The concentration of the acid decreases
When liquid state is being converted to solid state, atoms come closer to each other and their speed of movement also decreases. Instead of moving about in the substance, they are only able to vibrate about their positions in solid. Hence, this change leads to loss of energy. Similarly, atoms loose energy when state changes from gas to liquid.
The larger it is, the farther away the valence electrons is from the nucleus. This means that the pull is weaker, making it easier to pull the valence electrons away.
Could you attach the diagram please