Final answer:
The standard enthalpy change for the reaction 4a 2b → 2ab a2 is -138.4 kJ.
Step-by-step explanation:
The standard enthalpy change for the reaction 4a 2b → 2ab a2 can be calculated using the given information. The standard enthalpy change, ΔH°, is equal to -138.4 kJ. To calculate this value, you need to multiply the ΔH° of each product by its stoichiometric coefficient, and sum those values. Then, multiply the ΔH° of each reactant by its stoichiometric coefficient, and sum those values. Finally, subtract the result found in the reactants from the result found in the products.