In an exothermic reaction, is the heat of the reation less than, greater than, or equal to zero?
In an endothermic reaction, is the heat of reaction less than, greater than, or equal to zero?
Exothermic reaction - Energy is released - so it is less than zero. [Negative value]
Endothermic reaction - Energy is absorbed - greater than zero [Positive value]
Examples -
Exothermic Reactions
C(s) + O2(g) → CO2(g); ΔH = –393.5 kJ
H2(g) + 1/2 O2(g) → H2O(l); ΔH = –285.8 kJ
Endothermic Reactions
N2(g) + O2(g) → 2NO(g); ΔH = +180.5 kJ > 0
C(s) + 2S(s) → CS2(l); ΔH = +92.0 kJ > 0
In an exothermic reaction, the heat of the reaction is greater than zero. This means that the reaction releases heat to the surroundings.
In an endothermic reaction, the heat of the reaction is less than zero. This means that the reaction absorbs heat from the surroundings.
In an exothermic reaction, the heat of the reaction is less than zero. To understand why, we need to understand the concept of heat in a chemical reaction.
Heat is a form of energy that is either released or absorbed during a chemical reaction. It is a measure of the energy transferred between the system (the reacting substances) and the surroundings.
In an exothermic reaction, energy is released from the system to the surroundings. This energy is primarily in the form of heat. The heat is given off by the reaction, causing an increase in the temperature of the surroundings. Since heat is released, the heat of the reaction is negative.
On the other hand, in an endothermic reaction, the heat of the reaction is greater than zero. In this case, energy is absorbed by the system from the surroundings. The reaction requires an input of external energy in order to proceed. This energy is primarily in the form of heat, and as a result, the heat of the reaction is positive.
So, in summary:
- In an exothermic reaction, the heat of the reaction is less than zero (-ΔH).
- In an endothermic reaction, the heat of the reaction is greater than zero (+ΔH).