To determine which statement describes an exothermic reaction, we need to understand the basic concept of enthalpy (denoted as [tex]\( \Delta H \)[/tex]) and how it changes during a chemical reaction.
An exothermic reaction is one in which energy is released to the surroundings, usually in the form of heat. For such a reaction, the energy of the products is lower than the energy of the reactants. This indicates a loss of energy from the system to the environment.
### Analysis of Options:
1. Option A: [tex]\( \Delta H_{f, \text{reactants}} = \Delta H_{f, \text{products}} \)[/tex]
- This statement implies that the enthalpy of the reactants is equal to the enthalpy of the products. In a real chemical reaction, this would suggest no net change in energy, which does not characterize an exothermic reaction where energy is released.
2. Option B: [tex]\( \Delta H_{f, \text{reactants}} > \Delta H_{f, \text{products}} \)[/tex]
- This statement indicates that the enthalpy of the reactants is greater than the enthalpy of the products. During an exothermic reaction, because energy is released, the system's final energy (enthalpy) is lower than its initial energy. Therefore, this matches the condition for an exothermic reaction.
3. Option C: [tex]\( \Delta H_{f, \text{reactants}} = 0 \)[/tex]
- This statement suggests that the enthalpy of the reactants is zero, which is not generally descriptive of the nature of an exothermic reaction itself. If this were true, it wouldn't give us any insight into the energy comparison between reactants and products.
4. Option D: [tex]\( \Delta H_{f, \text{reactants}} < \Delta H_{f, \text{products}} \)[/tex]
- This statement indicates that the enthalpy of the reactants is less than the enthalpy of the products. This situation characterizes an endothermic reaction, where energy is absorbed from the surroundings, resulting in higher energy for the products.
### Conclusion
The correct statement that describes an exothermic reaction is:
Option B: [tex]\( \Delta H_{f, \text{reactants}} > \Delta H_{f, \text{products}} \)[/tex].
Thus, the answer is [tex]\( B \)[/tex].
