If fire is exothermic, can we put it out by adding heat?

Question

We are told that when heat is a product, at equilibrium, adding heat would favor reverse reaction, and if not, at least slow down the forward reaction.

Since combustion is an exergonic and exothermic reaction, doesn't it mean we should add heat to put out the fire instead of cooling it down?

What's the problem here? That fire never burns at equilibrium so we can never add heat on the product side? I understand we add heat to reach the burning point if we wanted combustion, but the reaction still produces even more heat, so shouldn't we add heat to put it out? (or, does "put it out" mean burning faster and consuming all flammable material rather than stop burning?)

Answer 1

Exothermic simply means the chemical reaction releases heat.  Rapid oxidation, often called fire, is exothermic and releases a heat of combustion.  For example C + O2 + q --> CO2 + Q; where Q is the heat of oxidizing (combusting) carbon.  q is heat added to cause the two elements to oxidize and q < Q.  That is, it takes less heat to burn the carbon than is produced by the burning; which is why we call it exothermic.

There is another reaction...2C + O2 + q ---> 2CO + Q, which is called incomplete combustion.  This is the one that sometimes kills because people use gas heaters that are not efficient and produce carbon monoxide that is extremely toxic.  CO is ordorless and tasteless; so it creeps up on people, especially when they are asleep.

In either case, CO2 or 2CO, adding more heat has no effect because q is all the elements need to oxidize.  So the additional heat appears on both sides of the reaction; for example, C + O2 + q + k ---> CO2 + Q + k since the k is not used for the oxidation.  This holds true for any material that is oxidized.  You can only put in so much heat to cause burning and the rest is superfluous.

The point is this...adding heat to a fire will not change a thing with the possible exception that you'll speed up burning that material that has not yet burned.  So you'll really create more fire by that additional heat if there is more fuel yet to burn.

Answer 2

Heat is liberated during an exothermic reaction, but is not a product in a classical sense (so it doesn't have the same impact on kinetics that a true chemical product would have).  Heat is a type of energy that is less useful than most other types (because not all of it can be turned into work).

When we heat a chemical mixture, the ignition temperature is the point at which the energy threshold is crossed and the reaction can begin.  In a fire, the heat liberated is enough to keep the reaction sustained until one of the reactants is consumed.  If we remove heat from the reactants faster than heat is generated in the reaction, the temperature of the reactants will drop until they are below the ignition temperature, at which point the fire is extinguished.

Answer 3

Although fire is exothermic, combustion only takes place when there is sufficient heat to allow the oxidation process to happen.  Even liquid petrol (gas) will not burn until it is hot enough to release vapour (although this happens easily at room temperature).  Take away the heat by cooling it with water and the fire goes out.  So adding more heat will not stop it.