Understanding how reaction enthalpies help predict if a chemical reaction can happen is very important for engineers and chemists. When we look at the energy involved in chemical reactions, it helps us decide if a reaction will happen naturally under certain conditions.

First, let's talk about the standard enthalpy of formation. This term, written as ($\Delta H_f^\circ$), tells us the energy change when one mole of a compound is made from its basic elements. If the $\Delta H_f^\circ$ values for the starting materials (reactants) are higher than those for the final products, it means the reaction gives off energy (this is called exothermic) and is likely to happen on its own. On the other hand, if the products have higher energy values, the reaction takes in energy (called endothermic), meaning we might need to add energy for the reaction to occur.

One good example of how these ideas work is in combustion reactions. The standard enthalpy of combustion, shown as ($\Delta H_c^\circ$), measures the energy released when something burns completely in oxygen. This information is really useful when we calculate energy for things like power generation. We usually prefer fuels that release a lot of energy when they burn, like those used in power plants and engines.

Also, we can figure out the total change in reaction enthalpy ($\Delta H_{reaction}$) using Hess's Law. This law lets us add up the enthalpy changes from different steps to find out the overall energy change for the whole reaction. It can be shown like this:

$$\Delta H_{reaction} = \sum \Delta H_f^\circ (products) - \sum \Delta H_f^\circ (reactants)$$

If the $\Delta H_{reaction}$ is negative, it means the reaction is favorable and likely to occur. If it’s positive, that means it’s not very likely to happen.

In summary, reaction enthalpies are a key tool for chemists and engineers. They give us important information to figure out if a chemical reaction will happen. By looking at these energy changes, we can better design chemical processes, making them efficient and safe for various applications.