation, a type of chemical reaction where oxidation and reduction processes occur simultaneously.
What is oxidation?
Oxidation is the process in which an element loses electrons, resulting in an increase in oxidation state.
What is reduction?
Reduction is the process in which an element gains electrons, resulting in a decrease in oxidation state.
What is an oxidizing agent?
An oxidizing agent is a substance that gains electrons and is reduced in a redox reaction. It causes the oxidation of another substance.
What is a reducing agent?
A reducing agent is a substance that loses electrons and is oxidized in a redox reaction. It causes the reduction of another substance.
What is an oxidation state?
The oxidation state (or number) of an element is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic.
What is the oxidation state of an element in its pure form (e.g., O₂, N₂)?
The oxidation state of an element in its pure form is 0.
What is the oxidation state of oxygen in most compounds?
The oxidation state of oxygen is typically -2, except in peroxides where it is -1.
What is the oxidation state of hydrogen in most compounds?
The oxidation state of hydrogen is typically +1, except in metal hydrides where it is -1.
What is the oxidation state of alkali metals in compounds?
The oxidation state of alkali metals (Group 1) in compounds is always +1.
What is the oxidation state of halogens in compounds?
The oxidation state of halogens is typically -1, except when they are bonded to oxygen or fluorine, where they can have positive oxidation states.
How do you determine the oxidation state of an atom in a compound?
The sum of the oxidation states of all atoms in a molecule or ion equals the charge on the molecule or ion. Use this rule, along with known oxidation states (e.g., hydrogen +1, oxygen -2), to determine the oxidation state of an unknown element.
What is the rule for oxidation states in compounds?
The sum of the oxidation states of all atoms in a molecule or ion must equal the overall charge of the compound or ion.
What is an example of a redox reaction?
A classic example of a redox reaction is the reaction between magnesium and oxygen to form magnesium oxide:
2Mg + O₂ → 2MgO
Here, magnesium is oxidized (loses electrons) and oxygen is reduced (gains electrons).
What is the general equation for a half-reaction?
A half-reaction shows either the oxidation or reduction process separately. For example:
Oxidation half-reaction: Mg → Mg²⁺ + 2e⁻
Reduction half-reaction: O₂ + 4e⁻ → 2O²⁻
How do you balance redox reactions in acidic solution using half-reactions?
To balance a redox reaction in acidic solution:
1- Write the oxidation and reduction half-reactions.
2- Balance atoms other than hydrogen and oxygen.
3- Balance oxygen atoms by adding H₂O molecules.
4- Balance hydrogen atoms by adding H⁺ ions.
5- Balance the charges by adding electrons.
How do you balance redox reactions in basic solution using half-reactions?
After balancing the redox reaction in acidic solution, add OH⁻ ions to both sides of the equation to neutralize H⁺ ions, creating water molecules if necessary, to balance the reaction in basic solution.
What is disproportionation in a redox reaction?
Disproportionation occurs when a substance is both oxidized and reduced in the same reaction. For example, in the reaction of chlorine with water:
Cl₂ + H₂O → HCl + HClO
What is the half-cell in a redox reaction?
A half-cell is a part of a galvanic cell in which either oxidation or reduction occurs. It consists of a metal electrode in contact with a solution of its ions.
What is the electrochemical series?
The electrochemical series is a list of elements or compounds ranked according to their ability to gain or lose electrons. The higher up the series, the stronger the oxidizing agent.
How does the electrochemical series help predict redox reactions?
In a redox reaction, the substance higher in the electrochemical series will be reduced (gain electrons), and the substance lower in the series will be oxidized (lose electrons).
What is the role of electrons in redox reactions?
In redox reactions, electrons are transferred from the reducing agent (which loses electrons and is oxidized) to the oxidizing agent (which gains electrons and is reduced).
How do you identify the oxidizing agent and reducing agent in a redox reaction?
- The oxidizing agent is the substance that gains electrons (reduced).
- The reducing agent is the substance that loses electrons (oxidized).
What is the oxidation number of chlorine in NaCl?
The oxidation number of chlorine in NaCl is -1, as it gains one electron to form the chloride ion (Cl⁻).
What is the oxidation number of oxygen in H₂O?
The oxidation number of oxygen in H₂O is -2, as oxygen typically has an oxidation state of -2 in most compounds.
