Thermodynamics Flashcards

Question

Derive the relationship between heat capacity at constant pressure (Cp) and heat capacity at constant volume (Cv) for an ideal gas.

Answer 1

At constant volume: q(v) = C(v) ∆T = ∆U (I) At constant pressure: q(p) = C(p) ∆T = ∆H (II) For mole of an ideal gas: ∆H = ∆U + p∆V ∆H = ∆U + ∆(pV) ∆H = ∆U + ∆(RT) ∆H = ∆U + R∆T (III) From (I) and (II) into (III): Cp∆T = Cv∆T + R∆T Cp = Cv + R Therefore, relationship: **Cp - Cv = R**

Answer 2

Reaction enthalpy is nothing but the enthalpy change accompanying a reaction. **∆H(r) = (Sum of enthalpies of products) - (Sum of enthalpies of reactants)**.

Answer 3

The standard enthalpy of reaction is the enthalpy change for a reaction when all the participating substances are in their standard states. The standard state of a substance at a specified temperature is it's pure form at one bar. Standard conditions are denoted by adding the superscript^ '°' symbol (∆H°).

Answer 4

A calorimeter measures the amount of heat absorbed or released during a chemical or physical reaction under two conditions: 1. Constant volume. (qv) 2. Constant pressure. (qp)

Answer 5

The standard enthalpy change for the formation of one mole of a compound from its element in the most stable states of aggregation (also known as references) is called standard Molar enthalpy of formation. (∆fH°) The reference state of an element is its most stable state of aggregation at 25 degree Celsius and 1bar pressure.

Answer 6

The enthalpy change that accompanies melting of one mole of a solid substance in the standard state is called **standard enthalpy of fusion** or **molar enthalpy of fusion**. Melting of a solid is endothermic so all enthalpy of fusion are positive. (∆fusH°) Amount of heat required to vaporize 1mole of a liquid at constant temperature and under standard pressure (1Bar) is called its **standard enthalpy of vaporization** or **molar enthalpy of vaporization**. (∆vapH°) **Standard enthalpy of sublimation** is the change of enthalpy when the mole of a solid substance sublimes at a constant temperature in under standard pressure (1 Bar). (∆subH°) The magnitude of the enthalpy change depends on the strength of the internal molecular interactions in the substance undergoing the phase transformations. For example: it requires less heat to vaporize 1 mole of acetone then it does to vaporize 1 mole of water.

Answer 7

False, it is actually the other way around. **Standard enthalpy of formation (∆fH°)** is just a special case of **Standard reaction enthalpy (∆rH°)** where obviously the compounds are formed from their reactants, but the elements must also be in their most stable forms.

Answer 8

Standard enthalpy of combustion is defined as the enthalpy change per mole of substance when it undergoes combustion and all the reactants and products being in the standard states at the specified temperature. (∆cH°) Ex: Cooking gas in cylinders contain mostly butane (C4H10) During complete combustion of one mole of butane, ≈2600 kJ of heat is released. C4H10 (g) + 13/2 O2 (g) **→** 4 CO2 (g) + 5 H2O (l); ∆cH° = -2658.0 kJ/mol

Answer 9

Thermochemical equations are Balanced chemical equations together with the value of their enthalpy of reaction, which specify the physical state along side allotropic state of the substance in the equation.. The coefficients in a balanced thermochemical equation always refer to the number of moles and never the number of molecules of reactants and products involved in the reaction. The numerical value of enthalpy of reaction refers to the number of moles of substance specified by an equation.

Answer 10

Enthalpy of atomization is the enthalpy change on breaking 1mole of bonds completely to obtain atoms in the gaseous phase. (∆aH°) In case of diatomic molecules like dihydrogen enthalpy of atomization is also the bond is also the bond dissociation enthalpy. Ex: Dihydrogen H2 (g) **→** 2H (g) ; ∆aH° = 435.0 kJ/mol

Answer 11

Hess's law of constant heat summation states that **if a reaction takes place in several steps then its standard reaction enthalpy is the sum of the standard enthalpy of the intermediate reactions into which the overall reaction may be divided at the same temperature. This means that because enthalpy is a state function, **enthalpy change for a reaction is the same whether it occurs in one step for in a series of steps**. ∆rH = ∆rH1 + ∆rH2 + ∆rH3 + ...... + ∆rHn Can be represented as a polygon.

Answer 12

Relation of heat of reaction to changes in energy **associated with breaking and making of chemical bonds** is known as Bond Enthalpy. Types: 1.**Bond dissociation enthalpy**: HD change in enthalpy when one mole of covalent bonds of a gaseous covalent compound is broken to warm products in the gas phase more that it is the same as the enthalpy of atomization of dihydrogen. Therefore, **we use bond dissociation enthalpy in diatomic molecules**. Ex: Cl2 (g) **→** 2Cl2 (g); ∆cl-clH° = -242 kJ/mol 2.**Mean Bond Enthalpy**: It is the same as bond dissociation enthalpy but in case of polyatomic molecules, **bond dissociation enthalpy is different for different bonds in the same molecule**. Therefore, **it is used for polyatomic molecules**._₹ Ex: CH4 (g) **→** C(g) +4H(g); ∆aH = -1665 kJ/mol However, for bond enthalpy; CH4 (g) **→** CH3(g) +H(g); ∆bondH° = +427 kJ/mol CH3 (g) **→** CH2(g) +H(g); ∆bondH° = +439 kJ/mol • • Therefore; **∆c-hH° = 1/4(∆aH°) = (1/4) × 1665 kJ/mol = 416 kJ/mol** Remember the symbol used to denote both types of Bond enthalpy is the same; ∆bondH°

Answer 13

Absolute zero, at any other temperature heat capacities for the reactants and products are to be taken into account.

Answer 14

Enthalpy of solution of a substance is the enthalpy change when one mole of it dissolves in a specified amount of solvent. The enthalpy of solution at infinite dilution is the enthalpy change observed on dissolving the substance in an infinite amount of solvent when the interactions between the ions (or solute molecules) are negligible. (∆solH°) When an ionic compound dissolves in a solvent, the ions leave their ordered positions onto the crystal lattice. **For most of the ionic compounds enthalpy of solution is positive and the dissociation process is endothermic**. Therefore, the solubility of most salts in water increases with rise of temperature. If the lattice enthalpy is very high, the dissolution of the compound may not take place at all. **∆solH° = ∆latticeH° + ∆hybH°**

Answer 15

Enthalpy of dilution, also known as heat of dilution, is the change in enthalpy that happens when a solution is diluted. As more and more solvent is used the enthalpy of solution approaches a limiting value; the value in infinitely dilute solution.

Answer 16

The latice enthalpy of an ionic compound is the enthalpy change which occurs when one mole of an ionic compound dissociates its ions in gaseous state. Ex: Na+ Cl- (s) **→** Na+(g) **+** Cl- (g); ∆latticeH° = +788 kJ/mol It's impossible to determine lattice enthalpy directly therefore we use born-haeber cycle. See ex. of NaCl

Answer 17

Spontaneity **doesn't** actually mean the ability to perform a reaction very quickly. **It means having the potential to proceed without the assistance of external agencies**. **It does not tell about the rate of reaction or process**. **Spontaneous processes is an irreversible process, which may only be reversed by some external agency**.

Answer 18

False. All natural occuring processes whether chemical or physical will tend to proceed spontaneously in one direction only.

Answer 19

No, it is true that while decrease in enthalpy may be a contributary factor for spontaneity but **it is not true for all cases**.

Answer 20

Entropy (S) maybe described as a measure of the degree of randomness or disorder in the system. The greater the disorder in an isolated system the higher is the entropy. As far is a chemical reaction concerned, these entropy changes can be attributed to rearrangement of atoms or ions from one pattern in the reactants to another. **If the structure of the products is very much disordered than that of the reactants, there will be resultant increase in entropy**. Decrease in regularity of structure would mean increase in entropy. **The crystalline solid state is the state of lowest entropy (most ordered). The gaseous state is the state of highest entropy.** **It's a state function. Independent of path.** Whenever heat is added to the system it increases molecular motions causing increases randomness in the system, therefore heat has randomising influence on the system. **A system in greater temperature has greater randomness than one at lower temperature**. Temperature is the measure of average chaotic motion of particles in the system. **Entropy change is inversely proportional to the temperature.** Entropy for reversible reaction: **∆S = qrev/T** Entropy for system and surroundings of spontaneous process: **∆Stotal = ∆Ssystem + ∆Ssurr > 0** When a system is in equilibrium the entropy is maximum and the change in entropy, ∆S = 0. We can say entropy for spontaneous process increases till it reaches maximum and at equilibrium the change in entropy is zero. Entropy for irreversible reaction: **∆Ssys = qsys,rev/T**

Answer 21

The Gibbs Energy or Gibbs function was created to determine direction of spontaneous change for systems. (G) **It's a state function**. G = H - TS **∆Gsys = ∆Hsys - T∆Ssys - Ssys∆T** For constant temperature, (∆T = 0) ∆Gsys = ∆Hsys - T∆Ssys **∆G = ∆H - T∆S** Gibbs equation: **Gibbs energy change = enthalpy change - temperature × entropy change** **Unit: J** At spontaneous process: ∆G < 0 (-ve At non-spontaneous process: ∆G > 0 (+ve)

Answer 22

It represents the maximum amount of energy within a thermodynamic system that is available to perform useful work under constant temperature and pressure conditions; essentially, the "free" part refers to the usable energy that can be extracted from a system to do work, not that the energy is "free" in the sense of costing nothing.

Answer 23

The second law of thermodynamics states that **the total entropy of an isolated system always increases over time**, meaning that in any spontaneous process, the disorder of the universe will always increase, and a system naturally moves towards a state of maximum entropy; it can never decrease in an isolated system. It explains why spontaneous exothermic reactions are so common. In exothermic reactions, heat released by the reaction increases the disorder of this surroundings and overall entropy change is positive which makes the reactions spontaneous.

Answer 24

**The entropy of any pure crystalline substance approaches zero as the temperature approaches absolute zero. This is called the third law of thermodynamics**. This is also because there is perfect order in a crystal at absolute zero. **This statement is confined to pure crystalline solids because theoretical arguments and practical evidences have shown that the entropy of solutions and super cooled liquids is not zero at 0K.**

Answer 25

Gibbs/Free Energy Change (∆G)

Answer 26

In reversible reactions, reactions in both the directions should proceed with the decrease in free energy, which seems impossible. It is only possible if at the equilibrium the free energy of the system is minimum, if it is not, the system would spontaneously change to configuration of lower free energy. Hence, ∆G ≈ 0 or ∆G = 0

Thermodynamics Flashcards

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