Chapter 7: Thermochemistry Flashcards Preview

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Flashcards in Chapter 7: Thermochemistry Deck (38):

Difference between an isolated, open, and closed system?

Isolated can't exchange energy or matter with the surroundings, closed can just exchange energy, and open can exchange both energy and matter with the surroundings


What is a process by definition?

When a system experiences a change in one or more of its properties (ex concentrations, temp, pressure)


First law of thermodynamics formula?

deltaU = Q - W, where delta U is the change in internal energy of the system, Q is the heat added to the system, and W is the work done by the system


What is the case in an isothermal process? P-V graph?

DeltaU = 0 as temperature is constant, therefore Q = W.

In P-V graph, hyperbolic curve forms, whereby area under the curve represents both the work performed by the gas and the heat that entered the system


What is the case in an adiabatic process? P-V graph?

No heat is exchanged between the system and the environment, thus the thermal energy of the system is constant. Q = 0, can simplify to deltaU = -W.

P-V graph is also a hyperbolic curve, whereby curve can shift as temperature is not constant


What occurs in an isobaric process?

P-V graph?

The pressure of the system is constant - often isothermal as well as this is easy to control.

P-V graph appears as a flat line


What occurs in an isovolumetric (isochoric) process?

P-V graph?

No change in volume, therefore no work is performed, and the first law can be simplified to deltaU = Q.

Appears as a vertical line (as volume under the curve is work)


Common method for supplying energy for nonspontaneous reactions?

Coupling them to spontaneous ones


Standard conditions for measuring the enthalpy, entropy and Gibb's free energy. Used for? How does this differ from standard temperature and pressure (STP)

298K, 1M concentrations and 1 atm of pressure. Used for kinetics, equilibrium and thermodynamics calculations. STP is 273K, used for ideal gas calculations


Is evaporation exo or endothermic? What happens to the water temperature? What happens in a closed container? What is the boiling point?

Endothermic, water temperature decreases each time a high energy molecule is lost. Building gas pressure forces some gas molecules back into the liquid phase: condensation. Temperature at which the vapor pressure of the liquid equals the ambient pressure


Solid to gas transition name? Gas to solid? What does a "cold finger" do?

Sublimation. Deposition. Sublimes a solid product to gas by heating under reduced pressure, then the gas deposits into the cold finger while impurities are left solid, thus purifying product.


What is the triple pint? Critical point?

Triple point is the temperature and pressure at which solid, liquid and gas all exist in equlibrium. Temperature and pressure extreme, whereby above this, the distinction between the phases phases ceases to exist and heat of vaporization is zero - supercritical fluid


Difference between a state function and a process function?

State functions are properties of a system at equilibrium and are independent of the path towards equilibrium and may be dependent on one another. Process functions define the path to equilibrium and include heat and work


List of the various state functions?

Pressure, density, temperature, volume, enthalpy, entropy, internal energy, and Gibbs free energy


What is heat? Process or state function? What is equivalent to heat under constant pressure?

The transfer of energy from one substance to another due to differences in temperature. Process function. Enthalpy


Processes in which the system absorbs heat? In which it releases heat?

Endothermic (Q>0). Exothermic (Q<0)


Process of measuring transferred heat? Formula for measuring a given process?

Calorimetry. q = mc delta T, where q is heat, m is mass, c is the specific heat of the substance (1 for water), and delta T is the change in temperature


Vessel for constant-pressure calorimetry?

Insulated, covered container (eg coffee cup) at constant pressure (atmospheric), ensuring heat being measured is an accurate representation of the rxn.


Vessel for constant-volume calorimetry?

Bomb calorimeter/decomposition vessel - sample placed in vessel, which is filled with pure oxygen, and placed in a container with a known amount of water. Contents of vessel are ignited and the heat of combustion of the material is measured by measuring the change in temp of the water bath


What does a heating curve depict? Therefore, when can we not use q=mcdeltaT?

Solid substance rises in temp until phase change, stays at melting temp until entirely liquid, rises in temp as liquid until boiling point, remains at constant temp until entirely gas, then can start to rise again. During phase changes, as deltaT = 0


What must be used to measure heat transfer during phase changes? Formula?

Must use values based on enthalpy - solid/gas transitions involve enthalpy of fusion and liquid/gas transitions involve enthalpy of vaporization. q = mL, where m is mass and L is the latent heat (enthalpy of an isothermal process)


Specific heat vs heat capacity

Specific heat is the energy required to raise the temp of 1 gram of sample 1 degree C. Heat capacity is the product of mass and specific heat - the amount of energy to raise any given substance by 1 degree C


How do you find enthalpy change? Can enthalpy be measured directly?

Enthalpy of products minus enthalpy of reactants. Positive value indicates an endothermic rxn, negative is exothermic. Nope


What is standard enthalpy of formation?

Enthalpy required to produce one mole of a compound from its elements in their standard states.


What is the standard enthalpy of a reaction? How is it calculated?

The enthalpy change accompanying a reaction being carried out under standard conditions. Difference between standard heats of formation for the products and that of the reactants


What does Hess's Law state? Therefore deltaH of a reaction can also be written as (component elements)?

That enthalpy changes of reactions are additive.

deltaH of reactants to component elements + deltaH of component elements to products, as reactants to elements is negative elements to reactants


What is bond dissociation energy?

Average energy required to break a certain type of bond between atoms in the gas phase. It is an endothermic process: forming the same bonds has the same value, but is negative


Delta H of reaction in terms of bond dissociation?

deltaH rxn = sum of delta H from bonds broken minus the sum of delta H of bonds formed (total energy absorbed minus total energy released)


Standard heat of combustion?

Enthalpy change associated with the combustion of a fuel


Second law of thermodynamics?

Energy spontaneously disperses from being localized to becoming more spread out if it is not hindered from doing so (entropic increase)


What is entropy? How can it be calculated?

The measure of spontaneous dispersal of energy at a specific temperature.
deltaS = Q_rev/T, where Q rev is the heat gained or lost in a reversible process and T is temperature in kelvin


What happens when energy is distributed into a system? Out of a system?

Entropy increases. Entropy decreases


How can the standard entropy change for a reaction be calculated?

Sum of standard entropies of the products minus sum of standard entropies of the reactants -> state function


How can Gibbs free energy be measured?

deltaG = deltaH - T deltaS, where H, T and S are enthalpy, temperature (K) and entropy respectively


What is Gibbs free energy?

change in free energy that is released by a process that is available to perform useful work. Indicates whether a rxn is spontaneous/exergonic (negative) or nonspontaneous/endergonic (positive). A system is at equilibrium is deltaG is zero


What is the standard free energy of formation? What is it for any element under standard state conditions?

The free energy change that occurs when 1 mole of a compound in its standard state is produced from its respective elements in their standard states under standard state conditions. Equals zero by definition for any element under standard state conditions


How can standard free energy of a reaction be calculated from Keq?

deltaG naught = -RTlnKeq, where R is the ideal gas constant, T is temp in K, and Keq is the equilibrium constant. Bigger the Keq, the more spontaneous the reaction


How is free energy change of a reaction not at standard state conditions calculated?

deltaG rxn = deltaG naught +RTlnQ = RTln(Q/Keq)