Chemistry: Thermochemistry Flashcards
Thermodynamics
Help determine whether a chemical reaction is spontaneous.
Spontaneous
If under a given set of conditions it can occur, by itself, without outside assistance.
A spontaneous reaction may or may not proceed to completion, depending upon the rate of the reaction, which is determined by chemical kinetics.
System
Particular part of the universe being studied.
Surroundings
Everything outside the environment.
Isolated System
It cannot exchange energy or matter with the surroundings, as with an insulated bomb reactor.
Closed System
It can exchange energy but not matter with the surroundings, as with a steam radiator.
Open System
It can exchange both matter and energy with the surroundings, as with a pot of boiling water.
Process
A system undergoes a process when one or more of its properties change. A process is associated with a change of state.
Isothermal Process
Occurs when the temperature of the system remains constant.
Adiabatic Process
Occurs when no heat exchange occurs.
Isobaric Process
Occurs when the pressure of the system remains constant.
Heat
Form of energy that can easily transfer to or form a system as the result of a temperature difference between the system and its surroundings; this transfer will occur spontaneously from a warmer system to a cooler system. According to convention, heat absorbed by a system (from its surroundings) is considered positive, while heat lose by a system (to its surroundings) is considered negative.
Calorimetry
Measures heat changes.
Constant-Volume and Constant-Pressure Calorimetry
Terms used to indicate the conditions under which the heat changes are measured.
Equation for Specific Heat
q = mcdeltaT
q = heat m = mass c = specific heat deltaT = change in temperature
This can only be used when the phase remains the same. If there’s a phase change, you must calculate the heat during transformation using the equation q = mL, where L is either heat of fusion or vaporization, depending on whether you are interchanging solid and liquid or liquid and gas respectively.
Constant-Volume Calorimetry
The volume of the containing holding the reacting mixture doesn’t change during the course of the reaction.
Constant-Volume Calorimetry: Bomb Calorimeter
The heat of reaction is measured using a device called a bomb calorimeter. This apparatus consists of a steel bomb into which the reactants are placed. The bomb is immersed in an insulated container containing a known amount of water. The reactants are electrically ignited and heat is absorbed or evolved as the reaction proceeds.
Constant-Volume Calorimetry: Determining Qrxn
The heat of the reaction, qrxn, can be determined as follows. Since no heat enters or leaves the system, the net heat change for the system is zero; therefore, the heat change for the reaction is compensated for by the heat change of the water and the bomb, which is easy to measure.
Constant-Volume Calorimetry: Type of Process
Note that the overall system is adiabatic, since no net heat gain or loss occurs. However, the heat exchange between the various components makes it possible to determine the heat of reaction.
State Function
Properties whose magnitude only depends on the initial and final states of the system, not on the path of the change.
Pressure, temperature, volume, enthalpy, entropy, free energy, and internal energy all examples. Although independent of path, state functions not necessarily independent of one another.
Standard State
Substance in its most stable form under standard conditions.
Enthalpy (H)
Express heat changes at constant pressure. The change in enthalpy (deltaH) of a process is equal to the heat absorbed or evolved by the system at constant pressure. The enthalpy of a process depends only on the enthalpies of the initial and final states, not on the path.
deltaH = Hproducts - Hreactants
Enthalpy Answer Interpretation
A positive answer is endothermic, negative exothermic.
Standard Heat of Formation
The enthalpy of formation of a compound, deltaHf, is the enthalpy change that would occur if one mole of a compound were formed directly from its elements in their standard states.
Note that deltaHf of an element in its standard state is zero. Then delta Hf of most known substances are tabulated.
Standard Heat of Reaction
deltaHrxn. It’s the hypothetical enthalpy change that would occur if the reaction were carried out under standard conditions; i.e., when reactants int heir standard states are converted to products in their standard states at 298K. It can be expressed as
deltaHrxn = (sum of deltaHf products) - (sum of deltaHf reactants)
Hess’s Law
States that enthalpies of reactions are additive. Specifically, Hess’s law states that when the reactants are transformed into products, the net change in enthalpy is the same irrespective if the reaction takes place in a series of steps or in a single step. When thermochemical equations (chemical equations for which energy changes are known) are added to give the net equation for a reaction, the corresponding heats of reaction are also added to give the net heat of reaction. Because enthalpy is a state function, the enthalpy of a reaction doesn’t depend on the path taken, only on the initial and final states.
Bond Dissociation Energy
Heats of reaction are related to changes in energy associated with the breakdown and formation of chemical bonds. Bond energy, or bond dissociation energy, is an average of the energy required to break a particular type of bond in one mole of gaseous molecules. It’s tabulated as the positive value of the energy absorbed as the bonds are broken.
Bond energies can be used to estimate enthalpies of reactions. The enthalpy change of reaction is
deltaHrxn = (deltaH of bonds broken) + (deltaH of bonds formed)
= total energy input - total energy released
Note: Since energy is released when bonds are formed, the deltaH of bonds formed will be negative.
Heats of Combustion
One more type of standard enthalpy change that’s often used is the standard heat of combustion. deltaHcomb. As stated earlier, a requirement for relatively easy measurement of deltaH is that the reaction be fast and spontaneous; combustion generally fits this description.
Entropy (S)
Measure of disorder, or randomness, of a system. The units of entropy are energy/temperature, commonly J/K or cal/K. The greater the order in a system, the lower the entropy; the greater the disorder or randomness, higher the entropy. At any given temperature, a solid will have lower entropy than a gas, because individual molecules int he gaseous state are moving randomly, while individual molecules in a solid are constrained in place. Entropy is a state function, so a change in entropy depends only on the initial and final states:
deltaS = Sfinal - Sinitial
Entropy: qrev
A change in entropy is also given by:
deltaS = qrev/T
where qrev is the heat added to the system undergoing a reversible process (a process that proceeds with infinitesimal changes in the system’s conditions) and T is the absolute temperature.
Standard Entropy
A standard entropy change for a reaction is calculated using the standard entropies of reactants and products:
deltaSrxn = (sum of deltaSproducts) - (sum of deltaSreactants)
2nd Law of Thermodynamics
The second law of thermodynamics states that all spontaneous processes proceed such that the entropy of the system plus its surroundings increases:
deltaSuniverse = deltaSsystem + deltaSsurroundings > 0
A system reaches its max entropy at equilibrium, a state in which no observable change takes place as time goes on. A system will spontaneously tend toward an equilibrium state if left alone.
Gibbs Free Energy (G)
Combines the two factors that affect the spotaneity of a reaction, changes in enthalpy and changes in entropy. The change in the free energy of a system, deltaG, represents the max amount of energy released by a process, occurring at constant temperature and pressure, that’s available to perform useful work.
deltaG = deltaH - TdeltaS
where T is the absolute temperature and TdeltaS represents the total amount of heat absorbed by a system when its entropy increases reversibly.
Gibbs Free Energy Answer Interpretation
In the equilibrium state, free energy is at a minimum. A process can occur spontaneously if the Gibbs function decreases (i.e., deltaG > 0).
- If deltaG is negative, the reaction is spontaneous.
- If deltaG is positive, the reaction is not spontaneous.
- If deltaG is zero, the system is in a state of quilibrium. Thus, if deltaG = 0, then deltaH = TdeltaS
The rate of a reaction depends on the activation energy, not the deltaG.
Standard Free Energy
Defined as the deltaG of a process occurring at 25 degrees Celsius and 1 atm pressure and for which the concentrations of any solutions involved are 1 M. The standard free energy of formation of a compound, deltaGf, is the free-energy change that occurs when one mole of a compound in its standard state is formed from its elements in their standard states under standard conditions. The standard free energy of formation of any element in its most stable form (and, therefore, its standard state) is zero. The standard free energy of a reaction, deltaGrxn, is the free energy change that occurs when that reaction is carried out under standard state conditions; i.e., when the reactants in their standard states are converted to the products in their standard states, at standard condition of T and P. For example, conversion of C(diamond) to C(graphite) is spontaneous under standard conditions. However, its rate is so slow that the rxn is never observed.
deltaGrxn = (sum of deltaGf of products) - (sum of deltaGf of reactants)
Reaction Quotient
deltaGrxn can also be derived from the quilibrium constant for the equation:
deltaGdegree = -RTlnKeq
where Keq is the equilibrium constant, R is the gas constant, and T is the temperature in K.
Once a reaction commences, however, the standard state conditions no longer hold. Keq must be repalced by another parameter, the reaction quotient (Q). For the reaction, aA + bB cC + dD:
{Q=[C]^c[D]^d}/{[A]^a[B]^b}
Likewise, deltaG must be used in place of deltaGdegree.
deltaG = deltaGdegree + RTlnQ
where R is the gas constant and T is the temperature in K.
