Question

_________ lets us keep track of electrons in oxidation-reduction reactions by assigning charges to the various atoms in a compound.

Short answer

Oxidation numbers are imaginary charges assigned to atoms in a molecule or ion, which help keep track of electrons during oxidation-reduction (redox) reactions. They indicate the extent of loss, gain, or sharing of electrons by an atom in a compound, and thus help us understand the electron flow in redox reactions. For example, in the redox reaction between hydrogen peroxide (H2O2) and sulfuric acid (H2SO4), the oxidation numbers help identify that the oxygen in H2O2 loses one electron while forming water, and the peroxide oxygen in SO5^2- gains one electron. This demonstrates the transfer of electrons in the reaction.

Step-by-step solution

Introduce oxidation numbers

Oxidation numbers are imaginary charges assigned to atoms in a molecule or ion, to help keep track of electrons during a redox reaction. These "bookkeeping" values indicate the extent of loss, gain, or sharing of electrons by an atom in a compound. Determining the oxidation numbers of atoms in a molecule or an ion assists in understanding the electron flow in redox reactions.

Rules for assigning oxidation numbers

The following rules will help assign oxidation numbers to atoms in a compound: 1. The oxidation number of a free (uncombined) element is zero. 2. The oxidation number of an atom in a simple ion is equal to its charge. 3. The oxidation number of hydrogen is usually +1 (except when in metal hydrides, where it is -1). 4. The oxidation number of oxygen is usually -2 (except in peroxides, where it is -1, or superoxides, where it is ½, or if chemically bonded to fluorine). 5. The sum of oxidation numbers of all atoms in a molecule or a polyatomic ion equals the charge of that molecule or ion.

Assign oxidation numbers to a sample compound

Let's use the compound H2SO4 (sulfuric acid) as a sample: 1. Hydrogen: As per rule 3, the oxidation number of hydrogen is +1. 2. Sulfur: There is just one sulfur atom, so we'll determine its oxidation number later. 3. Oxygen: As per rule 4, the oxidation number of oxygen is -2. Now, as per rule 5, the sum of the oxidation numbers must be equal to the charge of the molecule. In this case, H2SO4 is a neutral molecule, so the sum of oxidation numbers must be zero. Therefore, 2(+1) + 1(S) + 4(-2) = 0, giving S (sulfur) an oxidation number of +6. So, the oxidation numbers are: H: +1; S: +6; O: -2 in H2SO4.

Show how oxidation numbers help identify electron transfer

Now, let's consider the redox reaction between hydrogen peroxide (H2O2) and sulfuric acid (H2SO4), forming water (H2O) and peroxydisulfate (SO5^2-). H2O2 + 2H2SO4 -> 2H2O + SO5^2- We will first find the oxidation numbers of each species in the reactants and products. H2O2: H: +1, O: -1 (since it is a peroxide) H2SO4: H: +1, S: +6, O: -2 H2O: H: +1, O: -2 SO5^2-: S: +6 (will be determined later), O: -1 (1 of these oxygens is in a peroxide) Applying rule 5 for charge conservation to the SO5^2- ion, we get +6 - 4 - x = -2, which gives x = +2. Thus, the oxidation number for the peroxide oxygen in SO5^2- is +2. Now we can see that the oxygen in hydrogen peroxide (H2O2) loses one electron (as its oxidation number changes from -1 to -2) when it forms water. At the same time, the peroxide oxygen in SO5^2- gains one electron (as its oxidation number changes from -1 to +2). This exchange of electrons indicates the transfer of electrons in this redox reaction.

Key concepts

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, play a crucial role in chemistry. These reactions involve the transfer of electrons between substances. During a redox reaction, one substance undergoes oxidation (loses electrons), while another undergoes reduction (gains electrons). Understanding redox reactions helps chemists track the movement of electrons, which is vital for predicting reaction outcomes and understanding chemical changes.
Oxidation numbers provide a systematic way to track electron transfer. By assigning charges to atoms, chemists can determine which atoms gain or lose electrons during the reaction, making redox reactions easier to analyze and balance.

• Oxidation involves an increase in oxidation number.
• Reduction involves a decrease in oxidation number.

Visually, these processes can be tracked through reaction equations where reactants transform into products via electron transfer. This visual aid simplifies understanding complex reactions involved in processes like respiration and combustion.

Electron Transfer

Electron transfer is the underlying mechanism of redox reactions. It involves the movement of electrons from one atom or molecule to another. This process can be either direct or indirect, often mediated by chemical compounds that facilitate the transfer.
In the example of hydrogen peroxide reacting with sulfuric acid, electron transfer occurs as the oxygen in H2O2 undergoes oxidation, converting from a -1 oxidation number in the reactant to -2 in the product water. Conversely, the peroxide oxygen in the SO5^{2-} ion gains electrons, changing its oxidation state.

• Direct electron transfer involves the shuttling of electrons between reactants immediately.
• Indirect transfer involves intermediary compounds or ions.

This transfer is pivotal for balancing redox reactions, allowing us to ensure mass and charge conservation. The completion of electron transfer results in the stabilization of atoms involved, often leading to the formation of new compounds.

Chemical Compounds

Chemical compounds are substances made up of two or more types of atoms bonded together through chemical forces. They have unique properties derived from the specific arrangement of their atoms. Compounds like sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) are excellent examples to study oxidation and reduction properties.
When looking at any chemical compound, it is essential to know the roles of individual atoms. This includes knowing their oxidation numbers, which are crucial for predicting how they will behave in reactions.

• Sulfuric acid (H2SO4) is a strong acid used in various industrial processes and its sulfur and oxygen atoms play vital roles in redox reactions.
• Hydrogen peroxide (H2O2) serves as a common oxidizing agent due to the unstable peroxide bond, leading to its utility in multiple redox reactions.

Overall, understanding chemical compounds means understanding how they interact, react, and transform, often dictated by redox principles and the transfer of electrons.