How do i answer this

Question

How do you thing the energy exchange digram for the combustion of ethane differ from the digram for methane?

Answer 1

Same shape, different depth. Both reactions are combustions, so both diagrams look alike in the way that matters first: the products sit below the reactants and energy leaves the system. What changes is how far down the product line lands. Per mole of fuel, ethane falls about 1.75 times further than methane.

The two reactions, with the water condensed to liquid (what standard tables assume):

CH4(g) + 2 O2(g) -> CO2(g) + 2 H2O(l), delta H = -890 kJ per mole
C2H6(g) + 3.5 O2(g) -> 2 CO2(g) + 3 H2O(l), delta H = -1560 kJ per mole

Energy
  |
  |   CH4 + 2 O2  ----            C2H6 + 3.5 O2  ----
  |         |                            |
  |         |  890 kJ                    |
  |         v                            |  1560 kJ
  |   CO2 + 2 H2O ----                   |
  |                                      |
  |                                      v
  |                              2 CO2 + 3 H2O  ----
  |
  +--------------------------------------------- reaction progress

Both stacks are drawn from a common reactant line above, which is the usual convention when the whole point is to compare how much comes out. 1560 / 890 = 1.75, so if you draw methane with a 4 cm arrow, ethane needs 7 cm.

Now the part your teacher is really after: "ethane releases more" is only true one way. There are three fair ways to compare two fuels, and they do not agree.

1. Per mole of fuel - ethane wins, easily. 1560 kJ against 890 kJ. Ethane is the bigger molecule with more bonds to burn, so this is no surprise and it is what the diagram above shows.

2. Per gram - methane wins, and this reverses the answer. Molar masses are 16.04 and 30.07 g/mol, so methane gives 890 / 16.04 = 55.5 kJ per gram and ethane gives 1560 / 30.07 = 51.9 kJ per gram. Methane carries more energy per unit mass than any other hydrocarbon, because hydrogen is the energy-dense part and methane has the highest H-to-C ratio of the family. That is why rockets and gas grids care about methane and why "which releases more" is an incomplete question until you say per what.

3. Per mole of O2 consumed - a dead heat. 890 / 2 = 445 kJ for methane, 1560 / 3.5 = 446 kJ for ethane. That near-identity is not a coincidence and it is worth remembering: for ordinary hydrocarbons, combustion energy tracks the oxygen consumed at roughly 440-450 kJ per mole of O2, whatever the fuel. It is a fast sanity check on any combustion number you calculate.

What does not differ. Both diagrams still need an activation hump on the way down - neither gas ignites on its own at room temperature, which is why a spark is required and why a natural gas line is safe until it is not. And because delta H is a state function, the depth of the drop is set only by the difference between the two levels, never by the height of the hump or the route taken.

One more contrast, if you want the good mark. Count the carbon dioxide against the energy delivered. Methane makes 1 mol CO2 per 890 kJ, or 1.12 mol per megajoule. Ethane makes 2 mol per 1560 kJ, or 1.28 mol per megajoule - about 14 percent more CO2 for the same heat. Deeper arrow, dirtier fuel per joule. That is the quantitative version of the claim that natural gas is the cleanest of the fossil fuels.

If your course leaves the water as vapour, subtract 44 kJ for every mole of it: methane becomes -802 kJ and ethane -1428 kJ. Every ratio above shifts a little, none of the conclusions change. Just be sure both fuels are quoted on the same basis before you compare them - mixing a liquid-water value with a vapour one is the most common way these comparisons go wrong.

When you draw it, label four things and you have the full answer: the reactant level, the product level, the vertical gap with its kJ value and a minus sign, and an arrow pointing down for energy released.