Enthalpy (H) is the total heat content of a system at constant pressure. Enthalpy change (ΔH) is the difference in enthalpy between the products and reactants during a chemical reaction. Formula: ΔH = H(products) - H(reactants).
Breaking a bond is endothermic because energy is required to overcome the attractive forces between atoms. This energy input separates bonded atoms into individual entities.
The transition state involves partially broken H-H and Cl-Cl bonds and partially formed H-Cl bonds. Depiction: H---H⋯Cl---Cl → [H⋯H⋯Cl⋯Cl] → H-Cl + H-Cl. Dots (⋯) represent unstable intermediate bonds.
Faster reactions have a lower activation energy (Ea), resulting in a steeper energy drop. Slower reactions have a higher Ea. Both profiles show products at lower energy than reactants (exothermic).
Glycogen stores glucose in the liver and muscles. It is broken down into glucose when energy is needed.
Exothermic
Exothermic
Endothermic
Endothermic
Exothermic
Endothermic
Exothermic
Exothermic
The reaction requires high activation energy. Lightning provides enough energy to overcome this barrier, forming NO. The reaction stops when lightning ceases.
The reaction has high activation energy. A burning match provides initial energy, after which the exothermic reaction sustains itself until reactants are consumed.
ΔH = [958.38 + 498] - [2 × 626] = 1456.38 - 1252 = +204.38 kJ/mol (endothermic).
Heat is energy transferred due to temperature differences. Enthalpy (H) is total heat content at constant pressure. ΔH measures heat exchange during reactions.
Bond formation releases energy as atoms stabilize into lower-energy bonds, making the process exothermic.
Lipids store energy, form cell membranes, insulate organs, and aid hormone production (e.g., cholesterol).
1. Activation energy: Minimum energy for reactants to form products. 2. Transition state: High-energy intermediate during bond rearrangement. 3. Aerobic respiration: Glucose breakdown with O₂ to produce ATP, CO₂, and H₂O.
Cooking provides activation energy to break down complex molecules (e.g., denaturing proteins), making digestion easier.
Metal salts (e.g., strontium/red, copper/blue) emit colored light when heated. Oxidizers (KNO₃) and fuels (charcoal) drive explosive reactions.
Yes. Examples: Light (glow sticks), electricity (batteries), or sound.
Enthalpy depends on system state, not absolute values. Only ΔH (change) is measurable via heat exchange.
Endothermic. Heat energy is absorbed to break hydrogen bonds in liquid water.
Sodium’s high reactivity releases excess heat, igniting H₂ gas. Reaction: 2Na + 2H₂O → 2NaOH + H₂↑ + heat.
Endothermic. Heat is absorbed to break ice’s crystalline structure.
Yes. The reverse reaction is endothermic (e.g., 2H₂O → 2H₂ + O₂ requires energy input).
ΔH = (436 + 151) - (2 × 299) = 587 - 598 = -11 kJ/mol (exothermic).
Lightning provides energy to overcome the high activation energy of N₂’s triple bond, enabling NO formation.
