Chemistry: Thermochemistry

Question 1

Thermodynamics

Answer 1

Help determine whether a chemical reaction is spontaneous.

Question 2

Spontaneous

Answer 2

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.

Question 3

System

Answer 3

Particular part of the universe being studied.

Question 4

Surroundings

Answer 4

Everything outside the environment.

Question 5

Isolated System

Answer 5

It cannot exchange energy or matter with the surroundings, as with an insulated bomb reactor.

Question 6

Closed System

Answer 6

It can exchange energy but not matter with the surroundings, as with a steam radiator.

Question 7

Open System

Answer 7

It can exchange both matter and energy with the surroundings, as with a pot of boiling water.

Question 8

Process

Answer 8

A system undergoes a process when one or more of its properties change. A process is associated with a change of state.

Question 9

Isothermal Process

Answer 9

Occurs when the temperature of the system remains constant.

Question 10

Adiabatic Process

Answer 10

Occurs when no heat exchange occurs.

Question 11

Isobaric Process

Answer 11

Occurs when the pressure of the system remains constant.

Question 12

Heat

Answer 12

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.

Question 13

Calorimetry

Answer 13

Measures heat changes.

Question 14

Constant-Volume and Constant-Pressure Calorimetry

Answer 14

Terms used to indicate the conditions under which the heat changes are measured.

Question 15

Equation for Specific Heat

Answer 15

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.

Question 16

Constant-Volume Calorimetry

Answer 16

The volume of the containing holding the reacting mixture doesn’t change during the course of the reaction.

Question 17

Constant-Volume Calorimetry: Bomb Calorimeter

Answer 17

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.

Question 18

Constant-Volume Calorimetry: Determining Qrxn

Answer 18

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.

Question 19

Constant-Volume Calorimetry: Type of Process

Answer 19

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.

Question 20

State Function

Answer 20

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.

Question 21

Standard State

Answer 21

Substance in its most stable form under standard conditions.

Question 22

Enthalpy (H)

Answer 22

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

Question 23

Enthalpy Answer Interpretation

Answer 23

A positive answer is endothermic, negative exothermic.

Question 24

Standard Heat of Formation

Answer 24

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.

Question 25

Standard Heat of Reaction

Answer 25

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)

Question 26

Hess’s Law

Answer 26

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.

Question 27

Bond Dissociation Energy

Answer 27

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.

Question 28

Heats of Combustion

Answer 28

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.

Question 29

Entropy (S)

Answer 29

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

Question 30

Entropy: qrev

Answer 30

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.

Question 31

Standard Entropy

Answer 31

A standard entropy change for a reaction is calculated using the standard entropies of reactants and products:

deltaSrxn = (sum of deltaSproducts) - (sum of deltaSreactants)

Question 32

2nd Law of Thermodynamics

Answer 32

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.

Question 33

Gibbs Free Energy (G)

Answer 33

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.

Question 34

Gibbs Free Energy Answer Interpretation

Answer 34

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.

Question 35

Standard Free Energy

Answer 35

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)

Question 36

Reaction Quotient

Answer 36

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.