Thermodynamics And Kinetics Flashcards
Thermodynamics
Study of energy transfer and change
Can be used to predict how much energy a given process requires
Thermodynamic functions are based on probabilities and are valid only for systems composed of large number of molecules
Cannot usually be applied to specific microscopic phenomena such as a single collision between two molecules
Kinetics
Study of reaction rates and mechanisms
Can explain how essential chemical reactions can happen within milliseconds with the help of enzymes in the cell
Can be used to predict the rate of a chemical reaction
Often provides insight into the mechanism of the reaction at the molecular level
What can average speed of microscopic atoms and molecules contained in a space tell us?
Manifests as macroscopic observable properties such as pressure, temperature, and volume
What are the three categories of thermodynamic systems?
Open, closed, and isolated
System definitions are based on mass and energy exchange within the surroundings
Open: Can exchange both mass and energy with surroundings
Closed: can exchange energy, but not mass
Isolated: Cannot exchange energy or mass
What are two types of properties which can be used to describe the macroscopic state of a system?
Extensive: proportional to size of system (Volume, n moles)
Intensive: independent of size of system (Pressure P, Temperature T)
If two identical systems are combined and a property is the same for both the single and combined systems, then intensive
If property doubles when systems are combined, then extensive
Dividing one extensive property by another gives an intensive property
Temperature
Represents the amount of molecular movement in a substance
Motion of molecules divided into translational, rotational, and vibrational energies, which together describe overall energy of molecular motion
Sum of these energies is the thermal energy
Increase in thermal energy increases temperature
Temp can be thought of as the thermal energy per mole of molecules
- So temp is intensive property
- Combine two systems and thermal energy and moles double, temp same
How does the temperature of a gas or liquid relate to the translational kinetic energy of its molecules?
Directly proportional relationship
Translational motion
One of three energies of molecular motion
Can be divided into three degrees of freedom, or modes: 1. Along the x-axis, 2. Along the y-axis, and 3. Along the z-axis
Equipartition theory states that in a normal system each mode of motion will have the same average energy
- Energy of each mode is equal to 1/2 kT, k is Boltzmann’s constant (1.38 x 10^-23 J/K)
Boltzmann constant
Symbol k 1.38 x 10^-23 J/K Related to ideal gas constant R by Avogadros’ number N_A R = N_A k OR k = R/N_A
What is the average kinetic energy of a single molecule in any fluid? A mole of molecules in a fluid?
KE_molecule = 3/2 kT
KE_mole = 3/2 RT
T is temperature in Kelvin
Absolute Zero
Lowest possible temperature
-273 deg C
Or 0 deg K
Temp at which no molecular movement
At a pressure of 1 atm what temperatures does water freeze and boil at?
Freezes at 0 deg C or 273 K
Boils at 100 deg C or 373 K
What does Standard temperature and pressure refer to? What does standard ambient temperature and pressure refer to?
Stand temperature and pressure (STP): pressure of 1 atm, temperature of 273 K
Standard ambient temperature and pressure (SATP): pressure of 1 atm and temperature of 298 K (25 deg C)
What does the graph of volume vs. temperature look like for any given fixed pressure?
Exactly linear for any given pressure, but different slope
Higher slope for lower pressure, lower slope for higher pressure
Extrapolate line back to x intercept to get absolute zero temperature of 0 K or 273 deg C
What are the different ideal gas constants, R?
R = 0.0821 L atm / mol K R = 8.314 J atm / mol K
Pressure
Proportional to random translational kinetic energy of a group of molecules per volume occupied
At microscopic level, pressure results from molecules pushing against their container as they move randomly, exerting force that causes the container to stretch until equilibrium is reached through an opposing force
How does the rate of a given reaction usually compare to the frequency of collisions?
Rate of given reaction is usually much lower than frequency of collisions
Most collisions do not result in a reaction
Relative kinetic energies of a colliding compound must be greater than or equal to a threshold energy called activation energy
Atoms of both molecules must align in a specific way for collision to result in a reaction
- When molecules do not properly align, no reaction occurs, even if particles have sufficient kinetic energy to overcome activation energy
Arrhenius equation
k = zpe^{-Ea/RT}
Where z: collision frequency, p: fraction of collisions having effective spatial orientation (steric factor), and e^{-Ea/RT} fraction of collisions having sufficient relative energy
K is rate constant of reaction
Often written as k = A e^{-Ea/RT}
Increase in activation energy reduces value of k
Increase in either z or p (A) increases k
Value of rate constant is affected by pressure (relevant for gases), presence of catalysts, and temperature
How does increasing temperature affect the rate of a reaction?
Increases the number of possible collisions with sufficient activation energy, therefore increases the rate of a reaction
Higher temperature increases rate of forward and reverse reactions
Reaction rate
Describes how quickly the concentration of the reactants or products are changing over the course of the reaction
Rates are most often presented in units of molarity / s (M /s or mol / L s) because represent change in concentration of the reactants and the products over time
Concentration of substances can affect rate of reaction
Elementary reaction
Reaction that occurs in a single step
Stoichiometric coefficients of elementary equation give the molecularity of reaction (number of molecules that need to collide at one time for reaction to occur)
Molecularity of Reaction
Number of molecules that need to collide at one time for a reaction to occur
E.g. given elementary reaction aA + bB -> cC + dD
Molecularity is a + b (if both are 1, then bimolecular)
Most common molecularities are unimolecular, bimolecular, and termolecular
Most reactions represent the sum of multistep reactions
Average reaction rate over time for example: rate = - delta [A] / a t = - delta [B] / b t = delta [C] / c t = delta [D] / d t
Intermediates
Species that are products of one step and reactants of a later step in a multistep reaction
Get used up before end of reaction, so not shown in overall chemical equation
Often present in low concentrations
If sufficiently low, could pretend a reaction is elementary as an approximation for rate law
How do reverse reactions complicate calculation of reaction rates?
In early reaction, when reactants are high and concentration of products is zero, formation of products may have a different rate than later on, when there is a reverse reaction going to form reactants from the products
Rates rely on the concentration of reactants, so complicated
Tend to determine reaction rates based only on concentrations observed by experimenter in initial moments of reaction
Rate Law
Equation for reaction rate which incorporates only the concentration of reactants
Rate_forward kf [A]^{alpha} [B]^{beta}
Where kf is rate constant for forward reaction, alpha and beta are reaction order of each reactant, and sum of alpha + beta is overall order of reaction
If reaction is elementary then alpha = a and beta = b (stoichiometric coeffs), if not then alpha and beta must be determined experimentally
Remember that rate constant kf is not the rate of the reaction (proportional)
Reaction Order
Indicates how changes in the reactant concentrations influence the reaction rate
Order of each reactant indicates the particular influence of that reactant, while order of overall reaction provides more general information about relationships btwn reactant concentrations and reaction rate
Order of overall reaction is sum of exponents in rate law
Exponent of reactant in the rate law is reaction order for one reactant
What’s an example of a zero-order reaction in biology?
Occurs in enzyme-catalyzed reactions when concentration of substrate far outweighs concentration of enzyme
All enzyme catalytic sites are saturated and addition of additional substrate has no effect on reaction rate
Or multistep reaction where that particular reactant is in fast step of reaction
Rate-determining step
Rate of slowest elementary step determines rate of overall reaction
If second step in reaction, the first step also contributes (formation of reactants for slow step)
Catalyst
Substance that increases the rate of a reaction without being consumed or permanently altered
Increase rate of both the forward and reverse reactions by providing an alternative reaction mechanism that competes with uncatalyzed mechanism
- Can enhance product selectivity and reduce energy consumption
- May lower activation energy
- May increase steric factor
Most work by decreasing activation energy
Does not change equilibrium ratio of products and reactants
Types of catalysts
Heterogeneous or homogeneous
Heterogeneous: different phase than reactants and products (gas or aqueous particle react on a solid)
- particles can stick to or adsorb to surface of solid (IMFs)
- Rate of catalysis depends on strength of attraction btwn reactant and catalyst
- Rates can be enhanced by increasing SA of catalyst (grind solid into powder)
Homogeneous: Same phase as reactants and products (gas or liquid)
- Aqueous acid or base solutions
Auto-catalysis
Product of reaction acts as a catalyst for the reaction
E.g. Acid-catalyzed hydrolysis of ester, where product of carboxylic acid creates more acid to further catalyze reaction
Rate law of catalyst
Reactions with catalysts require separate rate constants
Total rate is given by sum of rates of both reactions (catalyzed and uncatalyzed)
Sometimes rate law includes conc of catalyst if catalyst in small concentration in reaction
E.g. uncatalyzed, rate = k0 [A]
Catalyzed, rate = kH+ [H+] [A]
Total rate law, rate = k0[A] + kH+ [H+][A]
Biological example of a catalyst
Enzyme: protein catalyst that speeds up almost every chemical reaction in human body
Enzymes mostly far more effective than catalysts found in lab due to specificity
Turnover rate: Number of reactions occurring at a single active site on one enzyme ~1000 / second
- Can be tens of thousands of times faster
How does a solvent effect the rate of a reaction?
Liquid molecules have 100 x more collisions / s than gas, but most collisions w/ solvent, so do not lead to reaction
Rate constant in liquid is function of both solvent characteristics and temperature
Solvent characteristics:
- dielectric: electrical insulation of reactants, making more stable
- solvation: reactants spread out and surrounded by solvent, decreasing collisions
- viscosity: reactants get trapped in solvent cages (cage effect), making rate of collisions of reactants equal to rate of collision of reactants in gas
Stirring or shaking liquids increases number of collisions
State
Physical condition of system as described by a specific set of thermodynamic properties
Macroscopic state of any one-component fluid system in equilibrium can be described completely by just three properties and all other properties can be derived
- at least one property must be extensive
State functions
Properties that describe the current state of a system Thermodynamic state functions are macroscopic properties of a system Internal energy (U), Temperature (T), Pressure (P), Volume (V), Enthalpy (H), Entropy (S), Gibbs energy (G)
