As you heat something up does it slowly start to dissipate? Water, for instance, boils at 100 degrees Celsius, but if you are heating it up at 90 degrees will its bonds be weaker?
Remember the energy of the bonds never increase or decrease. They remain the same. When things are heated what actually happens is the molecules begin to move around (in solids they just oscillate while in liquids and gases the movement becomes much more prominent) and their kinetic energy increases and the molecules bump into each other harder and quite often which makes it increasingly difficult for the bonds to hold the molecules together (intermolecular forces). Once a certain threshold is reached the bonds become inefficient at their job and hence small lumps of molecules move apart enough for us to perceive the action as a phase change.