Compounds and Chemical Formulae

Introduction

Key Concepts

Definition of Compounds

A compound is a substance formed when two or more different elements chemically bond in fixed proportions. Unlike mixtures, compounds have unique properties that are different from their constituent elements. For example, water (\$H_2O\$) is a compound made from hydrogen and oxygen, each of which has distinct characteristics.

Chemical Bonds in Compounds

Compounds are held together by chemical bonds, which are classified primarily into ionic and covalent bonds.

• Ionic Bonds: Formed when electrons are transferred from one atom to another, resulting in the formation of positively and negatively charged ions. An example is sodium chloride (\$NaCl\$), where sodium donates an electron to chlorine.
• Covalent Bonds: Occur when atoms share electrons to achieve a stable electron configuration. Water (\$H_2O\$) is a classic example, where each hydrogen shares electrons with oxygen.

Chemical Formulae

A chemical formula represents the elements in a compound and the ratio of their atoms. It provides crucial information about the composition of the substance.

• Molecular Formulas: Show the exact number of each type of atom in a molecule. For instance, glucose has the molecular formula \$C_6H_{12}O_6\$.
• Empirical Formulas: Indicate the simplest whole-number ratio of the atoms in a compound. For example, the empirical formula of hydrogen peroxide is \$HO\$.

Naming Compounds

Naming compounds follows specific nomenclature rules to ensure consistency and clarity.

• Ionic Compounds: Named by stating the cation (positive ion) first, followed by the anion (negative ion). If the cation can form more than one type of ion, a Roman numeral indicates its charge, such as iron(III) oxide (\$Fe_2O_3\$).
• Covalent Compounds: Use prefixes to denote the number of atoms of each element, such as carbon dioxide (\$CO_2\$) and sulfur trioxide (\$SO_3\$).

Balancing Chemical Equations

Balancing chemical equations ensures the law of conservation of mass is satisfied, meaning the number of atoms for each element is the same on both sides of the equation.

1. Write the unbalanced equation.
2. List the number of atoms for each element on both sides.
3. Use coefficients to balance the number of atoms.
4. Ensure the smallest possible whole-number coefficients are used.

For example, the unbalanced combustion of methane is: $$CH_4 + O_2 \rightarrow CO_2 + H_2O$$ Balancing it gives: $$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$$

Types of Compounds

Compounds can be broadly classified into several types based on their bonding and properties.

• Salts: Ionic compounds composed of metal and non-metal ions, such as sodium chloride (\$NaCl\$).
• Covalent Compounds: Made up of non-metals sharing electrons, like carbon dioxide (\$CO_2\$).
• Metallic Compounds: Formed between metal atoms and show properties like electrical conductivity, as seen in alloys like steel.

Molecular Geometry and Chemical Formulae

The arrangement of atoms in a compound affects its properties and reactivity. Understanding molecular geometry helps in predicting the behavior of compounds in different chemical reactions.

• Linear: Molecules like carbon dioxide have atoms arranged in a straight line.
• Trigonal Planar: Boron trifluoride (\$BF_3\$) has a flat, triangular shape.
• Tetrahedral: Methane (\$CH_4\$) has a three-dimensional shape with four hydrogen atoms evenly spaced around a central carbon atom.

Stoichiometry

Stoichiometry involves calculating the quantities of reactants and products in chemical reactions based on the balanced equations.

For example, in the reaction: $$2H_2 + O_2 \rightarrow 2H_2O$$ 2 molecules of hydrogen gas react with 1 molecule of oxygen gas to produce 2 molecules of water. This ratio is crucial for determining the amounts needed or produced in a reaction.

Empirical vs. Molecular Formulas

While empirical formulas represent the simplest ratio of elements, molecular formulas show the actual number of atoms in a molecule.

For example, the empirical formula of hydrogen peroxide is \$HO\$, suggesting a 1:1 ratio of hydrogen to oxygen. However, its actual molecular formula is \$H_2O_2\$, indicating two hydrogen atoms and two oxygen atoms.

Isomerism in Compounds

Isomers are compounds with the same molecular formula but different structural arrangements, leading to diverse properties.

• Structural Isomers: Differ in the connectivity of atoms. For instance, pentane can exist as n-pentane or isopentane.
• Stereoisomers: Have the same connectivity but differ in the spatial arrangement of atoms, such as cis-trans isomers in alkenes.

Applications of Chemical Compounds

Chemical compounds play essential roles in various fields, including medicine, industry, and environmental science.

• Medicine: Compounds like aspirin (\$C_9H_8O_4\$) are used as analgesics.
• Industry: Sodium hydroxide (\$NaOH\$) is vital in manufacturing processes.
• Environmental Science: Carbon dioxide (\$CO_2\$) is a greenhouse gas affecting global temperatures.

Chemical Reactions Involving Compounds

Compounds undergo various types of chemical reactions, including synthesis, decomposition, single displacement, and double displacement reactions.

• Synthesis Reaction: Two or more reactants combine to form a single product, e.g., \$N_2 + 3H_2 \rightarrow 2NH_3\$.
• Decomposition Reaction: A single compound breaks down into two or more simpler substances, e.g., \$2H_2O \rightarrow 2H_2 + O_2\$.
• Single Displacement Reaction: An element displaces another in a compound, e.g., \$Zn + 2HCl \rightarrow ZnCl_2 + H_2\$.
• Double Displacement Reaction: Exchange of ions between two compounds, e.g., \$AgNO_3 + NaCl \rightarrow AgCl + NaNO_3\$.

Determining Chemical Formulas from Names

Learning to deduce chemical formulas from compound names is a crucial skill.

For example, "Calcium carbonate" indicates a compound composed of calcium (\$Ca\$) and carbonate (\$CO_3\$). Calcium typically forms a \$2+\$ ion, and carbonate has a \$2-\$ charge, resulting in the formula \$CaCO_3\$.

Oxidation States and Chemical Formulas

Understanding oxidation states aids in determining the correct chemical formula of a compound.

For instance, in iron(III) chloride, the Roman numeral (III) indicates that iron has a \$3+\$ oxidation state. Chloride ions have a \$1-\$ charge, so three chloride ions balance the charge of one iron ion, resulting in \$FeCl_3\$.

The Role of Compounds in Everyday Life

Compounds are integral to daily activities and technologies.

• Food: Sodium chloride (\$NaCl\$) is essential for flavoring and preservation.
• Cleaning Products: Ammonium hydroxide (\$NH_4OH\$) is used in household cleaners.
• Electronics: Silicon dioxide (\$SiO_2\$) is a key component in semiconductors.

Safety and Handling of Chemical Compounds

Proper handling of chemical compounds is vital to ensure safety in laboratories and workplaces.

• Always wear appropriate personal protective equipment (PPE).
• Understand the properties and hazards associated with each compound.
• Store compounds according to their compatibility and stability requirements.

Comparison Table

Summary and Key Takeaways