Thermochemistry MCQs for MDCAT

Correct answer: D. Decreases

In the standard enthalpy of atomization, heat flows out of the system, so the heat content (temperature) of the surroundings decreases as the system absorbs energy to break bonds... actually the process is endothermic, so heat is absorbed from the surroundings, decreasing the surroundings' heat content.

Correct answer: A. -394 kJ/mole

The standard heat of formation of CO2 is approximately -394 kJ/mol, reflecting the strongly exothermic combustion of carbon in oxygen.

Correct answer: C. -205.5 kJ/mol

Since the reaction as written (2H2 + O2 to 2H2O) is given as ΔH = +285.5 kJ/mol but water formation is actually exothermic, the correct enthalpy change for the reaction as written should be negative, i.e. -205.5 kJ/mol (per the values provided in this question set).

Correct answer: B. -110 kJ mol⁻¹

Using Hess's law, the enthalpy of formation of CO is found by subtracting the enthalpy of CO to CO2 (-283) from the enthalpy of C to CO2 (-393.7), giving -110 kJ/mol.

Correct answer: C. ½H₂(g) → H(g) +218 kJ mol⁻¹

The standard enthalpy of atomization of hydrogen is represented by ½H2(g) to H(g), ΔH = +218 kJ/mol, the energy needed to produce one mole of gaseous H atoms.

Correct answer: D. +1.91

Since graphite is more stable (lower energy) than diamond, converting graphite to diamond requires energy input, giving a positive enthalpy change of about +1.91 kJ/mol (the difference between their combustion enthalpies).

Correct answer: D. ΔH°l

The formation of MgO from its elements at standard conditions, releasing -692 kJ/mol, is described as the standard enthalpy of formation (ΔH°f), abbreviated here as ΔH°l.

Correct answer: D. More product (NH₃) will be formed

Since the formation of NH3 from N2 and H2 is exothermic, cooling the system shifts the equilibrium (per Le Chatelier's principle) toward the exothermic direction, forming more product (NH3).

Correct answer: B. Oxidation

The specific heat of vaporization of water plays an important role in the regulation of heat produced by OxidationSpecific heat of vaporization refers to the amount of heat energy required to change a substance from a liquid to a gas without changing its temperature. In the case of water, a significant amount of heat energy is needed to convert liquid water into water vapor (gas) due to the hydrogen bonds between water molecules. This property of water helps regulate and stabilize temperatures in biological systems and prevents rapid temperature changes during chemical reactions, including oxidation processes, which involve the release of heat energy.

Correct answer: C. Products is more than that of reactants

This is the correct statement. In endothermic reactions, the products have greater energy (or heat content) than the reactants because heat is absorbed during the reaction.

Correct answer: A. Greater Ea

The correct option is A as the endothermic reactions have a greater activation energy because the products of this reaction have more energy than the reactants. On the otherhand products in exothermic reaction have lesser energy than the reactants, therefore, the activation energy is lesser. The graph is shown below

Correct answer: B. More than delta H

For an endothermic reaction, the enthalpy of reaction (ΔH) is positive, indicating that energy is absorbed during the reaction. The activation energy (Ea) is the minimum energy required for the reaction to occur.Since the reaction is endothermic, the activation energy must be greater than the enthalpy of reaction (ΔH) to ensure that the reaction can proceed. This is because the system needs to overcome the energy barrier (Ea) to absorb the energy (ΔH) required for the reaction.