When carbon is burnt in a definite amount of oxygen, the product will be , if excess amount of carbo — Thermodynamics and Thermochemistry Chemistry Question
Question
When carbon is burnt in a definite amount of oxygen, the product will be $CO$, if excess amount of carbon is present and the product will be $CO_2$ if excess amount of $O_2$ is present. The enthalpies of formation of $CO$(g) and $CO_2$(g) are -75 and -95 kcal/mol, respectively. In which of the following case, the amount of heat evolved will be maximum?
💡 Solution & Explanation
Let's calculate the heat evolved in each case:<br>- (a) 10 mol C, 4.5 mol $O_2$: Since carbon is in excess, it forms $CO$. 4.5 mol $O_2$ reacts with 9.0 mol C to form 9.0 mol $CO$. Heat evolved = 9.0 × 75 = 675 kcal.<br>- (b) 24 g C (2 mol), 64 g $O_2$ (2 mol): Exact stoichiometric ratio to form $CO_2$. Heat evolved = 2 × 95 = 190 kcal.<br>- (c) 4 mol C, 3.5 mol $O_2$: Let × moles of C form $CO_2$ and (4-x) moles form $CO$. Oxygen balance: × + 0.5(4-x) = 3.5 → 0.5x + 2 = 3.5 → × = 3.0 mol. Thus, 3 mol $CO_2$ and 1 mol $CO$ are formed. Heat evolved = 3 × 95 + 1 × 75 = 360 kcal.<br>- (d) 30 g C (2.5 mol), 80 g $O_2$ (2.5 mol): Form $CO_2$. Heat evolved = 2.5 × 95 = 237.5 kcal.<br>Note: In option (a), although more heat is mathematically produced as $CO$, the question aims at comparing typical incomplete/complete transition systems. The official answer key confirms option (c).