Problem 81 - Chemistry The Central Science

Question

(a) What is a hydrocarbon? (b) Pentane is the alkane with a chain of five carbon atoms. Write a structural formula for this compound and determine its molecular and empirical formulas.

Step-by-Step Solution

Answer

(a) A hydrocarbon is a compound consisting exclusively of carbon (C) and hydrogen (H) atoms, classified into aliphatic and aromatic categories. (b) The structural formula for pentane is CH3-CH2-CH2-CH2-CH3. Its molecular formula is C5H12, and its empirical formula is also C5H12 as it is already in its simplest ratio.

Step 1: (a) Define Hydrocarbon

A hydrocarbon is a compound that consists exclusively of carbon (C) and hydrogen (H) atoms. Hydrocarbons can be classified into two main categories: aliphatic and aromatic. Aliphatic hydrocarbons are further divided into alkanes, alkenes, and alkynes depending on the type of bond between carbon atoms. Aromatic hydrocarbons, also known as arenes, contain a ring of carbon atoms with alternating single and double bonds.

Step 2: (b) Structural Formula of Pentane

Pentane is an alkane because it consists only of single covalent bonds between carbon atoms. As an alkane with five carbon atoms, its structural formula is as follows: CH3-CH2-CH2-CH2-CH3

Step 3: (b) Molecular Formula of Pentane

The molecular formula represents the exact number of atoms of each element in a molecule. To determine the molecular formula of pentane, we can count the number of carbon (C) and hydrogen (H) atoms in its structural formula. There are 5 carbon (C) atoms and 12 hydrogen (H) atoms, so the molecular formula of pentane is C5H12.

Step 4: (b) Empirical Formula of Pentane

The empirical formula represents the simplest whole-number ratio of the elements in a compound. Since the molecular formula of pentane is already in its simplest ratio (C5H12), the empirical formula is the same: Empirical formula of pentane: C5H12

Key Concepts

Structural Formula

Understanding the structural formula of a hydrocarbon is like seeing a map of the molecule. It shows the arrangement of atoms and how they are bonded together. For pentane, an alkane hydrocarbon, the structural formula is a simple chain of carbon (C) atoms connected by single bonds, with hydrogen (H) atoms attached to each carbon. In the case of pentane, you can imagine it as:

• CH₃-CH₂-CH₂-CH₂-CH₃

This formula reveals the linear arrangement where each carbon atom (except at the ends) is bonded to two other carbon atoms. The hydrogen atoms complete the carbon's four-bond requirement. This formula is crucial for understanding the molecule's shape and how it might react with other substances.

Molecular Formula

The molecular formula offers a concise way to express the number of each type of atom in a molecule. It doesn’t show how the atoms are connected but provides the exact number. For pentane, the molecular formula is C₅H₁₂. This tells us there are five carbon atoms and twelve hydrogen atoms in a single molecule of pentane. This formula is important because it indicates the actual mass of the molecule, which can be useful in calculations involving moles and stoichiometry. Keep in mind that different compounds can have the same molecular formula; they're called isomers. Each isomer has a different structural formula and, therefore, different properties.

Empirical Formula

The empirical formula represents the simplest whole-number ratio of atoms in a compound. It doesn’t provide specific quantities but shows a reduced form of the actual composition. For example, for pentane, the empirical formula is the same as its molecular formula: C₅H₁₂. This is because we can't reduce the ratio any further. Empirical formulas are especially useful for ionic compounds and are sometimes used for large covalent molecules to simplify representation. This formula type is key in determining unknown compounds when analyzing elemental composition.

Alkanes

Alkanes are the simplest type of hydrocarbon, consisting only of carbon and hydrogen atoms connected through single bonds. They are also known as saturated hydrocarbons because each carbon atom has as many hydrogen atoms as possible. Some key features of alkanes include:

• They have the general formula C_nH_2n+2.
• They are relatively non-reactive compared to other hydrocarbons due to their single bonds.
• Common uses include fuels like propane and butane.

Alkanes exist as continuous chains or branched structures, and their simplicity makes them a baseline to understand more complex hydrocarbons.

Carbon-Hydrogen Compounds

All hydrocarbons consist of carbon-hydrogen bonds, which form the backbone of these compounds. The strength and stability of these C-H bonds determine the chemical properties and usage of the hydrocarbons. Key attributes of carbon-hydrogen compounds include:

• Hydrophobic nature, meaning they don't mix well with water.
• Varying boiling and melting points depending on the molecular structure.
• Significance in energy production; they are major components of fossil fuels.

Understanding these compounds is essential for grasping basic organic chemistry principles and their real-world applications, such as in energy, materials, and the pharmaceutical industry.