10th Grade Chemistry: Energy Changes in Chemical Reactions

Energy Changes in Chemical Reactions

• Chemical reactions involve the breaking and forming of bonds, which requires energy
• Bond energy or bond dissociation energy is the energy required to break a chemical bond, measured in kJ·mol⁻¹
• Enthalpy (H) is a measure of the total energy of a chemical system at a given pressure
• The change in enthalpy (ΔH) indicates whether the reaction absorbs or releases energy

Exothermic and Endothermic Reactions

• Exothermic reactions release energy, as the energy released when new bonds form in the products is greater than the energy required to break bonds in the reactants
• Endothermic reactions absorb energy, as the energy required to break bonds in the reactants is greater than the energy released when new bonds form in the products
• The heat of reaction (ΔH) is denoted by the symbol ΔH and represents the change in enthalpy
• ΔH is negative for exothermic reactions and positive for endothermic reactions

Activation Energy and the Activated Complex

• Activation energy is the minimum energy required to start a chemical reaction
• The activated complex or transition state is a transient structure where bonds in the reactants are breaking and new bonds in the products are forming
• The energy diagram for an exothermic reaction shows an initial rise in energy to the activated complex, followed by a fall to the products
• The energy diagram for an endothermic reaction shows an initial rise in energy to the activated complex, followed by a further rise to the products

Acids and Bases

• Acids and bases are classified based on their characteristics, such as sour taste and soapy feel
• Arrhenius definition: acids increase the concentration of H₃O⁺ ions in solution, and bases increase the concentration of OH⁻ ions in solution
• Bronsted-Lowry definition: acids are proton donors, and bases are proton acceptors
• Amphoteric substances can act as both acids and bases, depending on the reaction

Acid-Base Reactions

• Acid-base reactions involve the transfer of protons (H⁺ ions)
• Neutralization reactions occur when an acid and a base react to form a salt and water
• Conjugate acid-base pairs consist of two species that transform into each other by gain or loss of a proton

Redox Reactions

• Redox reactions involve the transfer of electrons between two substances
• Oxidation is the loss of electrons by a molecule, atom, or ion, resulting in an increase in oxidation state
• Reduction is the gain of electrons by a molecule, atom, or ion, resulting in a decrease in oxidation state
• Oxidation numbers are assigned to track electron transfer in redox reactions

Balancing Redox Reactions

• Balancing redox reactions involves ensuring that the number of electrons lost in oxidation equals the number of electrons gained in reduction
• Half-reactions are used to balance the reaction, with oxidation and reduction occurring separately
• The final balanced equation is obtained by adding the half-reactions together, canceling out electrons and any other species that appear on both sides of the equation

Molecular Shape

• Molecular shape determines how molecules interact and react with other molecules
• The valence shell electron pair repulsion (VSEPR) theory is used to predict the shape of molecules
• The geometry of a molecule is determined by repulsion among electron pairs (both bonding and non-bonding) around a central atom

Determining Molecular Shape

• Draw the Lewis diagram to show all valence electrons around the central atom
• Count the number of electron pairs (bonding and lone pairs) around the central atom
• Use the number of electron pairs to determine the basic geometry of the molecule using VSEPR theory### Common Molecular Shapes
• The shape of a molecule can be predicted based on the number of bonding and lone pairs around the central atom.
• Molecular shapes can be visualized using Lewis diagrams, where green balls represent lone pairs, white balls represent terminal atoms, and red balls represent the central atom.
• Steps to predict molecular shape:
  • Draw the Lewis diagram
  • Count electron pairs
  • Determine basic geometry using VSEPR theory
  • Write the final answer

Examples of Molecular Shapes

• BeCl₂: linear shape, 2 bonding pairs, 0 lone pairs
• BF₃: trigonal planar shape, 3 bonding pairs, 0 lone pairs
• NH₃: trigonal pyramidal shape, 3 bonding pairs, 1 lone pair

Electronegativity

• Electronegativity is the ability of an atom to attract electrons towards itself.
• It is a dimensionless quantity that influences the nature of bonds between atoms.
• Importance of electronegativity:
  • Predicts how atoms will interact in a molecule
  • Influences polarity of molecules, affecting properties like solubility, melting points, and boiling points
• Each element (except noble gases) has an electronegativity value between 0 and 4.
• Higher electronegativity values indicate a stronger ability to attract electrons.

Electronegativity and Bonding

• The difference in electronegativity values between two atoms can indicate the type of bond:
  • Non-polar covalent bond: electronegativity difference is 0
  • Weak polar covalent bond: electronegativity difference between 0.1 and 1
  • Strong polar covalent bond: electronegativity difference between 1.1 and 2
  • Ionic bond: electronegativity difference greater than 2.1

Polarity of Molecules

• Polar molecules have one end with a slightly positive charge and one end with a slightly negative charge due to uneven electron distribution.
• The presence of polar covalent bonds and the molecule's shape contribute to its overall polarity.
• Determining molecular polarity:
  • Molecular shape: use VSEPR theory to find the molecular geometry
  • Symmetry: assess if the molecule is symmetrical
  • Electronegativity differences: determine the differences for each bond in the molecule
  • Overall polarity: assess the distribution of charges to determine if the molecule is polar or non-polar

Energy and Bonding

• Bond length: the distance between the nuclei of two adjacent atoms when they form a bond.
• Bond energy: the amount of energy required to break a bond between two atoms.
• Bond strength: refers to how strongly one atom is held to another in a chemical bond.
• Factors influencing bond strength:
  • Bond length: shorter bond lengths typically correspond to stronger bonds
  • Atom size: smaller atoms form stronger bonds
  • Number of bonds: multiple bonds are stronger than single bonds

Energy Changes in Bond Formation

• When atoms approach each other, three main forces act:
  • Repulsive force between electrons
  • Attractive force between nucleus and electrons
  • Repulsive force between nuclei
• The interaction of these forces results in changes in energy:
  • Decreasing energy: as atoms move closer, attractive forces initially dominate
  • Minimum energy point (bond formation): where the system reaches minimum energy
  • Increasing energy: if atoms move closer than the bond length, repulsive forces dominate

Intermolecular Forces

• Intermolecular forces are forces that act between molecules.
• Types of intermolecular forces:
  • Ion-dipole forces: between an ion and a polar molecule
  • Ion-induced dipole forces: between ions and non-polar molecules
  • Dipole-dipole forces: between polar molecules
  • Induced dipole forces (London dispersion forces): in non-polar molecules
  • Dipole-induced dipole forces: between a polar molecule and a non-polar molecule
• Hydrogen bonds: a special type of dipole-dipole force that occurs in molecules with hydrogen bonded to highly electronegative atoms.

Properties Affected by Intermolecular Forces

• Phase of matter: strong intermolecular forces result in solids, while weak intermolecular forces result in gases
• Melting and boiling points: substances with strong intermolecular forces have high melting and boiling points
• Viscosity: substances with strong intermolecular forces are more viscous
• Density: the mass per unit volume
• Thermal expansion: as substances are heated, their molecules move more vigorously and expand
• Thermal conductivity: the ability of a substance to conduct heat

The Chemistry of Water

• Water's unique microscopic structure, molecular shape, polar nature, and intermolecular forces (hydrogen bonds) give it unique properties.
• Unique properties of water:
  • High specific heat: water absorbs a lot of energy before its temperature changes significantly
  • Absorption of infra-red radiation: water can absorb and store heat energy, acting as a heat reservoir and helping to moderate the Earth's climate.