As a general rule of thumb combustion reactions are exothermic. You can determine whether something is endothermic vs exothermic by looking at the sign of the ΔHreaction of the reaction, also known as enthalpy. A negative sign means an exothermic reaction, while a positive means endothermic. ΔH can be expressed as (ΔHproducts -ΔHreactants). A negative ΔHreaction would mean that the ΔHreactants is greater than that of the products, meaning that heat was given off in the reaction.
For this part, you must calculate the number of moles of each reactant. To do so, divide the grams of each reactant by its molar mass. For example, the molar mass of O2(oxygen gas) is 32 grams/mole. 50.8 g / 32 grams/mole is 1.59 moles. The molar mass of acetone is 58.08 grams/mole, so 7.93 grams/ 58.08 grams/mole is 0.137 moles of acetone. The limiting reactant in this case is acetone. There is only 0.137 moles of acetone that can react with 4 (0.137 moles of oxygen gas) which is 0.548 grams of acetone. However, we do not have to worry about the moles of oxygen gas in this problem. We can now simply calculate the amount of energy released by the reaction. The stochiometric equation is for 1 mole of acetone, not 0.137 moles. Therefore, we must multiply 0.137 moles by -1790KJ to get the amount of energy released by 0.137 moles of acetone instead of 1 mole of acetone. -245.23 KJ is the amount of energy released by the reaction in Question 2.
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