4.1 Types of chemical reactions

Chemical reactions are the heart of chemistry, transforming substances into new ones. They come in various types, each with unique characteristics. Understanding these reactions helps us predict outcomes and balance equations, key skills for any chemist.

From synthesis to combustion, chemical reactions shape our world. We'll explore how atoms rearrange, energy changes, and products form. This knowledge is crucial for grasping how matter behaves and transforms in countless processes around us.

Types of Chemical Reactions

Synthesis Reactions

• Involve two or more reactants combining to form a single product
• General equation: $A + B \rightarrow AB$
• Often release energy (exothermic)
• Examples:
  • $2H_2 + O_2 \rightarrow 2H_2O$ (formation of water)
  • $2Na + Cl_2 \rightarrow 2NaCl$ (formation of sodium chloride)

Decomposition Reactions

• Involve a single compound breaking down into two or more simpler substances
• General equation: $AB \rightarrow A + B$
• Often require energy input (endothermic)
• Examples:
  • $2H_2O \rightarrow 2H_2 + O_2$ (electrolysis of water)
  • $CaCO_3 \rightarrow CaO + CO_2$ (thermal decomposition of calcium carbonate)

Single Displacement Reactions

• Involve one element replacing another element in a compound
• General equation: $A + BC \rightarrow AC + B$
• More reactive element displaces a less reactive element in a compound
• Reactivity of elements determined using the activity series
• Examples:
  • $Zn + 2HCl \rightarrow ZnCl_2 + H_2$ (zinc displacing hydrogen in hydrochloric acid)
  • $2Al + 3CuSO_4 \rightarrow Al_2(SO_4)_3 + 3Cu$ (aluminum displacing copper in copper(II) sulfate)

Double Displacement Reactions

• Involve two compounds exchanging ions to form two new compounds
• General equation: $AB + CD \rightarrow AD + CB$
• Typically occur in aqueous solutions
• Formation of a precipitate, gas, or weak electrolyte (such as water) can drive the reaction to completion
• Examples:
  • $NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$ (formation of silver chloride precipitate)
  • $HCl + NaOH \rightarrow NaCl + H_2O$ (neutralization reaction forming water)

Combustion Reactions

• Involve a substance (fuel) reacting with oxygen to produce heat, light, and often carbon dioxide and water
• General equation: $Fuel + O_2 \rightarrow CO_2 + H_2O + Heat + Light$
• Hydrocarbons are common fuels, producing $CO_2$ and $H_2O$ as products
• Insufficient oxygen may result in the formation of carbon monoxide ($CO$)
• Examples:
  • $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ (combustion of methane)
  • $C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$ (combustion of propane)

Characteristics of Chemical Reactions

Reactants and Products

• Reactants are the starting materials that undergo a chemical change in a reaction
• Products are the substances formed as a result of the chemical reaction
• The physical states of reactants and products (solid, liquid, gas, or aqueous) are often indicated using parentheses $(s)$, $(l)$, $(g)$, or $(aq)$, respectively

Energy Changes

• Chemical reactions often involve energy changes, either releasing (exothermic) or absorbing (endothermic) energy
• Exothermic reactions release energy, often in the form of heat or light (combustion reactions)
• Endothermic reactions absorb energy, requiring an input of energy to proceed (decomposition reactions)

Driving Forces

• Some reactions are driven by the formation of stable products, such as precipitates, gases, or weak electrolytes (water)
• The solubility rules can predict whether a double displacement reaction will result in the formation of a precipitate
• The reactivity series (activity series) determines the likelihood of single displacement reactions occurring

Balancing Chemical Equations

Conservation of Mass

• The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction
• The total mass of the reactants must equal the total mass of the products
• A balanced chemical equation has equal numbers of each type of atom on both the reactant and product sides

Balancing Techniques

• Coefficients are used to balance the number of each type of atom on both sides of the equation
• Subscripts represent the number of atoms in a molecule and cannot be changed when balancing equations
• Steps for balancing equations:
  1. Identify the reactants and products
  2. Write the unbalanced equation
  3. Begin balancing with the most complex compound (often the one with the most elements)
  4. Balance the remaining atoms one at a time
  5. Verify that the equation is balanced by counting the number of each type of atom on both sides

Stoichiometry

• Stoichiometry is the quantitative study of reactants and products in a chemical reaction
• Balanced equations provide the relative amounts of reactants and products in mole ratios
• Mole ratios can be used to calculate the amount of product formed or reactant needed in a reaction
• Example: For the balanced equation $2H_2 + O_2 \rightarrow 2H_2O$, the mole ratio of $H_2$ to $O_2$ is 2:1, meaning that 2 moles of $H_2$ react with 1 mole of $O_2$ to produce 2 moles of $H_2O$

Predicting Reaction Products

Synthesis Reactions

• The product is a single compound formed by the combination of the reactants
• Example: $2Na + Cl_2 \rightarrow 2NaCl$ (sodium and chlorine combine to form sodium chloride)

Decomposition Reactions

• The products are two or more simpler substances formed by the breakdown of the reactant compound
• Example: $2H_2O \rightarrow 2H_2 + O_2$ (water decomposes into hydrogen and oxygen)

Single Displacement Reactions

• The more reactive element will replace the less reactive element in the compound, forming a new element and a new compound
• Example: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$ (zinc displaces hydrogen in hydrochloric acid to form zinc chloride and hydrogen gas)

Double Displacement Reactions

• The cations and anions of the reactant compounds will exchange partners to form two new compounds
• The solubility rules predict whether a precipitate will form
• Example: $NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$ (sodium chloride and silver nitrate react to form the precipitate silver chloride and the soluble compound sodium nitrate)

Combustion Reactions

• Hydrocarbons react with oxygen to produce carbon dioxide, water, heat, and light
• Insufficient oxygen may result in the formation of carbon monoxide
• Example: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ (methane reacts with oxygen to form carbon dioxide and water)