Considering the reaction shown below, how much thermal energy would be required to run the reaction with 5.0 g of hydrogen and 25 g of iodine?

Question

H2(g) + I2(g) --> 2HI(g)
deltaH(rxn) = +53 kJ

I happen to know the answer is 5.2 kJ, but I'm not sure how to get to it. I'm studying for finals and need help. Thank you!

Olivia Anton · Accepted Answer

#"5.2 kJ"#

The thermochemical equation given to you tells you how much heat is needed in order to produce #2# moles of hydrogen iodide.

In other words, you know that in order to produce #2# moles of hydrogen iodide, you need #1# mole of hydrogen gas, #1# mole of iodine, and #"53 kJ"# of heat.

#DeltaH_"rxn" = + "53 kJ"#

The plus sign tells you that this reaction taken is #"53 kJ"# of heat, i.e. the reaction is endothermic.

#color(blue)(ul(color(black)("1 mole I"_2 color(white)(.)"and 1 mole H"_2 -> "53 kJ of heat consumed")))#

So, convert the samples of hydrogen gas and iodine to moles by using the molar masses of the two reactants.

#5.0 color(red)(cancel(color(black)("g"))) * "1 mole H"_2/(2.016color(red)(cancel(color(black)("g")))) = "2.48 moles H"_2#

#25 color(red)(cancel(color(black)("g"))) * "1 mole I"_2/(253.81color(red)(cancel(color(black)("g")))) = "0.0985 moles I"_2#

The reaction consumes hydrogen gas and iodine in a #1:1# mole ratio, so you can say that iodine will act as a limiting reagent here because you have fewer moles of iodine than of hydrogen gas.

Therefore, the reaction will consume #0.0985# moles of iodine and of hydrogen gas and produce.

This means that the reaction will require

#0.0985 color(red)(cancel(color(black)("moles I"_2))) * "53 kJ"/(1color(red)(cancel(color(black)("mole I"_2)))) = color(darkgreen)(ul(color(black)("5.2 kJ")))#

of heat. The answer is rounded to two sig figs. You can thus say that you have

#DeltaH_ ("rxn for 0.0985 moles I"_2 color(white)(.)"and H"_2) = + "5.2 kJ"#

Luna Chen · Answer

undefined To determine the thermal energy required, you need to calculate the amount of energy released or absorbed in the reaction using the enthalpy change (∆H) of the reaction. Then, use the given amounts of reactants to find the limiting reactant and the theoretical yield of the product. Finally, use the stoichiometry of the balanced chemical equation to relate the amount of reactants consumed to the amount of energy required or released.

Considering the reaction shown below, how much thermal energy would be required to run the reaction with 5.0 g of hydrogen and 25 g of iodine?

H2(g) + I2(g) --> 2HI(g) deltaH(rxn) = +53 kJ I happen to know the answer is 5.2 kJ, but I'm not sure how to get to it. I'm studying for finals and need help. Thank you!

H2(g) + I2(g) --> 2HI(g)
deltaH(rxn) = +53 kJ

I happen to know the answer is 5.2 kJ, but I'm not sure how to get to it. I'm studying for finals and need help. Thank you!