Standard enthalpy changes — NEET & JEE Questions

Question 1 · difficulty L1 · recall

The standard enthalpy of formation of a compound is the enthalpy change when

• A. one mole of the compound forms from its elements in their standard states
• B. one mole of the compound burns completely
• C. one mole of gas condenses
• D. one mole of ions dissolves

Question 2 · difficulty L2 · understanding

The standard enthalpy of formation of an element in its most stable standard state is

• A. zero
• B. always positive
• C. always negative
• D. equal to its atomic number

Question 3 · difficulty L2 · understanding

Standard enthalpy of combustion refers to the enthalpy change when

• A. one mole of substance burns completely in oxygen under standard conditions
• B. one mole of oxygen is formed
• C. a substance freezes
• D. a salt dissolves

Question 4 · difficulty L2 · understanding

A negative standard enthalpy of combustion indicates that combustion is

• A. exothermic
• B. endothermic
• C. thermally neutral
• D. impossible

Question 5 · difficulty L3 · application

If $\Delta H_f^\circ(CO_2)=-394$ kJ mol$^{-1}$, what is the enthalpy change for forming 2 mol of $CO_2$ from standard-state elements?

• A. $-788$ kJ
• B. $-394$ kJ
• C. $+788$ kJ
• D. $-197$ kJ

Question 6 · difficulty L3 · application

Which reaction represents the standard formation of liquid water?

• A. $H_2(g)+\frac12O_2(g)\rightarrow H_2O(l)$
• B. $2H_2+O_2\rightarrow2H_2O$
• C. $H_2O(l)\rightarrow H_2+\frac12O_2$
• D. $H(g)+O(g)\rightarrow H_2O(l)$

Question 7 · difficulty L3 · application

Using standard enthalpies of formation, reaction enthalpy is calculated as

• A. sum of product formation enthalpies minus sum of reactant formation enthalpies
• B. reactants minus products always
• C. sum of all absolute values
• D. product molar masses minus reactant molar masses

Question 8 · difficulty L3 · application

For $CH_4+2O_2\rightarrow CO_2+2H_2O(l)$, use $\Delta H_f^\circ$: CH4 = -75, CO2 = -394, H2O(l) = -286 kJ mol$^{-1}$, and O2 = 0. What is $\Delta H^\circ$?

• A. $-891$ kJ
• B. $+891$ kJ
• C. $-605$ kJ
• D. $-319$ kJ

Question 9 · difficulty L4 · application

The standard enthalpy of atomisation of an element corresponds to formation of

• A. gaseous atoms from the element in its standard state
• B. one mole of liquid compound from atoms
• C. ions from a solid only
• D. a molecule from gaseous atoms

Question 10 · difficulty L4 · application

If the standard enthalpy of formation of product P is much more negative than that of its reactants, the formation of P from those reactants is likely

• A. exothermic
• B. endothermic
• C. zero enthalpy
• D. unrelated to enthalpy differences