Consider the following intermediate chemical equations. 3 equations. 1: upper C solid plus upper o subscript 2 gas right arrow upper C upper O subscript 2 gas Delta H 1 equals negative 393.5 kilojoules. 2: 2 upper C upper O gas plus upper O subscript 2 gas right arrow 2 upper C upper O subscript 2 gas Delta H 2 equals negative 566.0 kilojoules. 3: 2 upper H subscript 2 upper o gas right arrow 2 upper H subscript 2 gas plus upper O subscript 2 gas delta H 3 equals 483.6 kilojoules. The overall chemical equation is Upper C (s) plus upper H subscript 2 upper O (g) right arrow upper C upper o (g) plus upper H subscript 2 (g).. To calculate the final enthalpy of the overall chemical equation, which step must occur? Reverse the first equation, and change the sign of the enthalpy. Then, add. Reverse the second equation, and change the sign of the enthalpy. Then, add. Multiply the first equation by three, and triple the enthalpy. Then, add. Divide the third equation by two, and double the enthalpy. Then, add.