Chemistry Combustion Reaction and Heat

Question

I'm having problems with these questions (photo attached below) #1 and #2, if someone can help me with them please

Answer 1

Hey Joseph,

1. 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 ΔH_reaction of the reaction, also known as enthalpy. A negative sign means an exothermic reaction, while a positive means endothermic. ΔH can be expressed as (ΔH_products -ΔH_reactants). A negative ΔH_reaction would mean that the ΔH_reactants is greater than that of the products, meaning that heat was given off in the reaction.
2. 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.

Hope this helps! Please let me know if you need any other help.