Classify each of the following reactions as addition or substitution and write its chemical equation: (a) hydrogen reacts with 2-pentene in the presence of a nickel catalyst; (b) hydrogen chloride reacts with propene.

Understanding chemical equations is fundamental in studying chemical reactions. A chemical equation represents the process that changes reactants into products. It's like a recipe that describes the ingredients (reactants) and the final dish (products). In the context of our exercise, addition reactions are demonstrated through chemical equations.
For instance, the reaction of hydrogen with 2-pentene can be depicted by the equation \( \text{C}_5\text{H}_{10} + \text{H}_2 \rightarrow \text{C}_5\text{H}_{12} \). This succinctly shows how two hydrogen atoms are added to the alkene, resulting in an alkane. Similarly, the reaction of hydrogen chloride with propene is represented as \( \text{C}_3\text{H}_6 + \text{HCl} \rightarrow \text{C}_3\text{H}_7\text{Cl} \), displaying the addition of hydrogen and chlorine atoms across the double bond. These equations are crucial for visualizing the changes in atomic connectivity and to confirm the conservation of atoms before and after the reaction.

Substitution reactions are another common type of chemical process where an atom or a group of atoms in a molecule is replaced by another atom or group of atoms. Unlike addition reactions, substitution reactions involve the exchange of parts rather than adding new ones. An everyday example is when chlorine gas reacts with methane, resulting in the substitution of a hydrogen atom with a chlorine atom to form chloromethane.
However, the reactions from our exercise, such as the interaction between hydrogen and 2-pentene or propene with hydrogen chloride, do not involve any substitution as there's no exchange but rather the addition of new atoms to the existing double bond in the alkenes.

Alkenes are hydrocarbons with one or more carbon-carbon double bonds. The double bond is a region of high electron density, making alkenes highly reactive towards agents that can accept or share these electrons, such as hydrogen in the presence of a nickel catalyst, or hydrogen chloride.

In our given problems, 2-pentene and propene are alkenes, and their reactivity is showcased in the ease with which they participate in addition reactions. The electron-rich double bonds attract the slightly positive hydrogen (in case a) and positive hydrogen in HCl (in case b), leading to the opening of the double bond and the formation of more saturated compounds, namely alkanes or haloalkanes.

Catalysts are substances that speed up chemical reactions without being consumed in the process. Their role can be likened to that of a facilitator, making it easier for reactants to convert into products. In our example of hydrogen reacting with 2-pentene, nickel serves as a catalyst. It accelerates the reaction by providing a surface where the reactants can come together more efficiently, thereby breaking the pi bond in the alkene and facilitating the addition of hydrogen atoms. This is vital for industrially important reactions, as the use of a catalyst can drastically improve the rate and yield of the product while sometimes also allowing the reaction to occur at lower temperatures and pressures.