Bond forming

Bond forming is the stage of a reaction that releases energy, because energy is given out whenever new bonds form in the products. Malay: pembentukan ikatan · Chinese: 成键.

When atoms come together to make a bond, they settle into a lower, more stable energy state, and the energy they lose is released to the surroundings as heat. Forming bonds is therefore always an exothermic step, the exact opposite of breaking them. In a reaction, after the reactant bonds have broken, the free atoms rearrange and new bonds form in the products, releasing energy at this stage.

Example. In the burning of hydrogen, once the H–H and O=O bonds have been broken, new O–H bonds form as water molecules are made, and this bond forming releases a large amount of energy. Because the energy released in forming the O–H bonds is greater than the energy absorbed in breaking the H–H and O=O bonds, the reaction is exothermic overall, with a negative ΔH.

Confusion to avoid. Do not mix up which step is which: forming bonds releases energy, breaking bonds absorbs it. A reaction is exothermic when bond forming releases more energy than bond breaking absorbs, and endothermic when it releases less. Reversing these two, or comparing them the wrong way, is the classic error in explaining why a reaction is exothermic or endothermic.

Bond forming completes the pair with bond breaking, and the comparison between the two is exactly how the sign of ΔH is explained on the energy level diagram. In a full answer, name both stages: how much energy is absorbed to break the reactant bonds, and how much is released when the product bonds form, then compare the two to fix the sign. Our teachers train students to weigh energy released against energy absorbed, so SPM Chemistry explanations reach the right conclusion without reversing either stage.