Determine the amount of heat released when 5 g of ethane (C2H6) are combusted. ΔHcomb. of ethane = -1423 kJ/mol

Question

Determine the amount of heat released when 5 g of ethane (C₂H₆) are combusted.

ΔH_comb. of ethane = -1423 kJ/mol

Answer 1

The question asks how much heat is released when 5 gms of C2H6 is combusted with O2.

Since this is a combustion reaction and given the sign of the delta H comb being negative this is an exothermic reaction,

First you need to balance the combustion reaction involving ethane.

2C2H6 + 7O2 >>>> 6H2O + 4CO2

Next step is to convert the 5 grams of C2H6 into moles

5 gms X 1mole/30 gms >>> 0.1667 moles

The combustion of ethane releases -1423KJ/mol, however if you notice in the balanced equation, 2 moles of ethane is required to combust with 7 moles of O2 to produce 6 moles of H2O and 4 moles CO2 not one mole of ethane. Therefore you multiply the Delta H comb of -1423 by 2 = -2846 KJ.

This now equates with the balanced equation: 2 moles of C2H6 releases -2846KJ, and 7 moles of O2 when combusted with ethane releases -2846 KJ, further, the combustion of 2 moles of C2H6 will produce 6 moles of H20 where -2846 KJ are released and the production of 4 moles of CO2 where -2846 kj are released,

Now using molar ratios:

0.1667 moles C2H6 x -2846 KJ/2 moles C2H6 = -237.21KJ:

Answer 2

Your basic combustion reaction takes a molecule and puts Oxygen or O2 to it to yield CO2 and water or H2O.

The problem gives you the amount of energy related to this reaction with in 1 mol or in other words -1423 kJ is what is released per mole of ethane and having this reaction take place.

So if you are given 5g of ethane, would you be able to figure out how many moles this corresponds to, and therefore how many kJ of heat then being released? Of course we can.

C2H6 + O2 => CO2 + H2O

Balanced equation will look something like:

C2H6 + 3.5O2 => 2CO2 + 3H2O

OR

2C2H6 + 7O2 => 4CO2 + 6H2O

Now we can follow the stoichiometry properly,

It tells you specifically that 1 mole of ethane has that much energy of release corresponding with it.

So if you take the atomic weight of ethane and then put it to the 5 grams given in the problem, you will have the amount of moles connected to this amount of ethane.

atomic weight of ethane:

C = 12.01 x 2 = 24.02

H = 1.01 x 6 = 6.06

total: 30.08 g/mol

now you can take it to moles by getting grams to cancel:

5g X 1mol/30.08g grams cancels grams top to bottom and you have moles in the numerator. So you have:

0.1662 mol of the molecule ethane C2H6. So if 1 mole is to -1423 kJ release of energy

how much energy would be released for 0.1662 moles?

set up a ratio and solve:

1mol = -1423 kJ

0.1662 mol = X

X= 0.1662(-1423)

X = -236.5 kJ

It's negative because it is still giving off heat but just not as much.

You can give it to least sig figs if that makes sense at -200 kJ, but if for reporting based on kJ given in problem -236.5 kJ