To raise the T, we must have raised the K.E. of the system. This means that we must have lowered the potential energy of the system, by altering the chemical bonds. Below is a Potential Energy Diagram for an Exothermic reaction. The P.E. of the reactants is greater than the P.E. of the products. This is an Endothermic reaction.
Water placed under the beaker freezes the container to the board beneath it.
Heat has flowed into the system from the surroundings. We must have lowered the T of the system during the reaction. A lowered K.E. leads to an increase in the P.E. Above is a P.E. diagram for an Endothermic reaction. The P.E. of the products is greater than that of the reactants. Suppose we want to measure the Internal Energy changes associated with these reactions.