Answer
See the explanation below
Work Step by Step
When we calculate the enthalpy change of a reaction aA +bB --> cC +dD, $ΔH_{R}$, we can try look up the values of enthalpy of formation ($ΔH_{f}$) for all the substances A, B, C & D online or from our textbook and blog into the formula (c$ΔH_{f(C)}$+d$ΔH_{f(D)}$-a$ΔH_{f(A)}$-b$ΔH_{f(B)}$). As we know, enthalpy (H) is temperature and pressure dependent and the values of $ΔH_{f}$ for various substances we find on the resources are specified at 25°C and 1 bar. So this state of temperature and pressure is called the standard state for the formation of the substances. In this way, we know we calculate Δ$H_{R}^{^{\circ}}$ from the given Δ$H_{f}^{^{\circ}}$ of different substances at 25°C and 1 bar, but not other temperatures or pressures. If we need calculate $ΔH_{R}$ at other temperatures or pressures, we need convert Δ$H_{R}^{^{\circ}}$ at 25°C and 1 bar to $ΔH_{R}$ at other temperatures or pressures as we will learn in the chapters later.
In addition, the value of enthalpy of formation ($ΔH_{f}$) for a substance is relative to that of a defined substance’s state. Like $ΔH_{f}$ for CO2(g) is -393.5KJ/mol at standard state, it means $ΔH_{f}$ for CO2(g) is 393.5KJ/mol lower than that of another substance’s state with zero $ΔH_{f}$. So, the reference state is such a substance’s state with the value of $ΔH_{f}$ to be 0, and scientists adopt the most stable state of an element to be a reference state. With the reference state, a certain substance has its unique value of $ΔH_{f}$. Otherwise, if different people set $ΔH_{f}$ of a substance in relative to different states, they can give different values of $ΔH_{f}$ for the same substance as well as different values of enthalpy change of a reaction ($ΔH_{R}$), and this doesn’t make sense.
