CHEM204 - Physical Chemistry: Thermodynamics & Kinetics Flashcards

Question 1

What is the First Law of Thermodynamics?

Accepted Answer

The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy cannot be created or destroyed, only transferred or changed from one form to another. Mathematically, it's expressed as $\Delta U = Q + W$, where $\Delta U$ is the change in internal energy, $Q$ is heat, and $W$ is work.

Question 2

Define Enthalpy ($\Delta H$) and its significance.

Accepted Answer

Enthalpy ($\Delta H$) is a thermodynamic property representing the total heat content of a system at constant pressure. It is defined as $H = U + PV$. The change in enthalpy ($\Delta H$) for a process at constant pressure is equal to the heat absorbed or released: $\Delta H = Q_p$. A negative $\Delta H$ indicates an exothermic reaction (releases heat), while a positive $\Delta H$ indicates an endothermic reaction (absorbs heat).

Question 3

State Hess's Law.

Accepted Answer

Hess's Law states that if a reaction can be expressed as the sum of two or more steps, the enthalpy change for the overall reaction is the sum of the enthalpy changes for the individual steps. This allows for the calculation of $\Delta H$ for reactions that are difficult to measure directly.

Question 4

What is the Standard Enthalpy of Formation ($\Delta H_f^\circ$)?

Accepted Answer

The Standard Enthalpy of Formation ($\Delta H_f^\circ$) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states (298 K, 1 atm) under standard conditions. The $\Delta H_f^\circ$ for an element in its standard state is defined as zero.

Question 5

What is the Second Law of Thermodynamics?

Accepted Answer

The Second Law of Thermodynamics states that the total entropy of an isolated system can only increase over time, or remain constant in ideal cases where the system is in a steady state or undergoing a reversible process. It implies that spontaneous processes tend to increase the overall disorder (entropy) of the universe.

Question 6

Define Entropy ($\Delta S$) and its relation to spontaneity.

Accepted Answer

Entropy ($\Delta S$) is a measure of the disorder or randomness of a system. For a reversible process, the change in entropy is given by $\Delta S = Q_{rev}/T$, where $Q_{rev}$ is the heat transferred reversibly and $T$ is the absolute temperature. An increase in entropy ($\Delta S > 0$) generally favors spontaneity, especially for processes that are endothermic.

Question 7

State the Third Law of Thermodynamics.

Accepted Answer

The Third Law of Thermodynamics states that the entropy of a perfect crystal at absolute zero (0 Kelvin) is exactly zero. This law provides a reference point for determining absolute entropies.

Question 8

Define Gibbs Free Energy ($\Delta G$) and its formula.

Accepted Answer

Gibbs Free Energy ($\Delta G$) is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system. Its formula is $\Delta G = \Delta H - T\Delta S$, where $\Delta H$ is enthalpy change, $T$ is absolute temperature, and $\Delta S$ is entropy change.

Question 9

How does $\Delta G$ predict the spontaneity of a reaction?

Accepted Answer

If $\Delta G < 0$: The reaction is spontaneous (exergonic) under the given conditions.
If $\Delta G > 0$: The reaction is non-spontaneous (endergonic) and requires energy input. The reverse reaction is spontaneous.
If $\Delta G = 0$: The system is at equilibrium, and there is no net change.

Question 10

What is the relationship between $\Delta G^\circ$ and the equilibrium constant $K$?

Accepted Answer

The standard Gibbs Free Energy change ($\Delta G^\circ$) is related to the equilibrium constant ($K$) by the equation: $\Delta G^\circ = -RT \ln K$, where $R$ is the ideal gas constant ($8.314 	ext{ J mol}^{-1} 	ext{ K}^{-1}$) and $T$ is the absolute temperature in Kelvin.

Question 11

What is an ideal gas, and what is the Ideal Gas Law?

Accepted Answer

An ideal gas is a theoretical gas composed of randomly moving point particles that do not interact with each other except for elastic collisions. The Ideal Gas Law describes the behavior of ideal gases: $PV = nRT$, where $P$ is pressure, $V$ is volume, $n$ is the number of moles, $R$ is the ideal gas constant, and $T$ is absolute temperature.

Question 12

Define Reaction Rate.

Accepted Answer

Reaction rate is the speed at which a chemical reaction proceeds, typically expressed as the change in concentration of a reactant or product per unit time. For a reaction $aA + bB 	o cC + dD$, the rate can be expressed as $Rate = -\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = \frac{1}{c}\frac{d[C]}{dt} = \frac{1}{d}\frac{d[D]}{dt}$.

Question 13

What is a Rate Law, and what is its general form?

Accepted Answer

A Rate Law (or rate equation) expresses the relationship between the rate of a reaction and the concentrations of its reactants. Its general form for a reaction $A + B 	o Products$ is $Rate = k[A]^m[B]^n$, where $k$ is the rate constant, and $m$ and $n$ are the reaction orders with respect to reactants A and B, respectively. The exponents $m$ and $n$ must be determined experimentally.

Question 14

What is Reaction Order?

Accepted Answer

Reaction order is the exponent to which the concentration of a reactant is raised in the rate law. It indicates how the rate of a reaction is affected by changes in the concentration of that specific reactant. The overall reaction order is the sum of the individual orders with respect to each reactant ($m+n$ in $Rate = k[A]^m[B]^n$).

Question 15

Describe the characteristics of a Zero-Order Reaction.

Accepted Answer

In a zero-order reaction, the rate of reaction is independent of the concentration of the reactant.
*   **Rate Law:** $Rate = k$
*   **Integrated Rate Law:** $[A]_t = -kt + [A]_0$
*   **Half-life ($t_{1/2}$):** $t_{1/2} = \frac{[A]_0}{2k}$ (depends on initial concentration)
*   Plot of $[A]$ vs. time is linear with a negative slope.

Question 16

Describe the characteristics of a First-Order Reaction.

Accepted Answer

In a first-order reaction, the rate of reaction is directly proportional to the concentration of one reactant.
*   **Rate Law:** $Rate = k[A]$
*   **Integrated Rate Law:** $\ln[A]_t = -kt + \ln[A]_0$ or $[A]_t = [A]_0 e^{-kt}$
*   **Half-life ($t_{1/2}$):** $t_{1/2} = \frac{\ln 2}{k}$ (independent of initial concentration)
*   Plot of $\ln[A]$ vs. time is linear with a negative slope.

Question 17

Describe the characteristics of a Second-Order Reaction.

Accepted Answer

In a second-order reaction, the rate of reaction is proportional to the square of the concentration of one reactant, or to the product of the concentrations of two reactants.
*   **Rate Law (single reactant):** $Rate = k[A]^2$
*   **Integrated Rate Law (single reactant):** $\frac{1}{[A]_t} = kt + \frac{1}{[A]_0}$
*   **Half-life ($t_{1/2}$):** $t_{1/2} = \frac{1}{k[A]_0}$ (depends on initial concentration)
*   Plot of $1/[A]$ vs. time is linear with a positive slope.

Question 18

What is the Rate Constant ($k$)?

Accepted Answer

The rate constant ($k$) is a proportionality constant in the rate law that relates the rate of a reaction to the concentrations of the reactants. Its value is specific for a given reaction at a particular temperature and reflects the intrinsic speed of the reaction. The units of $k$ depend on the overall reaction order (e.g., $	ext{s}^{-1}$ for first-order, $	ext{M}^{-1}	ext{s}^{-1}$ for second-order).

Question 19

Define Activation Energy ($E_a$).

Accepted Answer

Activation Energy ($E_a$) is the minimum amount of energy that reactant molecules must possess to undergo a chemical reaction. It represents the energy barrier that must be overcome for reactants to transform into products, typically by forming a high-energy transition state.

Question 20

What is the Arrhenius Equation, and what do its terms represent?

Accepted Answer

The Arrhenius Equation describes the temperature dependence of reaction rates: $k = A e^{-E_a/RT}$.
*   $k$: rate constant
*   $A$: pre-exponential factor (or frequency factor), related to the frequency of collisions and orientation of molecules.
*   $E_a$: activation energy ($	ext{J mol}^{-1}$)
*   $R$: ideal gas constant ($8.314 	ext{ J mol}^{-1} 	ext{ K}^{-1}$)
*   $T$: absolute temperature (Kelvin)

Question 21

How does a catalyst affect a chemical reaction?

Accepted Answer

A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It achieves this by providing an alternative reaction pathway with a lower activation energy ($E_a$), thereby increasing the fraction of molecules that can overcome the energy barrier at a given temperature. Catalysts do not change the overall $\Delta G$ or the equilibrium position of a reaction.

Question 22

List factors that affect the rate of a chemical reaction.

Accepted Answer

1.  **Concentration of Reactants:** Higher concentration generally leads to more frequent collisions and thus a faster rate.
2.  **Temperature:** Higher temperature increases kinetic energy, leading to more frequent and energetic collisions, thus a faster rate.
3.  **Surface Area:** For heterogeneous reactions, increased surface area of solid reactants allows for more contact points, increasing the rate.
4.  **Presence of a Catalyst:** Catalysts lower activation energy, speeding up the reaction.
5.  **Nature of Reactants:** The inherent chemical properties and bond strengths of reactants influence reaction speed.

Question 23

How is the standard Gibbs Free Energy of formation ($\Delta G_f^\circ$) used?

Accepted Answer

The standard Gibbs Free Energy of formation ($\Delta G_f^\circ$) is the change in Gibbs Free Energy when one mole of a compound is formed from its constituent elements in their standard states. It's used to calculate the standard Gibbs Free Energy change for a reaction ($\Delta G_{rxn}^\circ$) using the formula: $\Delta G_{rxn}^\circ = \sum n \Delta G_f^\circ (products) - \sum m \Delta G_f^\circ (reactants)$.

Question 24

Explain the concept of a "state function" in thermodynamics.

Accepted Answer

A state function is a property of a system that depends only on the current state of the system, not on the path taken to reach that state. Examples include internal energy ($\Delta U$), enthalpy ($\Delta H$), entropy ($\Delta S$), and Gibbs Free Energy ($\Delta G$). Work ($W$) and heat ($Q$) are *not* state functions.

Question 25

What is the difference between $\Delta G$ and $\Delta G^\circ$?

Accepted Answer

*   **$\Delta G$:** Gibbs Free Energy change under *any* given conditions (non-standard concentrations/pressures, non-298K temperature). It predicts spontaneity under those specific conditions.
*   **$\Delta G^\circ$:** Standard Gibbs Free Energy change, calculated under *standard conditions* (1 M concentration for solutes, 1 atm partial pressure for gases, 298 K temperature). It predicts spontaneity if the reaction starts under standard conditions and is related to the equilibrium constant $K$.

CHEM204 - Physical Chemistry: Thermodynamics & Kinetics

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