Are all combustions endothermic?

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Saira Purewal 3E
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Joined: Fri Sep 26, 2014 2:02 pm

Are all combustions endothermic?

Postby Saira Purewal 3E » Sat Jan 10, 2015 8:36 pm

I thought all combustion reactions would be endothermic because you're always combusting something in the presence of heat. The example we did on page 15 of the course reader on this past Friday's lecture was the combustion of methane. The standard enthalpy came to -891 kJ which means its strongly exothermic. How does this make sense?

Neil DSilva 1L
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Joined: Fri Sep 26, 2014 2:02 pm

Re: Are all combustions endothermic?

Postby Neil DSilva 1L » Sat Jan 10, 2015 9:10 pm

Recall Dr. Lavelle's example of an endothermic reaction: melting ice. Ice melts because it takes the energy from its surroundings; that's why water gets cooler when it has ice melting in it. The same kind of thing doesn't happen when you combust something; the surroundings aren't getting colder.

Combustion reactions don't occur in the presence of heat. Combustion reactions are processes that RELEASE heat. The standard enthalpy of the combustion reaction is negative because it gives us the change in the energy of the system. The system (molecules involved in the reaction) is releasing energy into the surroundings, so it is losing energy.

I think you might be remembering a bit of kinetics. A combustion reaction requires a spark of heat to overcome the activation energy and begin the forward reaction. Then once the reaction starts, it is continuously releasing energy in the form of heat. So yes, heat is required to START a combustion reaction, but the reaction itself is RELEASING (not consuming) heat (again, this is kinetics and we're only dealing with thermodynamics right now).

Vivien Ho 3L
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Joined: Fri Sep 26, 2014 2:02 pm

Re: Are all combustions endothermic?

Postby Vivien Ho 3L » Sun Jan 11, 2015 3:33 pm

You may be confused between the heat of the system and the heat of the surroundings. "Exothermic" and "endothermic" are used to describe the heat of a system.

When combustion occurs, the heat of the surroundings INCREASES. This is due to the heat RELEASED from the system. The system has lost heat, which is why it is EXOTHERMIC. For ENDOTHERMIC reactions, it's the other way round. When we put ice into a glass of water, the heat of the surroundings DECREASES (the water becomes cold). The system has ABSORBED heat (which is used to break the bonds of ice).

Therefore, all combustions are EXOTHERMIC.

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