Why are combustion's standard enthalpy changes usually negative? Because heat is constantly present, the reaction is exothermic, which means that heat is lost to the surroundings. The amount of energy required to break bonds is always less than the amount of energy generated when bonds form. Therefore, in order for there to be no net change in energy, energy must be added to or removed from the system. Addition of energy can come from many sources including chemical reactions, radiation, electricity, and heat transfer.
Enthalpy changes are often referred to as "heat losses" because they represent a loss of useful energy. As the reaction proceeds, more high-energy molecular bonds are formed and broken each time another molecule joins the chain. This results in the formation of low-energy products. During this process, energy is lost due to heat production by the reacting molecules and by other mechanisms such as light emission or radioactivity.
In conclusion, combustion is an exothermic reaction that results in the loss of useful energy in the form of heat. Enthalpy changes are negative because heat is lost not gained within the system.
In an exothermic reaction, the change in enthalpy is negative because energy is "lost" throughout the process (because there is more energy on the product side than on the reactant side). The DH [difference in enthalpy] is -484 kJ. (you can calculate this using standard enthalpy of formation numbers).
In an endothermic reaction, the change in enthalpy is positive because energy is "gained" throughout the process (because there is less energy on the product side than on the reactant side). The DE [differential enthalpy] is +504 kJ.
Differentiating both equations with respect to temperature gives us:
DH/dt = Q/(R T)
Where dH is differential enthalpy, Q is the heat flow and R is the gas constant. If we divide both sides by T then we get:
D(HRT)/dt = Q/V
Where V is the volume of the sample. Integrating both sides from T=0 to T=Tsurr gives us:
HRT = Q/V + C
C is a constant.
The total energy of the products in an exothermic reaction is less than the total energy of the reactants. As a result, the change in enthalpy is negative, and heat is discharged into the surrounding environment.
For example, water vapor will leave a room if a glass jar containing a chemical reaction that produces gas is placed on top of it. This occurs because the production of gas increases the entropy of the system, which leads to a release of heat.
At first glance, this seems contradictory to our previous statement that all reactions must increase entropy. But this is only because we are viewing the reaction from one side; if we were to view it from the other side, we would see that it is actually consistent with our earlier statement. The reaction on its own cannot increase or decrease entropy; it can only do so as a part of a larger process. In this case, the release of gas increases entropy because there now exist more possible configurations of gases inside the jar than before the reaction took place.
So, changing systems often lead to changes in entropy, and these changes can be positive or negative. But remember that the overall process is always conservative: The net number of particles remains the same, and the net amount of matter is also constant. Only the state of those particles has changed.
Because heat is lost during combustion processes, the overall heat content must fall. If it falls, the end value will be less than the beginning value, resulting in a negative value for DH.
DH can also be negative if there is an increase in entropy. For example, if we burn gasoline to produce energy and CO2, then without further work the system will eventually return to its original state with more entropy than when it started. In this case, DH would be negative.
Finally, DH can be negative if there is absorption of radiation. For example, some chemicals absorb infrared radiation from the surface of the earth and re-radiate it at lower temperatures. This loses energy which must come from elsewhere; otherwise the chemical would heat up and break down faster than it could be replaced by conduction from the ground.
Negative DH values indicate that energy has been absorbed rather than produced. Most chemical reactions produce energy gain (the exception being photosynthesis), so any time you measure DH to be less than zero you know that an energy-absorbing process is taking place.