Question

Compare and contrast combination reactions and decomposition reactions. Give an example of each.

Short answer

Combination reactions, also known as synthesis reactions, involve two or more reactants combining to form a single product and are exothermic, releasing energy. Decomposition reactions, on the other hand, involve a single compound breaking down into two or more simpler substances and are endothermic, requiring an input of energy. An example of a combination reaction is the formation of water: \(2H_{2}(g) + O_{2}(g) \rightarrow 2H_{2}O(l)\). An example of a decomposition reaction is the decomposition of calcium carbonate upon heating: \(CaCO_{3}(s) \rightarrow CaO(s) + CO_{2}(g)\).

Step-by-step solution

Definition of Combination Reactions

A combination reaction (also called a synthesis reaction) is a chemical reaction in which two or more substances combine to form a single product. The reactants can be elements or compounds, and the resulting product is usually a compound. Combination reactions generally release energy (typically in the form of heat) and are thus classified as exothermic reactions.

Definition of Decomposition Reactions

A decomposition reaction is a chemical reaction in which a single compound breaks down into two or more simpler substances. The products of a decomposition reaction can be elements or compounds. Decomposition reactions generally require an input of energy to break the chemical bonds, making them endothermic reactions.

Comparing Combination and Decomposition Reactions

Both combination and decomposition reactions involve the formation or breaking of chemical bonds, resulting in a change of substances. 1. Energy Change: Combination reactions are exothermic, meaning they release energy in the form of heat or light, whereas decomposition reactions are endothermic, requiring an input of energy to break the bonds in the reactants. 2. Number of Reactants and Products: Combination reactions involve two or more reactants combining to form a single product, while decomposition reactions involve a single reactant breaking down into two or more products.

Example of a Combination Reaction

One example of a combination reaction is the formation of water from hydrogen gas and oxygen gas: \[ 2H_{2}(g) + O_{2}(g) \rightarrow 2H_{2}O(l) \] In this reaction, two hydrogen molecules (H₂) and one oxygen molecule (O₂) combine to form two water molecules (H₂O). This reaction is exothermic, releasing energy in the form of heat as the bonds in the hydrogen and oxygen molecules are broken and new bonds are formed in the water molecules.

Example of a Decomposition Reaction

One example of a decomposition reaction is the decomposition of calcium carbonate (limestone) to calcium oxide and carbon dioxide upon heating: \[ CaCO_{3}(s) \rightarrow CaO(s) + CO_{2}(g) \] In this reaction, calcium carbonate breaks down into calcium oxide and carbon dioxide when heated. This reaction is endothermic, requiring an input of energy (heat) to break the bonds in calcium carbonate and form new bonds in the products.

Key concepts

Combination Reactions

Combination reactions are fundamentally a process where two or more substances merge to create a single, new compound. Think of this like a dance where separate individuals (the reactants) join hands to form a circle (the product). For instance, when you mix iron filings with sulfur and heat them, they fuse into iron sulfide, a compound distinctly different from its initial parts.

In daily life, you witness a combination reaction when magnesium ribbon burns in the air to form magnesium oxide. A striking luminescence accompanies this reaction, signaling the release of energy. This is a hallmark of exothermic reactions - more on that later.

Decomposition Reactions

Now, imagine the reverse of a combination reaction: this is called a decomposition reaction. It's like taking a complex toy apart; what was once a single item becomes several separate pieces. These reactions typically occur when a single compound is subjected to energy, such as electricity or heat, causing it to break down.

A classic example is the electrolysis of water, where electrical energy splits water into hydrogen and oxygen gases. These are typically endothermic, where the substance at hand absorbs energy to fragment into simpler molecules.

Exothermic and Endothermic Reactions

Chemical reactions involve a trade of energy, and they are classified based on how this energy flows. Exothermic reactions, like the previously mentioned combination of hydrogen and oxygen to form water, release energy, usually in the shape of heat or light. It's exo- or 'outward,' because the energy is given off to the surroundings.

Conversely, endothermic reactions soak up energy from their environment. Picture a sports gel pack that turns cold when activated - this is an example of an endothermic process. Decomposition reactions, such as the endothermic breakdown of calcium carbonate into calcium oxide and carbon dioxide, require energy to proceed; they absorb it, typically as heat, to break down compounds into simpler substances.

Chemical Bonding

At the heart of these reactions is chemical bonding, the very glue that holds atoms together within molecules. Chemical bonds form when atoms share or transfer electrons, and they can be likened to different types of relationships, each with unique characteristics. Ionic bonds occur when one atom donates electrons to another, while covalent bonds are the result of atoms sharing electrons.

The creation or disruption of these bonds is what drives combination and decomposition reactions. For example, water formation involves the creation of covalent bonds as hydrogen and oxygen share electrons. On the other hand, the decomposition of calcium carbonate entails breaking ionic bonds. Understanding the intricacies of chemical bonding is key to comprehending how and why substances react as they do.