Chemical Thermodynamics NEET 2006: Hydrogenation Answer

What is the correct answer to the chemical thermodynamics NEET 2006 hydrogenation question?

Option A is correct: hydrogenating one mole of cyclohexene releases 121 kJ. The chemical thermodynamics NEET 2006 question asks for the signed enthalpy change, not merely the amount of heat released. The chapter is Chemical Thermodynamics, and the method is Hess’s law.

Complete combustion of one mole each of hydrogen, cyclohexene and cyclohexane gives these enthalpy changes, respectively:

Determine the enthalpy change when cyclohexene reacts with hydrogen to form cyclohexane. The supplied options are:

• A)

• B)

• C)

• D)

The question bank tags this as hard and sets a 90-second target. Neither is an official exam classification or a measured student result.

How do you make both combustion routes end at the same products?

Burn cyclohexene and hydrogen directly, or hydrogenate first and then burn cyclohexane. Both routes finish at the same carbon dioxide and water endpoints. Start with the target reaction to fix how much hydrogen belongs in the calculation:

One mole of cyclohexene consumes one mole of hydrogen to form one mole of cyclohexane. There is no factor of two in this hydrogenation.

Cyclohexene combustion is:

$\Delta H=-3800\mathrm{k}\mathrm{J}$ Hydrogen combustion is:

$\Delta H=-241\mathrm{k}\mathrm{J}$ Each enthalpy applies to its reaction as written. Adding the two reactions gives the direct-combustion route:

For the other route, hydrogenation produces cyclohexane. Its combustion reaction and enthalpy change as written are:

$\Delta H=-3920\mathrm{k}\mathrm{J}$ Both routes consume nine moles of oxygen and end at six moles each of carbon dioxide and water. Cyclohexane is an intermediate in the second route: it forms during hydrogenation and is consumed during combustion.

Use the supplied combustion values throughout. Keep the water-state convention unchanged between routes, with no imported tabulated values or extra phase-change terms.

How does Hess’s law give option A?

The hydrogenation enthalpy is the direct-route total minus the cyclohexane-combustion enthalpy. Hess’s law states that the total enthalpy change is identical for routes connecting the same initial and final states. The balanced reactions establish those matching endpoints, so write their equality before substituting numbers.

The supplied solution’s relationship is:

Substitute the given molar values:

Add the two combustion terms on the right: $\Delta H_{\mathrm{h}\mathrm{y}\mathrm{d}}-3920=-4041$

Move the cyclohexane-combustion term to the other side: $\Delta H_{\mathrm{h}\mathrm{y}\mathrm{d}}=-4041+3920$

Therefore:

Option A is correct. The negative sign means that 121 kJ is released per mole of cyclohexene hydrogenated. “Heat released” describes a positive amount, but the question asks for the signed enthalpy change, so do not replace the negative answer with option B.

Check the complete second route: $-121+(-3920)=-4041$

This matches direct combustion of the reactants. The check tests the sign against the two-route equality, not just the expectation that hydrogenation releases heat.

Why does reversing the subtraction produce option B?

Option B results from subtracting the reactants’ combined combustion enthalpy from the product’s combustion enthalpy. That reverses the difference needed for forward hydrogenation and gives the enthalpy of the reverse reaction instead.

The incorrect calculation for forward hydrogenation is:

This value belongs to the reverse reaction:

Reversing a reaction changes the sign of its enthalpy change. The positive value is meaningful, but it answers a different question.

Do not confuse combustion data with standard enthalpies of formation. The familiar products-minus-reactants rule applies when using formation enthalpies with their stoichiometric coefficients. Here, the supplied values are combustion enthalpies: combustion takes each substance to common endpoints, so the two-route equality determines the subtraction order.

Write that equality before moving any numerical term across the equals sign. This is safer than memorising a subtraction shortcut because it keeps the reaction direction visible.

How do reversal, doubling and constant-pressure heat change the answer?

Reversal changes the sign, doubling the reaction doubles its total enthalpy change, and the amount hydrogenated determines the total heat. These are original practice questions based on the supplied data, not additional verified PYQs. Keep the reaction direction and amount separate.

1. What is the enthalpy change when cyclohexane forms cyclohexene and hydrogen?

The enthalpy change is positive because this is exactly the reverse of the original hydrogenation:

The reversed process absorbs the amount of heat that forward hydrogenation releases. Only the direction changes; the stoichiometric amounts remain unchanged.

2. What is the enthalpy change when the hydrogenation equation is doubled?

The total enthalpy change doubles because every stoichiometric coefficient doubles:

This is the enthalpy change for the doubled reaction as written. Per mole of cyclohexene, the value remains:

3. What is the constant-pressure heat when 0.50 mol of cyclohexene is hydrogenated?

The system releases 60.5 kJ when the sample is completely hydrogenated. At constant pressure, with only pressure–volume work, heat equals the total enthalpy change under the same conditions:

The negative sign identifies heat leaving the reacting system. Before selecting an answer, check which change the question makes:

• Reaction reversal: change the sign, not the magnitude.
• Stoichiometric scaling: multiply the total enthalpy by the same factor as the coefficients.
• A specified sample amount: calculate total heat in kilojoules; do not label it as molar enthalpy in kilojoules per mole.

Next step: the past-paper archive on NEET JEEnius AI and search past NEET papers by year, subject or chapter, each with a worked solution (100 free searches a month).

For a worked example of the same idea, see Banking of Roads NEET 2026: Angle and Minimum Tyre Wear.