Comprehensive notes, solved MCQs, Short Questions, and Numericals for Class 9 Chemistry Chapter 5 Energetics. Covers Enthalpy, Exothermic/Endothermic Reactions, Activation Energy, Bond Energy, and Respiration.
Energetics: The branch of chemistry which deals with the energy changes during a chemical reaction is called energetics.
Forms of Energy:
Principle: Heat is absorbed when bonds are broken and released when bonds are formed.
System: Any physical or chemical change under study. E.g., Boiling water in a beaker.
Surrounding: Everything else which does not fall in the system. E.g., The beaker, air.
Thermodynamics: The branch of science dealing with how energy changes during a chemical reaction affect the properties of a chemical system.
Definition: The total heat content of a body is called enthalpy. It is the measurement of energy in a thermodynamic system.
Heat Content: The total amount of heat energy present in a molecule under standard conditions ($0^{\circ}C$, 760 mmHg).
Enthalpy Change ($\Delta H$): The heat transferred into or out of a system during a reaction. $\Delta H = H_{products} - H_{reactants}$. Unit: kJ/mol.
Definition: The enthalpy change when reactants in their standard states undergo a reaction to produce products in their standard states. Standard conditions: 25$^{o}$C (298K) and 1 atm pressure.
Example: $2H_2(g) + O_2(g) \to 2H_2O(g)$ has $\Delta H$ depending on states.
Definition: Chemical reactions in which heat is released (given out) to the surroundings. The surroundings become hot.
Enthalpy Change: $\Delta H$ is negative ($-$). $H_{products} < H_{reactants}$.
Examples:
Definition: Chemical reactions in which heat is absorbed from the surroundings. The surroundings become cold.
Enthalpy Change: $\Delta H$ is positive ($+$). $H_{products} > H_{reactants}$.
Examples:
Activation Energy ($E_a$): The minimum amount of energy required by reactant molecules to convert into products (collision to transition state). Represented by $E_a$.
Transition State: High energy state where old bonds are breaking and new bonds are forming. $E_{transition} > E_{reactants}$.
Catalyst: A substance that increases the rate of reaction by lowering the activation energy path, without being consumed. E.g., $Pt$ in $H_2SO_4$ production.
Aerobic Respiration: Respiration in presence of oxygen. Exothermic process. Equation: $C_6H_{12}O_6 + 6O_2 \to 6CO_2 + 6H_2O + Energy$.
Anaerobic Respiration: Respiration in absence of oxygen (e.g., bacteria). Exothermic. Equation: $C_6H_{12}O_6 \to 2CO_2 + 2C_2H_5OH + Energy$.
Glycolysis: Breakdown of glucose into two pyruvate molecules to release energy (2 ATP net).
Lipids: Energy reserves (fats/waxes). Provide more energy than carbohydrates.