Endothermic and Exothermic Reactions

Key Law: Conservation of Energy

Energy cannot be created or destroyed, only changed in form. During bond breaking and reforming, heat energy is released or absorbed.

Exothermic Reactions

Reactions where reactants have more energy than products, releasing energy (heat, light, sound) and increasing the temperature of surroundings.
General formula: Reactants → Products + Energy. ΔH is negative.
Example: 2Mg(s) + O2(g) \rightarrow 2MgO(s) + light energy, Mg(s) + 2HCl(aq) \rightarrow MgCl2(aq) + H2(g) + heat energy.

Endothermic Reactions

Reactions where products have more energy than reactants, requiring continuous energy input and decreasing the temperature of surroundings.
General formula: Reactants + Energy → Products. ΔH is positive.
Example: CuCO3(s) + heat energy → CuO(s) + CO2(g).

Enthalpy (ΔH)

Total energy possessed by a chemical substance. ΔH signifies the change in enthalpy, measured in kJ.
ΔH = H(products) - H(reactants)

  • Positive ΔH: Endothermic reaction.

  • Negative ΔH: Exothermic reaction.

Thermochemical Equations

Balanced chemical equations including states with the ΔH value next to the equation (negative for exothermic, positive for endothermic).

Calculating ΔH

ΔH = (energy required to break bonds) - (energy released when bonds are formed)

Heat, Energy, and Temperature

Heat is a form of energy.
Temperature measures the average kinetic energy.

Forms of Energy

  • Translational Kinetic Energy: KE due to movement through space. KE = \frac{1}{2} mv^2

  • Rotational Kinetic Energy: KE in rotational motion.

  • Vibrational Kinetic Energy: KE from oscillation of atoms.

Chemical Bond Energy (Bond Enthalpy)

Energy to break 1 mole of bonds (kJ/mol).

Calculating Enthalpy of Reaction Using Bond Enthalpies

ΔH = Σ(bond enthalpies of broken bonds) - Σ(bond enthalpies of formed bonds)

Temperature Conversion

  • Temperature (^oC) = Temperature (K) – 273

  • Temperature (K) = 273 + Temperature (^oC)

Heat Energy (q)

q = mcΔT

  • m = mass (g)

  • C = specific heat capacity (J/g⋅K)

  • ΔT = change in temperature (K)

Specific Heat Capacity

Heat energy required to raise the temperature of 1g of a substance by 1K.

Calorimetry

Experimental method to measure heat energy changes in a chemical reaction.