Chemical Reactions

Introduction to Chemical Reactions

Chemistry is fundamentally about change, specifically the rearrangement of atoms in substances. Chemical reactions are the processes by which these changes occur, resulting in new substances with different properties.

• Definition: A chemical reaction is a process in which one or more substances (reactants) are converted into one or more different substances (products).

• Evidence of Chemical Reactions:

  • Visual indications such as rusting of steel, curling of hair, burning (flames).

  • Other evidence includes color change, formation of solids, bubble formation (gas evolution), and heat exchange (endothermic/exothermic reactions).

Reactants and Products

In a chemical reaction, the substances that undergo change are called reactants, and the new substances formed are called products.

• Example:

• Law of Mass Conservation: The total number and type of atoms are conserved in a chemical reaction.

Balancing Chemical Equations

Rules for Balancing Equations

Balancing chemical equations ensures that the same number of each type of atom appears on both sides of the equation, in accordance with the law of conservation of mass.

1. Write correct formulas for reactants and products.

2. List all atoms present under the equilibrium arrow.

3. Adjust coefficients of the simplest compounds first.

4. Delete any coefficients equal to 1.

5. If all coefficients share a common denominator, divide through by it.

Examples of Balanced Equations

Practice: Writing and Balancing Equations

• Mercury(II) oxide decomposes:

• Zinc reacts with hydrochloric acid:

• Ethanol combustion:

• Potassium iodate with potassium iodide and hydrochloric acid:

Types of Chemical Reactions

Classification of Reactions

Chemical reactions can be classified into several types based on the nature of the reactants and products.

• Combination (Synthesis) Reaction: Two or more substances combine to form one product. Example:

• Decomposition Reaction: One reactant breaks down into two or more products. Example:

• Displacement (Substitution) Reaction: An element in a compound is replaced by another element. Simple: Double:

• Combustion Reaction: Rapid reaction of a substance with oxygen. Example:

Possible Outcomes of Chemical Reactions

• Formation of a solid

• Formation of water

• Transfer of electrons

• Formation of a gas

These changes generally lower the energy of the system and increase the entropy of the universe.

Molecular Structure and Molecular Mass

Understanding Molecular Formulas

A molecular formula indicates the types and numbers of atoms in a molecule. Each element symbol represents an atom, and the subscript shows the number of each atom type.

• Atomic Mass: The weighted average mass of an atom (including isotopes).

• Molecular Mass (Molecular Weight): The sum of the atomic masses of all atoms in a molecule.

Calculations Using Molecular Mass

• A chemical formula provides the ratio of one atom type to others in a compound.

• It also provides the ratio of moles of one atom type to moles of others.

• Moles are the key unit in dimensional analysis for relating species.

Avogadro's number () is essential for converting between number of atoms/molecules and moles.

Example Calculations

• Calculate the number of molecules in 0.445 mol of .

• Calculate the mass of 0.445 mol of ( g/mol).

• Calculate the number of moles in 41.9 g of ( g/mol).

• Calculate the number of moles in a sample containing molecules of .

Additional Calculations

• Calculate the number of H atoms in 6.170 mol of .

• Calculate the number of moles of aspirin () that contains oxygen atoms.

• Calculate the number of grams of F in 7.197 g of .

• Calculate the mass of that contains 7.197 g of Fluorine.

• Calculate the mass of P in 1.50 kg of .

Stoichiometry

Introduction to Stoichiometry

Stoichiometry is the quantitative study of reactants and products in chemical reactions. It is based on balanced chemical equations and relates the masses and moles of substances involved.

• Stoichiometry: Relationship of reactants and products in a chemical reaction, based on mass (g) and/or moles (number of molecules).

• Always relates two different substances via their mole ratios.

Representations of Stoichiometry

Stoichiometry can be represented in three ways: molecular, molar, and mass.

Stoichiometric Calculations

• Given moles of , calculate moles of needed to react completely.

• Given mass of , calculate mass of needed.

• Given desired mass of , calculate mass of required.

Example Problems

• Iron ore () reacts with CO to produce Fe and . Calculate kg of Fe produced from 1.00 kg of :

• Chlorine gas () produced from HCl and . Calculate molecules of HCl that react with 5.00 g :

Limiting Reactant

Concept of Limiting Reactant

The limiting reactant is the reactant that is completely consumed first in a chemical reaction, thus determining the maximum amount of product formed.

• Always use moles to compare chemical species.

• Example:

Practice: Given 0.30 mol Zn and 0.52 mol HCl, identify the limiting reactant and calculate moles of produced.

Theoretical Yield and Percent Yield

Calculating Yield

Theoretical yield is the maximum amount of product that can be formed from the limiting reactant. Percent yield compares the actual yield from experiment to the theoretical yield.

• Percent Yield Formula:

• Example: If 0.23 mol is produced when 0.30 mol Zn reacts with 0.52 mol HCl, calculate percent yield.

Percent Composition

Calculating Percent Composition

Percent composition is the percentage by mass of each element in a compound.

• Calculate percent composition of each element in .

• Calculate percent composition in Iron(II) Nitrate and Ammonium Sulfite.

Empirical Formula

Definition and Calculation

The empirical formula gives the lowest whole number ratio of atoms of each element in a compound.

• Example: Empirical formula of fructose () is .

• Practice: Write empirical formulas for compounds with given ratios of C and H.

Empirical Formula from Percent Composition

Given percent composition data, determine the empirical formula by converting percentages to moles and finding the simplest ratio.

• Example: Find empirical formula for a compound with 37% S and 63% O.

Molecular Formula from Empirical Formula and Percent Composition

Determining Molecular Formula

The molecular formula is a multiple of the empirical formula, determined by comparing the empirical mass to the molecular mass.

• Example: Given percent composition and molecular mass, determine the molecular formula for a compound.