Test: Thermochemistry - MCAT MCQ

Helium has some peculiar features in its phase diagram. A typical phase diagram looks like the following:

At room temperature, helium exists as a gas, and as pressure increases it eventually becomes a liquid. Looking at the line boundary between solid and the two liquid phases, the line curves up. Since the graph is using the logarithmic scale, the line curves more steeply than it would seem.

At atmospheric pressure, helium can exist as a supercritical fluid (He-II), normal liquid (He-I), and gas, but not as a solid.

Helium does not exist as a solid below 25 atmospheres due to the weak London dispersion forces that exist between noble gas atoms.

This is a P-V graph, so the plotted lines represent isotherms, whereby each line represents a different temperature. Normally, the plot is hyperbolic in shape, but as the temperature decreases, the plot flattens into a liquid-vapor equilibrium region or condensation region.

Moving from point 1 to 2, carbon dioxide is undergoing evaporation not condensation since the liquid region is to the left and the vapor region is to the right.

To the left of point 1, the pressure increases drastically because carbon dioxide has condensed into an incompressible liquid not solid.

Point 3 is the critical point of carbon dioxide, and the isotherm through point 3 is the critical isotherm.

Above the isotherm through point 3, carbon dioxide exists only as a supercritical fluid.

We are looking for a negative value of ∆G^∘ to ascertain spontaneity. However, the condition of high temperatures would preclude some options.

When the sign for ∆H^∘ and ∆S^∘ are different, the system is either always spontaneous or non-spontaneous. When the signs are the same, there is the temperature-dependent spontaneity.

Only when both sign are positive is ∆G^∘ spontaneous at high temperatures. We are looking for an endothermic reaction with a positive entropy.

Evaluating the change in entropy should allow us to more easily eliminate. The formation of ammonia and nitrogen dioxide both show a negative change in entropy, whereby moles of gases are reduced on the product side.

Of the remaining two reactions, one is a combustion reaction, which is strongly exothermic, and can be eliminated.

The decomposition of dinitrogen tetraoxide shows an increase in entropy.

Thermochemistry is a branch of chemistry that deals with the study of heat energy changes that occur during chemical reactions and phase transitions. It involves the measurement and calculation of quantities such as heat capacity, enthalpy, and entropy.

Hess's law states that the enthalpy change of a reaction is independent of the pathway taken and depends only on the initial and final states of the reaction. This means that the overall enthalpy change of a reaction can be calculated by summing up the enthalpy changes of the individual steps involved.

The standard enthalpy of formation (ΔHf°) of an element in its most stable form is defined as zero. This is because the formation of an element from its constituent elements does not involve any net energy change.

The standard enthalpy of formation of a compound is defined as the heat associated with the formation of one mole of the compound from its elements in their standard states. Most formation reactions are exothermic, but some are endothermic.

Carbon monoxide (CO) is not an element, in its reaction with oxygen gas, so the reaction is not a formation reaction.

In the reaction with hydrogen and bromine gases reacting, bromine is not in its standard state, which is liquid not gas. Bromine and mercury are the only two elements that are liquid in its standard state. The formation reaction of HBr would look like this:

H₂ (g) + Br₂ (1) + 2BHr (g)

In the reaction with hydrogen and oxygen gases reacting, water on the product state is not in its standard state. The formation reaction of H₂O would look like this:

In the reaction with rhombic sulfur and chlorine gas, there are elements in their standard state on the reactant side and only one product in its standard state. Sulfur is peculiar in that sulfur is not monoatomic and additionally has two forms, rhombic and monoclinic, and rhombic is the more stable form. The formation reaction of is SC1₂ correct as stated.

The dissolution of a gas in water is associated with a positive entropy change. When a gas dissolves, its particles become dispersed in the liquid, increasing the disorder or randomness of the system, which corresponds to an increase in entropy.

The first law of thermodynamics, also known as the law of conservation of energy, states that energy is conserved in a chemical reaction. It cannot be created or destroyed, but it can be converted from one form to another (e.g., from heat to work or vice versa).

Heat capacity is a measure of the amount of heat energy required to raise the temperature of a substance by a certain amount. It is expressed in units of energy per degree Celsius (or Kelvin) and represents the ability of a substance to absorb heat.