In both systems, how did the starting thermal energy compare to the ending thermal energy of the system?

According to the (second) law of thermodynamics, the thermal energy of a system changes when work is done on the system. The thermal energy of a material (increases) when the temperature of the material increases. ... At the same temperature, 1 kg of water has (the same amount) of thermal energy as 2 kg of water.

beta decay is the answer

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.