Chemical Reactions and Stoichiometry Review

Chemical Reactions and Stoichiometry

Definitions

• Chemical Change: Formation of new substances with a new chemical composition. Example: Photosynthesis.

• Physical Change: Changes that do not result in the formation of new substances. Example: Melting of ice.

Characteristics of Chemical Reactions

• Formation of new substances.

• Production of heat, light, or both.

• Change in color.

• Change in temperature.

Chemical Reactions and Equations

• Reactants: Substances that undergo change.

• Products: New substances formed from a reaction.

• A chemical reaction is represented by a chemical equation.

  • Example: $S + O<em>2 ightarrow SO</em>2$

• A balanced equation maintains equal numbers of atoms for each element on both sides of the equation.

Stoichiometry

• The quantitative study of the relationships between reactants and products in a chemical reaction.

Types of Chemical Reactions

1. Combination (Synthesis) Reaction: Two or more reactants combine to form a single product.

   • Example: $2Na + Cl_2 <br>ightarrow 2NaCl$

2. Decomposition Reaction: A single compound breaks down into two or more products.

   • Example: $2HgO <br>ightarrow 2Hg + O_2$

3. Single Displacement Reaction: One element displaces another in a compound.

   • Example: $Zn + 2HCl <br>ightarrow ZnCl<em>2 + H</em>2$

4. Double Displacement Reaction: Exchange of ions between two compounds.

   • Example: $Na<em>2SO</em>4 + Ba(NO<em>3)</em>2 <br>ightarrow BaSO<em>4 + 2NaNO</em>3$

Writing and Balancing Equations

1. Write the word equation first.

2. Convert to a chemical equation using symbols.

3. Balance the equation using coefficients to equalize the number of atoms of each element on both sides.

Balancing Methods

1. Inspection Method: Manually adjusting coefficients through trial and error.

2. Least Common Multiple (LCM) Method: Using LCM of the total valencies.

3. Algebraic Method: Assigning variables to coefficients and solving equations based on atom counts.

Example Problem: Balancing using Inspection

• Example Reaction: $Fe + H<em>2O ightarrow Fe</em>2O<em>3 + H</em>2$

• Balanced: $4Fe + 6H<em>2O ightarrow 2Fe</em>2O<em>3 + 3H</em>2$

Redox Reactions

• Oxidation: Loss of electrons; occurs when an element's oxidation state increases.

• Reduction: Gain of electrons; occurs when an element's oxidation state decreases.

• Oxidizing Agent: Causes oxidation and is reduced itself.

• Reducing Agent: Causes reduction and is oxidized itself.

Oxidation Numbers and Rules

1. Oxidation number of an uncombined element: 0.

2. Monatomic ion oxidation number equals its charge.

3. Oxygen usually has an oxidation number of -2; exceptions: peroxides, superoxides.

4. Hydrogen usually has an oxidation number of +1; exceptions in metal hydrides.

5. The sum of oxidation states in a neutral compound is 0.

6. The sum of oxidation states in polyatomic ions equals the ion’s charge.

Mole Concept

• A mole: Amount of substance that contains $6.022 imes 10^{23}$ entities (Avogadro's Number).

• Molar Mass: Mass of one mole of a substance. E.g., H2O molar mass = 18 g/mol.

• Empirical Formula: Simplest whole number ratio of elements in a compound (e.g., for benzene: CH).

• Molecular Formula: Actual number of atoms in a molecule (e.g., C6H6 for benzene).

Percent Composition

• The percent by mass calculation of each element in a compound.

  • Example: ext{%C} = rac{n imes ext{molar mass of C}}{ ext{molar mass of compound}} imes 100

Theoretical vs. Actual Yield

• Theoretical Yield: Maximum amount of product that can be formed from a given amount of reactants.

• Actual Yield: Amount of product actually produced when the reaction is completed.

• Percentage Yield: $rac{ ext{Actual Yield}}{ ext{Theoretical Yield}} imes 100$

Stoichiometric Calculations

• Use molar ratios from balanced equations to convert between amounts of reactants and products.

• Mass-Mass: Convert mass of one substance to moles, then to mass of another substance.

  • Example: $ext{grams A} <br>ightarrow ext{moles A} <br>ightarrow ext{moles B} <br>ightarrow ext{grams B}$

This knowledge is crucial for understanding various chemical reactions and their implications in chemistry, including practical applications in labs and industries.