Chap5 Chemical Reactions: Classification, Balancing, and Stoichiometry

Chapter 5: Chemical Reactions

Chemical Equations

Reactants, Products, and Balancing

Chemical equations represent the transformation of reactants into products. Each side of the equation must have the same number of atoms of each element, reflecting the Law of Conservation of Matter.

• Reactants: Substances present before the reaction.

• Products: Substances formed as a result of the reaction.

• Coefficients: Numbers placed before formulas to balance the equation.

Example:

$2H_2 + O_2 \rightarrow 2H_2O$

Here, 2 molecules of hydrogen react with 1 molecule of oxygen to produce 2 molecules of water.

Law of Conservation of Matter

The total mass and number of atoms are conserved in a chemical reaction. Matter cannot be created or destroyed, only changed from one form to another.

• Each element must have the same number of atoms on both sides of the equation.

• Example: $CH_4 + O_2 \rightarrow CO_2 + H_2O$

Types of Chemical Reactions

Synthesis (Combination) Reactions

Synthesis reactions involve two or more simpler substances combining to form a more complex substance.

• General form: $A + B \rightarrow AB$

• Example: $2H_2 + O_2 \rightarrow 2H_2O$

• Example: $2Ca + O_2 \rightarrow 2CaO$

Decomposition Reactions

Decomposition reactions involve a complex substance breaking down into two or more simpler substances.

• General form: $AB \rightarrow A + B$

• Example: $2H_2O \rightarrow 2H_2 + O_2$

Combustion Reactions

Combustion reactions typically involve a hydrocarbon reacting with oxygen to produce carbon dioxide and water.

• General form: Hydrocarbon + $O_2 \rightarrow CO_2 + H_2O$

• Example: $C_3H_8 (g) + O_2 (g) \rightarrow CO_2 (g) + H_2O (g)$

Single Replacement (Displacement) Reactions

In single replacement reactions, one element replaces another in a compound.

• General form: $A + BC \rightarrow AC + B$

• Example: $Zn + CuCl_2 \rightarrow ZnCl_2 + Cu$

• Example: $Br_2 + 2KI \rightarrow 2KBr + I_2$

Double Replacement (Displacement) Reactions

Double replacement reactions involve the exchange of ions between two compounds, often resulting in the formation of a precipitate or water.

• General form: $AB + CD \rightarrow AD + CB$

• Example: $Na_2SO_4 + Mg(OH)_2 \rightarrow MgSO_4 + 2NaOH$

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions react to form an insoluble solid (precipitate).

• Example: $Pb(NO_3)_2 (aq) + 2NaI (aq) \rightarrow PbI_2 (s) + 2NaNO_3 (aq)$

Acid-Base (Neutralization) Reactions

Acid-base reactions involve an acid and a base reacting to produce a salt and water.

• General form: Acid + Base $\rightarrow$ Salt + Water

• Example: $HCl + NaOH \rightarrow NaCl + H_2O$

• Example: $HBr + KOH \rightarrow KBr + H_2O$

Redox (Oxidation-Reduction) Reactions

Redox reactions involve the transfer of electrons between substances, resulting in changes in oxidation states.

• Oxidation: Loss of electrons, gain of oxygen, or loss of hydrogen.

• Reduction: Gain of electrons, loss of oxygen, or gain of hydrogen.

• Example: Cellular respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

Stoichiometry and Mole Concept

Mole-Mole Ratios

Stoichiometry uses balanced chemical equations to determine the relationships between reactants and products in moles.

• Mole-mole ratio: The ratio of coefficients from a balanced equation.

• Example: In $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$, the ratio of $CH_4$ to $CO_2$ is 1:1.

Mass-Mole Conversions

To relate masses of reactants and products, convert grams to moles using molar mass, apply the mole ratio, and convert back to grams if needed.

• Step 1: Convert grams of substance A to moles using molar mass.

• Step 2: Use the mole-mole ratio from the balanced equation to find moles of substance B.

• Step 3: Convert moles of substance B to grams using its molar mass.

Example:

Given $32\,g$ of $CH_4$, how many moles of $CO_2$ will you make?

Given $48\,g$ of $O_2$, how many grams of $CO_2$ will you make?

Given $52\,g$ of $H_2O$, how many moles of $CH_4$ did you start with?

Sample Calculation

For the reaction $2H_2 + O_2 \rightarrow 2H_2O$, how many grams of $H_2O$ are produced from $16.0\,g$ of $O_2$?

• Molar mass of $O_2 = 32\,g/mol$

• Molar mass of $H_2O = 18\,g/mol$

• Step 1: $16.0\,g\,O_2 \times \frac{1\,mol\,O_2}{32\,g\,O_2} = 0.5\,mol\,O_2$

• Step 2: Use mole ratio ($O_2:H_2O = 1:2$): $0.5\,mol\,O_2 \times \frac{2\,mol\,H_2O}{1\,mol\,O_2} = 1.0\,mol\,H_2O$

• Step 3: $1.0\,mol\,H_2O \times 18\,g/mol = 18\,g\,H_2O$

Energy in Chemical Reactions

Exothermic and Endothermic Reactions

Chemical reactions can either release or absorb heat energy.

• Exothermic reactions: Release heat; heat is a product. Example: Combustion.

• Endothermic reactions: Absorb heat; heat is a reactant. Example: Photosynthesis, cooking an egg.

Classification Table: Types of Chemical Reactions

Type	General Form	Example	Key Feature
Synthesis	$A + B \rightarrow AB$	$2H_2 + O_2 \rightarrow 2H_2O$	Combining simpler substances
Decomposition	$AB \rightarrow A + B$	$2H_2O \rightarrow 2H_2 + O_2$	Breaking down complex substances
Combustion	Hydrocarbon + $O_2 \rightarrow CO_2 + H_2O$	$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$	Produces $CO_2$ and $H_2O$
Single Replacement	$A + BC \rightarrow AC + B$	$Zn + CuCl_2 \rightarrow ZnCl_2 + Cu$	One element replaces another
Double Replacement	$AB + CD \rightarrow AD + CB$	$Na_2SO_4 + Mg(OH)_2 \rightarrow MgSO_4 + 2NaOH$	Exchange of ions
Precipitation	Aqueous reactants $\rightarrow$ Solid product	$Pb(NO_3)_2 + 2NaI \rightarrow PbI_2 + 2NaNO_3$	Formation of a precipitate
Acid-Base	Acid + Base $\rightarrow$ Salt + Water	$HCl + NaOH \rightarrow NaCl + H_2O$	Neutralization
Redox	Electron transfer	$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$	Change in oxidation state

Summary

• Chemical reactions are classified by the changes in substances and energy involved.

• Balancing equations ensures conservation of mass and correct stoichiometric relationships.

• Stoichiometry allows calculation of reactant and product quantities using mole ratios and molar masses.

• Understanding reaction types and energy changes is essential for predicting chemical behavior.