The inversion of configuration in SN₂ reactions is a consequence of the concerted mechanism in which these reactions occur. During the SN₂ process, the nucleophile attacks the substrate carbon atom from the side opposite to the leaving group. As the nucleophile approaches, it pushes the electrons of the carbon-leaving group bond towards the leaving group, causing it to depart. This backside attack results in an inversion of the spatial arrangement of the groups around the carbon atom, akin to an umbrella being flipped inside out. This inverted configuration is distinct from the starting configuration of the substrate, leading to the observed inversion of stereochemistry.

Yes, polar protic solvents can significantly influence the rate of SN₁ and SN₂ reactions. Polar protic solvents, like water and alcohols, have hydrogen atoms attached to strongly electronegative atoms, allowing them to form hydrogen bonds. In SN₁ reactions, polar protic solvents can stabilise the carbocation intermediate through solvation, accelerating the reaction. However, in SN₂ reactions, these solvents can solvate and 'shield' the nucleophile, reducing its nucleophilicity. This solvation makes it harder for the nucleophile to attack the substrate, thus slowing down SN₂ reactions. So, while polar protic solvents promote SN1 reactions, they are not ideal for SN₂ reactions.

The leaving group's ability plays a crucial role in determining the rate and feasibility of SN₁ and SN₂ reactions. A good leaving group is one that can depart as a stable entity, often as a weak base. The stability typically arises due to resonance or electron delocalisation, which allows the leaving group to accommodate the negative charge more effectively. Common good leaving groups include iodide, bromide, and tosylate. A competent leaving group will accelerate both SN₁ and SN₂ reactions. In SN₁, its departure forms the carbocation intermediate, and in SN₂, its departure is simultaneous with the nucleophilic attack.

The rate equation reflects the mechanism of the reaction. For SN₁ reactions, the slow, rate-determining step involves only the departure of the leaving group from the substrate to form a carbocation intermediate. Since this step does not involve the nucleophile, the rate of the reaction is dependent solely on the concentration of the substrate, leading to a unimolecular rate equation. Conversely, in SN₂ reactions, the nucleophile's attack and the leaving group's departure occur simultaneously in a single, concerted step. This bimolecular process means that the rate of the reaction is dependent on the concentrations of both the substrate and the nucleophile, leading to a bimolecular rate equation.

Tertiary halogenoalkanes do not undergo SN₂ reactions primarily because of steric hindrance. The tertiary carbon atom is surrounded by three bulky alkyl groups, making it difficult for a nucleophile to approach and attack the central carbon atom directly. In the concerted SN₂ mechanism, the nucleophile must be able to approach the carbon atom being substituted to displace the leaving group in one step. The spatial bulkiness of tertiary halogenoalkanes impedes this direct approach, making the reaction kinetically unfavourable. Instead, tertiary halogenoalkanes favour the SN₁ mechanism, where the leaving group departs first, forming a carbocation intermediate which is then attacked by the nucleophile.