For each of the reactions given below, calculate the oxidation number of each of the elements in the reactants and the products and determine if the reaction involves oxidation-reduction. If it is a redox reaction, identify the elements that have been oxidized and reduced. a. \(\mathrm{Cu}_{2} \mathrm{~S} \rightarrow 2 \mathrm{Cu}+\mathrm{S}\) Reactants: Cu ___________ \(\mathrm{S}\) ___________ Products: Cu ___________ \(\mathrm{S}\) ___________ Element oxidized: ___________ Element Reduced ___________ b. \(\mathrm{CaCO}_{3} \rightarrow \mathrm{CaO}+\mathrm{CO}_{2}\) Reactants: Ca ___________ C ___________ O ___________ Products: \(\mathrm{Ca}\) ___________ \(C\) ___________ O ___________ Element oxidized: ___________ Element Reduced ___________ c. \(\mathrm{Fe}_{2} \mathrm{O}_{3}+3 \mathrm{H}_{2} \rightarrow 2 \mathrm{Fe}+3 \mathrm{H}_{2} \mathrm{O}\) Reactants: Fe ___________ O ___________ \(\mathrm{H}\) ___________ Products: Fe ___________ O ___________ \(\mathrm{H}\) ___________ Element oxidized: ___________ Element Reduced ___________ \(\mathrm{d} . \mathrm{AgNO}_{3}+\mathrm{NaCl} \rightarrow \mathrm{AgCl}(\mathrm{s})+\mathrm{NaNO}_{3}\) Reactants: Ag ___________ \(\mathrm{N}\) ___________ \(\mathrm{O}\) ___________ \(\mathrm{Na}\) ___________ \(\mathrm{Cl}\) ___________ Products: \(\mathrm{Ag} \) ___________ \( \mathrm{N}\) ___________ \( \mathrm{O}\) ___________ \( \mathrm{Na}\) ___________ Element oxidized: ___________ Element Reduced ___________

Step by step solution

01

Assigning Oxidation Numbers for Reaction a

For the reactant Cu2S: Copper (Cu) in Cu2S is in the +1 oxidation state as it typically has a +2 oxidation state and therefore needs to balance the -2 charge of sulfur (S). Sulfur in Cu2S is in the -2 oxidation state as it is typically found with a -2 charge in binary compounds. For the products: Copper (Cu) is in the 0 oxidation state as it is in its elemental form. Sulfur (S) is in the 0 oxidation state as it is in its elemental form.

02

Identifying Redox Pairs for Reaction a

Since the oxidation state of Cu changes from +1 in Cu2S to 0 in elemental Cu, and the oxidation state of S changes from -2 in Cu2S to 0 in elemental S, oxidation and reduction have occurred. Thus, this is a redox reaction.

03

Determining Oxidized and Reduced Elements for Reaction a

The element that has been oxidized is S because its oxidation number increased from -2 to 0. The element that has been reduced is Cu because its oxidation number decreased from +1 to 0.

04

Assigning Oxidation Numbers for Reaction b

For the reactant CaCO3: Ca is in the +2 oxidation state, C is in the +4 oxidation state (considering CO32- has a -2 overall charge and oxygen has a typical -2 oxidation state), O is in the -2 oxidation state. For the products: In CaO, Ca is +2 and O is -2. In CO2, C is +4 and O is -2.

05

Checking for Redox in Reaction b

The oxidation numbers of all the elements remain the same from reactants to products. Therefore, there is no redox reaction in this case.

06

Assigning Oxidation Numbers for Reaction c

For the reactants Fe2O3 and H2: Fe in Fe2O3 is in the +3 oxidation state, O is in the -2 oxidation state, and H in H2 is in the 0 oxidation state. For the products: Fe is in the 0 oxidation state, and O and H in H2O are in the -2 and +1 oxidation states respectively.

07

Identifying Redox Pairs for Reaction c

Since the oxidation state of Fe changes from +3 in Fe2O3 to 0 in elemental Fe, and the oxidation state of H changes from 0 in H2 to +1 in H2O, reduction and oxidation have occurred, respectively. This is a redox reaction.

08

Determining Oxidized and Reduced Elements for Reaction c

The element that has been oxidized is H because its oxidation number increased from 0 to +1. The element that has been reduced is Fe because its oxidation number decreased from +3 to 0.

09

Assigning Oxidation Numbers for Reaction d

For the reactants AgNO3 and NaCl: Ag is in the +1 oxidation state, N is in the +5 oxidation state, O is in the -2 oxidation state, Na is in the +1 oxidation state, and Cl is in the -1 oxidation state. For the products AgCl and NaNO3: In AgCl, Ag is +1 and Cl is -1. In NaNO3, Na is +1, N is +5, and O is -2.

10

Checking for Redox in Reaction d

The oxidation numbers are the same for all elements in the reactants and products. Therefore, there is no redox reaction in this case.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Oxidation Number

Understanding the oxidation number of an element in a compound is crucial for identifying redox reactions. The oxidation number, also known as oxidation state, is a number that represents the total number of electrons an atom either gains or loses to form a chemical bond with another atom. It's an essential tool for keeping track of electron transfer during chemical reactions.

For example, in a simple ion like fluoride, F-, the oxidation number of fluorine is -1, indicating it has gained an electron. Conversely, in the sodium ion, Na+, the oxidation number for sodium is +1, showing it has lost an electron. In molecular compounds, the oxidation numbers of the atoms add up to zero, while in ionic compounds, they sum up to the charge of the ion.

Rules for Assigning Oxidation Numbers

• The oxidation number of an atom in its elemental form is always zero.
• For monoatomic ions, the oxidation number is equal to the charge of the ion.
• Oxygen usually has an oxidation number of -2, except in peroxides like H2O2, where it's -1.
• Hydrogen is normally +1 when bonded to non-metals and -1 when bonded to metals.
• The algebraic sum of oxidation numbers in a neutral compound is zero, while in a polyatomic ion, it's equal to the ion's charge.

Redox Pairs

Redox pairs are two elements that undergo changes in oxidation number during a redox reaction. These changes reflect the transfer of electrons between the elements involved. In a redox reaction, one element is oxidized (loses electrons) and increases its oxidation number, while another is reduced (gains electrons) and decreases its oxidation number.

The substance that donates electrons (gets oxidized) is called the reducing agent, and the one that accepts electrons (gets reduced) is the oxidizing agent. Redox pairs are critical for balancing redox reactions because they ensure that the number of electrons lost is equal to the number gained, following the law of conservation of mass.

Element Oxidized Reduced

In a redox reaction, identifying which element is oxidized and which is reduced is a key step. This is because redox reactions are all about the movement of electrons from one species to another.

The element that loses electrons during the reaction is said to be oxidized. As a result, its oxidation number increases. Conversely, the element that gains electrons is said to be reduced, and its oxidation number decreases. Remember that oxidation is not necessarily about oxygen. Any time an element loses electrons, it is experiencing oxidation, irrespective of whether oxygen is present in the reaction.

Assigning Oxidation States

Assigning oxidation states to the atoms within molecules or ions is based on a set of rules. In a step-by-step process, one can systematically determine the oxidation state of each atom in a chemical formula. Consider the overall charge of the compound and the typical values of oxidation numbers for different elements, based on their position in the periodic table and known charge tendencies in ionic or covalent bonds.

For example, in the compound H2O, we know that oxygen usually has an oxidation number of -2, and hydrogen is typically +1. Since there are two hydrogens, their total charge is +2, which balances the -2 from the oxygen, making the compound neutral. Assigning oxidation states is a foundational skill for all students studying redox chemistry, as it is the first step in identifying whether a reaction is a redox reaction and understanding how electrons are transferred.