U-world: Gen-Chem: Thermo chemistry#1

Kinetic and thermodynamic products

-During product formation, the compound with the lowest energy transition state (lowest energy barrier for formation) forms fastest, making it the kinetic product. -The compound with the most stable (lowest energy) structure is the thermodynamic product, but because it has a higher energy transition state (higher energy barrier for formation), it forms more slowly

enthalpy

-If the heat released by bond formation exceeds the heat consumed by bond cleavage, the overall reaction is exothermic and has a negative value for ΔH°. -Conversely, if more heat is consumed(bond broken) than produced, the reaction is endothermic with a positive ΔH°.

activation energy Ea

-the minimum energy needed to reach the highest energy transition state that enables the completion of a reaction. -In a reaction free-energy diagram, the activation energy is the energy difference between the tallest peak (relative to the reactants) and the reactants (peak energy minus reactant energy).

Assuming ΔH° and ΔS° are constant at all temperatures, at approximately which temperature (in degrees Celsius) will ΔG° of Reaction 1 equal zero (ie, switch from spontaneous to nonspontaneous)?

95°C **0 kJ/mol = −55 kJ/mol − T(−0.15 kJ/mol·K) T=367 − 273 = 94°C ≈ 95°C

endergonic reaction

A non-spontaneous chemical reaction in which free energy is absorbed from the surroundings.

exergonic reaction

A spontaneous chemical reaction in which there is a net release of free energy.

At a given temperature and injection composition, if researchers allowed each engine to carry out only one complete combustion reaction, which engine would produce more water?

Both engines would produce the same amount because the equilibrium constant is the same for both. **The amount of product formed in a reaction is dependent on the equilibrium constant of the reaction. This constant is not changed by a catalyst, which only changes the rate at which equilibrium is achieved.

In the absence of platinum, a spark initiates combustion by doing which of the following?

It increases the average kinetic energy of nearby molecules. ** Increased temperature increases the kinetic energy of the molecules in a system, and therefore increases the rate of a reaction.

Assuming the combustion of hydrogen behaves like an elementary reaction with respect to its rate, which of the following explains why an increase in the H2/O2 ratio causes an increase in the reaction rate?

Law of mass action **The law of mass action states that the rate of a reaction is proportional to the molar amount of each reaction component raised to the power of its reaction order. For elementary reactions, the reaction order of each species is equal to its stoichiometric coefficient.

If large amounts of ammonia and bleach were mixed, emergency responders could help stop the reaction by doing which of the following? 2 NaOCl(aq)+NH3(aq)⇆ NHCl2(aq)+2 NaOH(aq)

Mixing sodium hydroxide into the solution

Which of the following affects the freezing point(physical property) of an ideal solution?

The strength of intermolecular forces **Incorrect answers are -The strength of intramolecular forces -The reactivity of the solute(chemical property) Freezing occurs when the kinetic energy of a molecule can no longer overcome the intermolecular forces binding it to nearby molecules.

Which of the following does not explain water's ability to act as a solvent?

Water has a relatively high surface tension **Incorrect answers were: -Water has the geometry of a bent molecule. -Hydrogen is less electronegative than oxygen. -Water is a relatively small molecule

Hydrogen bonding

a strong intermolecular force that occurs between a hydrogen atom on one molecule and an electronegative atom with a lone electron pair on another molecule.

catalyst

a substance that increases the rate of a reaction without being consumed by the reaction

phase diagram of water

decrease pressure while keeping temp constant 3 possibilities happen: solid to gas- sublimation liquid to solid- crystallization liquid to gas- vaporization

Compared to the boiling point of a sample of pure water on Mars, the boiling point of a sample of briny water(more solute) under the same atmospheric conditions would be:

higher, because the vapor pressure of briny water is lower than the vapor pressure of pure water. **The boiling point temperature is the temperature at which the vapor pressure is equal to ambient pressure. The addition of solute lowers the vapor pressure of a solution at all temperatures, and therefore raises the temperature required for the vapor pressure to become equal ambient pressure and begin to boil (the boiling point).

During the formation of dichloramine and NaOH, the order (organization) of the system:

increases because the change in entropy is negative

colligative properties

properties that depend on the concentration of solute particles but not on their identity

Raoult law states

the addition of solute to a pure substance lowers the freezing point and saturation vapor pressure of the resulting solution

Henry's law

the amount of a gas that dissolves in a liquid is proportional to the partial pressure of that gas

Surface tension

the force that acts on the surface of a liquid and that tends to minimize the area of the surface

reaction free-energy diagram

y-axis: Gibbs free energy x-axis: reaction coordinate(reaction pathways) Peaks: Transition states(highest energy)

Based on the information given in the passage, which of the following is the standard molar enthalpy of formation for sodium hydroxide? 2 NaOCl(aq)+NH3(aq)⇆ NHCl2(aq)+2 NaOH(aq)

−470 kJ/mol **According to Hess law, the overall enthalpy change ΔH° of a reaction is equal to the sum of changes in enthalpy of each component. -The signs of the reactants must be changed. Table 1 gives enthalpies of formation, but ammonia and bleach are consumed, not formed. Therefore, they contribute +350 kJ/mol and +45 kJ/mol of enthalpy, respectively. 2(350 kJ/mol)+45 kJ/mol+140 kJ/mol+2x=−55 kJ/mol