[Blah DTMM]

I have a question here regarding molar enthalpy and different campfire fuels that I need help with. I am asked to select the most suitable fuel to be used at a campsite for cooking from this chart:



I need to support my choice with scientific, technological and ecological perspectives. I think ethanol would be the right choice since it is cleaner and non toxic but I don't know how molar mass and molar enthalpy effect whether or not it would be a good choice. Is a higher molar mass/enthalpy better for a campfire or a lower one?

[Dan]

I don't know how molar mass and molar enthalpy effect whether or not it would be a good choice. Is a higher molar mass/enthalpy better for a campfire or a lower one?

Well, from a practical point of view, you want to get the most out of your fuel - so you want a fuel which releases more heat when burned. What you have is molar enthalpy, so this tells you how much heat is released per mole - this may not be the most practical expression of the heat released for the purpose of choosing a camping fuel.

Here's a hint:

You are going to hike for 2 weeks in the wilderness. You need camping fuel to cook, but will not be able to refuel for the whole 2 weeks. You can choose to take 1 kg of either of these:

Fuel A: :delta: _cH_m = -1000; Molar mass = 100
Fuel B: :delta: _cH_m = -2500; Molar mass = 500

Which would you go for?

[Blah DTMM]

Is it fuel A because it would last longer? Sorry if that's a stupid answer, all of this enthalpy stuff is confusing me so far.

[Dan]

I would agree with you, not stupid at all.

[Blah DTMM]

Okay so regarding the original question. Assuming the other four options are out because of their toxicity, ethanol is better than wood for cooking at a campsite because it would last longer. Is that because it releases less heat per mole compared to wood or because it has a lower molar mass? Or a combination of the two? I'm just having trouble pinpointing the exact scientific reason why this is.

[Dan]

What I'm getting at is I think you'd be better off considering the fuel efficiency in terms of kJ/g rather than kJ/mol, because I think this is more relevant to the scenario of storage and handling in a camping scenario. Using the molar mass you can calculate these figures - so it is a combination.

If we take ethanol and wood as an example:

ethanol is better than wood for cooking at a campsite because it would last longer. Is that because it releases less heat per mole compared to wood or because it has a lower molar mass?

Wood releases more heat per mole, but one mole of wood weighs about 3.5 times the mass of one mole of ethanol. If you do the calculation, you can see that even though one mole of ethanol provides less heat than one mole of wood, one gram of ethanol will release about twice as much heat as one gram of wood.

There is no right answer to the question - you could argue in favour of a few different options - remember that wood, kerosene and propane are all commonly used as camping fuels (as far as I know ethanol is not). It's a question of balancing various advantages and disadvantages. To say anything with toxicity can be disregarded is a bit heavy handed I think - but it's a subjective question so if you think that is the top priority then that's fine.

My approach to this question would be:



1. Calculate energy return in kJ/g for all fuels and rank them- for me this is top priority
2. Now consider other factors - if you have two fuels with similar energy returns, but with clear differences in stability/cleanliness etc. you can clearly argue against the "dirty" fuel and eliminate it. Alternatively if you have two clean, renewable fuels but one gives a better energy return, then you can eliminate the less efficient fuel.
3. I would argue the case by eliminating options one by one (with justification) until you are left with what you think is the best option. When you are down to the last 2 or 3 options it could be sensibly argued in different ways depending on what you think are the most important factors.

[Blah DTMM]

Okay that helps a lot. Thank you very much!

Edit: Just to be 100% certain on this, to convert these numbers to kJ/g I just need to divide molar enthalpy(kJ/mol) by molar mass(g/mol)? And if propane gives off -50.3 kJ/g and wood gives off -15.5 kJ/g, that means that propane gives off more energy per gram right? I just want to make sure I'm not being confused by the negative symbol. That is mostly signifying that the energy is being given off in an exothermic reaction, yes?

[Dan]

to convert these numbers to kJ/g I just need to divide molar enthalpy(kJ/mol) by molar mass(g/mol)?

Yup

And if propane gives off -50.3 kJ/g and wood gives off -15.5 kJ/g, that means that propane gives off more energy per gram right? I just want to make sure I'm not being confused by the negative symbol.

Yeah, more negative means more exothermic (more heat given out).

[Borek]

Specific energy is the key word here.

[Blah DTMM]

I have another kind of related question here regarding molar enthalpy and Hess's law. I'm given the data for an experiment with two parts. Part A involves adding Mg to 1.0 mol/L hydrochloric acid and Part B is a separate reaction where we add MgO to 1.0 mol/L hydrochloric acid. Here are the resulsts.

Part A:
Mass of Mg(s) = 0.31 g
Initial temp of calorimeter contents = 24.1 C
Final temp of calorimeter contents = 36.8 C

Part B:
Mass of MgO(s) = 1.22 g
Initial temp of calorimeter contents = 24.0 C
Final temp of calorimeter contents = 31.9 C

The question says: Calculate the molar enthalpies of reaction for the reactions occurring in parts A and B. Does this question use Hess's law? Do I just add together the molar enthalpies of formation for the products and subtract them from the sum of the molar enthalpies of the reactants for each part? Or is it a different equation that uses the masses and temperature changes, like Q=mc :delta: t?