U5: Kinetics

Kinetics

Rate of Reaction

○ Speed at which a chemical reaction occurs

○ Average rate = change in concentration / change in time

Instantaneous Rate

○ Rate at a specific moment in reaction

○ Rates tends to decrease as reaction continues

○ Initial rate is the reaction rate when reaction first begans

Activation Energy

○ Minimum energy needed for a reaction to occur

○ Lower activation energy = faster rate

○ f = e-Ea/RT

■ f= fraction of molecules with energy equal to or greater than Ea

■ R = Gas Constant

■ T = absolute temperature

Arrhenius Equation

○ Use it to show the effect of change in temperature on the rate constant and the rate reaction

○ Ex. If the rate constant doubles, so does the rate of reaction

Rate Law

○ rate = k[𝐴]m[𝐵]n

■ k = rate constant, changes w/ temperature (units of rate / units of concentration) 

■ exponents m and n = reaction order

■ Shows how concentration affects rate (0 = no change when concentration changes)

■ Determined through experiment, not predicted from equation or coefficients

○ First Order Reaction

■ Reaction where rate depends on concentration of a single reactant

■ ln[A]t = -kt + ln[A]0

■ y = mx + b

■ Only needs 3 quantities to solve for the 4th

■ Ex. For a first order reaction involving popcorn, 6 kernels pop every 5 seconds when there are 150 kernels. How long until 75 of the kernels pop?

● Given:

○ ln[A]t = -kt + ln[A]0

○ k = rate / concentration

○ [A]t = 75

○ [A]0 = 150

■ rate = 6/5

■ concentration = 150

■ k = 6/5 / 150 = 1 / 125

■ ln (75) = -1/125 t + ln (150)

■ ln (75) - ln (150) = -1 / 125 t (t = 87 sec)

○ Second Order Reaction

■ Reaction where rate depends on concentration of 2 reactants

■ 1/[A]t = kt + 1/[A]0

■ Ex. For a second order reaction, the rate constant is 25 L/mol - s at 20 C.

Find the time it takes for the concentration to go from 0.025 M to 0.010 M

● Given:

○ 1/[A]t = kt + 1/[A]0

○ k = 25

○ [A]t = 0.010

○ [A]0 = 0.025

● 1 / 0.01 = 25t + 1 / 0.025

● 25t = 100 - 40 = 60

● t = 60/25 = 2.4 sec

● Collision Theory

○ In order for a reaction to occur, reactant particles must collide

○ Colliding particles must be oriented correctly

○ Collisions must have enough energy to react (activation energy)

○ Increasing temperature causes particles to move faster, which causes more energy and more frequent collisions, and ‘k’ increases with temp., catalyst, surface area

Factors affecting Rate of Reaction

○ Temperature

■ When it is increased, the molecules bounce around a lot more, which means there is more energy and more likely to collide

■ When it is lowered, the molecules are slower and move less

○ Concentration

■ If there is more of a concentration, there is a greater chance that the molecules will collide and have a faster rate of reaction

■ If there is less of a concentration, there is a lesser chance that the molecules will collide and it will have a slower reaction rate

○ Pressure

■ When pressure is increased, molecules have less space to move. That greater density allows it to increase the number of collisions

■ When pressure is decreased, molecules don’t hit each other as often and rate of reaction decreases

■ Pressure is correlated with volume and concentration. Decreasing the volume available to the molecules, increases the concentrations of molecules in a specific place

■ Changing pressure works for gases only, reaction rate for solids or liquids don’t change

○ Catalysts

■ Help reactions go faster without being consumed

■ Provide a different pathway to products and lower the reaction’s activation energy

■ Increase “k”

○ Inhibitors

■ Exact opposite of catalysts, slow or stop the rate of reaction

■ Use an inhibitor to make the reaction more controllable

■ Without inhibitors, some reactions can keep going on and on, which means all molecules would be used up and that would be very bad.

■ 2 types of inhibitors: Competitive and Non-competitive

○ Competitive Inhibition: the binding of the inhibitor to the active site of the enzyme

■ Similar to the structure of the substrate

■ Molecules compete with substrate to bind with active sites

■ Also increases the substrate concentrate

○ Noncompetitive Inhibition: the binding of the inhibitor to the enzyme at any point other than the active site

○ It causes distortion in size/shape of the active site on the enzyme

○ Have a different structure than the substrate

■ It doesn’t change the substrate concentration

○ For ex. Ethanol is a competitive inhibitor of methanol to alcohol dehydrogenase

■ Ethanol competes with methanol for the active site

■ When ethanol is there, less methanol binds to alcohol dehydrogenase active sites

Reaction energies

○ During chemical reactions, chemical bond break or form

○ Exothermic reactions

■ Heat is absorbed

■ Occurs when a chemical bond forms

■ Delta H is positive

■ Ex. Heat is absorbed from the pan to cook food

○ Endothermic reaction

■ Heat is released

■ Occurs when chemical bonds are broken

■ Delta H is negative

■ Ex. Combustion

Equilibrium

○ Two reactants combine to make a product

○ Products like to break apart and turn back into reactants

○ There is a point where reactions look like its finished but its not because some of the reactants are turning into products and some products are breaking back into reactants

○ The overall reaction is happy and equal

○ Reached by itself with no outside forces

○ A system at equilibrium has neutral charge because all the positive and negatives cancel out and equal 0

○ K is the equilibrium constant

○ Equilibrium is reached when molecules are left alone but that can change when something happens to the molecules

■ New molecules or substance are added that isn’t a part of the main reaction

■ Temperature, Concentration, Pressure, Volume is changed