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21 Cards in this Set

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For an exothermic system, as heat (q) leaves the system its internal energy (E) decreases. In classical physics, the result of heat movement from a hot mass (A) to a cold mass (B) is correctly interpreted as A cooling and B warming. in such a case the heat flow for A is negative (-q) and for B is positive (+q). The amount of heat flow between the masses is equal but opposite in sign.For a reaction, the internal energy released as heat to the surroundings is not a simple movement of heat from one mass to another mass. In a reaction the internal energy also includes bonding and intermolecular interaction energies of the reactants. The total bond and intermolecular interaction energies decrease during an exothermic reaction. That decrease in internal energy is lost as heat to the surroundings. The effect of the reaction's lost heat entering the surroundings is to raise the temperature of the surroundings.Given this description of internal energy and heat flow for an exothermic reaction, which of the following is the best description of an endothermic reaction?

During an endothermic reaction, heat flowing into the system is used to raise the internal energy by increasing the total bonding and intermolecular interaction energies.

During an exothermic reaction heat energy leaves the system and enters the surroundings, heating and thus raising the temperature of the surroundings. During an endothermic reaction heat energy leaves the surroundings and enters the system, cooling and thus lowering the temperature of the surroundings. Typically a thermometer is used to monitor the temperature and thus the heat flow of a reaction. Should the thermometer part considered as?

part of the surroundings

If a reaction is monitored with a thermometer and the temperature measured falls, did heat flow into the system (qsys is positive) or out of system (qsys is negative)?

heat flowed into the system

If a reaction is monitored with a thermometer and the temperature measured falls, did heat flow into the surroundings (qsurr is positive) or out of surroundings (qsurr is negative)?

heat flowed out of the surroundings

The heat energy lost by an exothermic reaction is characterized as a negative change of enthalpy of the reaction (ΔH°rxn ). Heat energy is measured in joules while the closely related change in enthalpy is measured in kilojoules/mole accounting for the amount of heat produced by each molecule. If 410 J of heat (q) is produced by a given reaction of 0.031 moles of a given reactant, what is the molar enthalpy change (ΔH°rxn) relative to that reactant in kJ/mol?

molar enthalpy change for the system ΔH°rxn=q/n.




-13

info

Probably one of the most needlessly confusing issues in chemistry is the minimally discussed difference in the standard temperature and pressure (STP) used for gas law calculations and the standard states which are considered the reference states in thermodynamic calculations. The confusion mainly has to do with temperature.STP: standard temperature and pressure of 0 °C and 1 atm at which 1 mol of an ideal gas occupies 22.4 L.Standard State (for a reaction): each solid is a pure solid, each liquid is a pure liquid, each gas is at 1 atm of pressure, each dissolved substance is at 1 M concentration, the standard enthalpy of formation, ΔH°f, of each element in its standard state is defined to be zero and the standard free energy of formation, ΔG°f, of each element in its standard state is defined to be zero. Typically these values are tabulated at 25 °C but they can be calculated at any other temperature.

Which of the following would be considered at STP, standard state or both?

a gas at 1 atm pressure and 298.15 K temperature C. standard state C. standard statea pure solid under 1 atm pressure and 323 K temperature C. standard state C. standard statea 1 M solution under 1 atm pressure and 273.15 K C. standard state C. standard statea 1 M solution under 1 atm pressure and 298.15 K

An important relationship exists between the free energy of a reaction at standard state conditions, ΔG°rxn, and the equilibrium constant, Keq, for that reaction:

ΔG°rxn = -RT ln Keq

an experimentally determined value of Keq can be used to determine the standard state free energy of a reaction. R is the gas constant (0.008314 kJ/mol) and T is the temperature in K.What is the standard free energy of a reaction at 28 °C having a measured Keq = 2.3?Your answer should be in kJ/mol.

ΔG°rxn = -RT ln Keq




-2.1

Which of the following is the best definition of a spontaneous process?

A process that occurs on its own.

Which of the following processes are spontaneous? Do not fail to consider your own experiences when answering this question.

Water freezing in surroundings below 0 °C. B. spontaneous B. spontaneousIce melting in surroundings above 0 °C. B. spontaneous B. spontaneousA nail rusting in a moist atmosphere. B. spontaneous B. spontaneousConversion of CO2 and H2O into carbohydrates. C. not spontaneous C. not spontaneous

Which of the following is the best description of the second law of thermodynamics?

A spontaneous process will increase the total entropy of the universe.

entropy of the universe

Every process has an effect on the total entropy of the universe, Suniv. That change can be characterized by the amount of change in the entropy of the universe, ΔSuniv, it produces.

ΔSuniv = ΔSsurr + ΔSsys

The entropy change of the universe can be be separated into the entropy change of the surroundings and the entropy change of the system according the equation




Since it is difficult to measure the entire universe or even the surroundings of a system, it is helpful to recast the relationship above in terms of the system alone.

The change in the entropy of the surroundings is due...

to heat flow into or out of the system

isothermal

(costant temperature, T)

For an isothermal reaction the entropy change of the surroundings can be determine from...

the enthalpy change of the system according to the equation:ΔSsurr = -ΔHsys/T

an isothermal system can be used to determine the change in the entropy of the universe based only on changes in the system as:

ΔSuniv = -ΔHsys/T + ΔSsys




-TΔSuniv = ΔHsys - TΔSsys

-TΔSuniv is called

the Gibbs free energy of the system, ΔGsys, which is the energy available to the process.

If the change in entropy of the universe must increase, what must happen to the Gibbs free energy of the system for a spontaneous process?

ΔGsys must decrease for a spontaneous process.

q

q=cm*m*dT