Select the correct statement(s) about standard enthalpy changes at $298 K$ .

AThe standard enthalpy of combustion of graphite equals $Δ_{f} H^{\circ} (CO X_{2} (g))$ .
BThe standard enthalpy of combustion of $H X_{2} (g)$ to liquid water equals $Δ_{f} H^{\circ} (H X_{2} O (l))$ .
CThe standard enthalpy of combustion of graphite equals $Δ_{f} H^{\circ} (CO (g))$ .
D $Δ_{f} H^{\circ} (O X_{3} (g))$ is zero because it is an element.

Answer: A, B

Burning graphite gives

CO X_{2}

from elements in their reference states, so

Δ_{c} H^{\circ} (C) = Δ_{f} H^{\circ} (CO X_{2})

(statement 1). Likewise

H X_{2} + \frac{1}{2} O X_{2} H X_{2} O (l)

is both the combustion of

H X_{2}

and the formation of liquid water (statement 2). Graphite burns to

CO X_{2}

, not

CO

, so statement 3 is false. The reference form of oxygen is

O X_{2}

, so

Δ_{f} H^{\circ} (O X_{3}) \neq = 0

and statement 4 is false.