Chemical Reactions: Types, Balancing, and Energy Changes – GOB Chemistry Study Notes

Chemical Reactions and Energy Changes

Introduction to Chemical Reactions

Chemical reactions occur when bonds between atoms in reactant molecules are broken and new bonds are formed to create products. For a reaction to take place, reactant molecules must collide with sufficient energy to overcome the activation energy barrier.

• Collision Theory: Reactant molecules must collide with enough energy to break existing bonds and form new ones.

• Activation Energy: The minimum energy required for a reaction to occur. Represented as a peak in energy diagrams.

• Energy Diagram: Shows the energy changes during a reaction, including activation energy and the difference between reactants and products.

Example: The reaction between nitrogen (N2) and oxygen (O2) to form nitrogen monoxide (NO).

Exothermic and Endothermic Reactions

Chemical reactions can be classified based on their energy changes as exothermic or endothermic.

• Exothermic Reaction: Releases heat; the energy of the products is lower than that of the reactants. Heat is a product.

• Endothermic Reaction: Absorbs heat; the energy of the products is higher than that of the reactants. Heat is a reactant.

Examples:

• Exothermic: $\mathrm{C(s) + O_2(g) \rightarrow CO_2(g) + \text{heat}}$

• Endothermic: $\mathrm{N_2(g) + O_2(g) + \text{heat} \rightarrow 2NO(g)}$

Energy Diagram: Exothermic reactions show a drop in energy from reactants to products; endothermic reactions show an increase.

Balancing Chemical Equations

Law of Mass Conservation

The law of mass conservation states that the total mass of reactants equals the total mass of products in a chemical reaction. The number of atoms of each element must be the same on both sides of the equation.

• Original Definition: Mass is neither created nor destroyed in a chemical reaction.

• Modern Definition: The number of atoms of each element is conserved.

Law of Constant Composition

A chemical compound always contains the same proportion of elements by mass. Subscripts in chemical formulas must not be changed when balancing equations.

• Incorrect: Changing subscripts to balance equations violates this law.

• Correct: Use coefficients to balance equations.

Steps to Balance Chemical Equations

Balancing chemical equations ensures the law of mass conservation is obeyed. The following steps are recommended:

1. Do not change subscripts of chemical formulas.

2. Change only the coefficients in front of formulas.

3. Balance elements that appear in only one reactant and one product first.

4. Balance polyatomic ions as single units if they appear unchanged on both sides.

5. Multiply all coefficients to get the simplest whole number ratio.

Symbols Used in Chemical Equations

Symbol	Meaning
(s)	Solid
(l)	Liquid
(g)	Gas
(aq)	Aqueous (dissolved in water)
→	Yields (produces)
+	Separates reactants or products

Types of Chemical Reactions

Combination (Synthesis) Reactions

In a combination reaction, two or more reactants combine to form a single product.

• General Pattern: $\mathrm{A + B \rightarrow AB}$

• Example: $\mathrm{2Mg(s) + O_2(g) \rightarrow 2MgO(s)}$

Decomposition Reactions

In a decomposition reaction, a single compound breaks down into two or more simpler substances.

• General Pattern: $\mathrm{AB \rightarrow A + B}$

• Example: $\mathrm{2KClO_3(s) \rightarrow 2KCl(s) + 3O_2(g)}$

Single Replacement Reactions

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

• General Pattern: $\mathrm{A + BC \rightarrow AC + B}$

• Example: $\mathrm{Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)}$

Double Replacement Reactions

In a double replacement reaction, two elements in different compounds exchange places.

• General Pattern: $\mathrm{AB + CD \rightarrow AD + CB}$

• Example: $\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$

Combustion Reactions

Combustion reactions involve a substance (often a hydrocarbon) reacting with oxygen to produce heat and light, typically forming carbon dioxide and water.

• General Pattern: $\mathrm{C_xH_y + O_2 \rightarrow CO_2 + H_2O}$

• Example: $\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)}$

Oxidation-Reduction (Redox) Reactions

Definition and Electron Transfer

Redox reactions involve the transfer of electrons from one substance to another. Oxidation is the loss of electrons, while reduction is the gain of electrons.

• Oxidation: Element loses electrons; oxidation state increases.

• Reduction: Element gains electrons; oxidation state decreases.

Example: $\mathrm{Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)}$

Oxidation States

Oxidation state (or oxidation number) is a value assigned to an atom to indicate its degree of oxidation or reduction.

• Element in its natural state: oxidation state = 0

• Oxygen in compounds: usually -2

• Hydrogen in compounds: usually +1

Example: In $\mathrm{Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)}$, Zn is oxidized (loses electrons), H is reduced (gains electrons).

Summary Table: Reaction Types

Type	General Pattern	Example
Combination	$A + B \rightarrow AB$	$2Mg + O_2 \rightarrow 2MgO$
Decomposition	$AB \rightarrow A + B$	$2KClO_3 \rightarrow 2KCl + 3O_2$
Single Replacement	$A + BC \rightarrow AC + B$	$Zn + 2HCl \rightarrow ZnCl_2 + H_2$
Double Replacement	$AB + CD \rightarrow AD + CB$	$AgNO_3 + NaCl \rightarrow AgCl + NaNO_3$
Combustion	$C_xH_y + O_2 \rightarrow CO_2 + H_2O$	$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Additional info:

• Energy diagrams are essential for visualizing activation energy and heat of reaction.

• Balancing equations is a foundational skill in chemistry, ensuring the conservation of mass and correct stoichiometry.

• Redox reactions are central to many biological and industrial processes.