R1.1.3 – Energetic stability and the direction of change
📌 Direction of change
- Chemical reactions change to decrease their enthalpy (ie the chemical potential energy)
- The natural change in enthalpy is generally meant to reach a more stable state
- The change in enthalpy is denoted using the positive (+) and negative (-) signs in relation to Δ𝐻
- In an exothermic reaction , the reactants have greater energy than the products. This is because the system less energy to the surroundings in the form of heat during the reaction. This means that many exothermic reactions occur spontaneously in an attempt to reach a state of stability.
- Conversely, in an endothermic reaction, the products have greater energy than the reactants. This is because the system is gaining energy in the form of heat from the surroundings. Endothermic reactions often require external effort as products may be less stable than reactants.
- A positive change in enthalpy is associated with endothermic reactions and a negative enthalpy change is associated with exothermic reactions
⭐️ some exceptions to this rule do exist – certain endothermic reactions can occur spontaneously [eg. : SOCl(I) + FeCl3.6H,O(s) → FeCl(s) + 6SO,(g) + 12HCl(g) is endothermic but highly spontaneous ]
！the concept of spontaneous reactions is further discussed in R1.4 (HL)
The following diagrams demonstrate the change in enthalpy in endothermic and exothermic reactions :
[image from Chemistry for the IB Diploma Programme]