A+B → C
Starting materials, enthalpy change, transition state, Activation Energy, products
CH3I + OH→ CH3OH + I-
- favorable reaction
- Can happen 2 different ways Rate = K[OH-] [CH3I]
Only way to know what is happening is by finding the rate of the reaction
Enthalpy change is difference between the reactions (Eab)
Collision Model
- Reactions occur whne the particles involved collide with each other with enough kinetic energy to overcome Activation Energy
- Temperature is a measure of average kinetic energy
- At any temperature some material will have enough kinetic energy to do the reaction
Molecules have to have both the same amount of energy and speed to hit, and have to have the same orientation
Kinetics refers to how fast a chemical reactiono ccurs. In general 4 things determine the rate of reaction
- Physical state of the reactants or how well mixed they are. They have to touch each other to be able to interact. Aqueous solutions usually react the fastest or two Gases depend on rate of mixing or how big a hole is between the containers
- Cocnentration of reactants. Usually as the concentration of reactants the rate of reaction will also increase. More particles = more collisions
- The temperature at which it occurs. At higher temperatures the collisions will occur with more kinetic energy making it more likely to break the initial bonds in the reaction
- The presence of a catalyst or Activation Energy. Catalysts somehow modify the reactants so that breaking the initial bonds in a reaction is easier and takes less energy. Catalysts new undergo an overall reaction themselves, they will always be reformed as part of the reaction
H2O + HCOCl + HBr (catalyst) → HCOOH + HCl
HCOCl + HBr → H2CO+Cl + Br- HC + OH+ Cl + Br - → HBr + HCl + HCOOH
The Br added an extra step to lower the Activation Energy (peak goes down)
ln(k) = -(Ea/RT)+ln(A) k- rate factpr Ea - Activation Energy R - ideal gas constant T - temperature in K A - frequency factor
Surface area is used as an example of mixing, giant ionic crystal vs tiny fragments
- fragments dissolve faster because more contact and more collisions
Reaction Rates
- Measure the change in concentration of reactions and products
- delta molarity rxn/delta T in molarity per second is the rate
All of the rates have coefficients of 1
How to measure rate
- to be able to measure rate of reaction
- track change in concentration of a substance over time
- Calculate change in concentration divided by chabge in time over smallest time intervals possible
- Beer’s Law
- Uses absorption of light to determine the concentration of a particular species in a container
- Relates photon colors to concentrations of solutions
- Shine light through solution, know how much goes in and comes out
- Use equation to turn it into a percentage of how much occured
- If you continually measure absorption vs time you can measure reactions vs time or rate
- Can create graph of concentration vs time
- Decreases over time, so you know which is a reactant
- Calculate slopes to determine the reaction rate, uses calculus but cb gives formula
- Considering general equation:
aA + bB -⇒ cC dD
the rate would be: k[A]^n [B]
for any multi step chemical reaction rate and rate order dont have to be the same
The exponent that the concentration is raised to is known as the order of the reaction for that particle. The order is often the stoichiometric coefficient but not always. You must determine the order empirically from experiments
Negative slope
Reaction order: exponent that concentration is raised to is the reaction order rate = k(x)(x)
Find reactions that are zero, first, or second order
Consider the following general composition reaction
A---⇒ B + C rate = -∆A/∆ t = kA
- differential rate law, do math to get rid of the delta signs
A=-Kt + Ao is the integrated rate law, slope = -K
First order: ∆(A)/∆r = k(A)
- slows down as it proceeds
- Has constant half lifes for the slope
ln(A) is just a straight line down, so first order reaction. Slope is -K when lnA = =Kt + ln(Ao)
Equations are in order on the sheet, 0,1,2 rate law equations
1/x curve for 2nd order equations
- Half life gets longer over time because it slows down faster than materials are lost
Half life is the time it takes for half of any material in the container to undergo the reaction. It is the time when (A) = (A)o /2
Zero order:
Ao/2 = -Kt1/2 + Ao
First order:
ln(A) = -Kt + ln(Ao)
ln(Ao/2) - ln(Ao) = -Kt1/2
-ln(2) = -Kt1/2
t1/2 = ln(2)/K = .693/K
Second order: t1/2 = 1/KAo
2019 ap chem frq number 6
https://apcentral.collegeboard.org/media/pdf/ap19-apc-chemistry-q6.pdf
rate law doesnt include liquids
Catalysis
A catalyst is a subsyance that lowers the Activation Energy to make a reaction go faster
- The catalyst itself isnt used in the reaction
Homogeneous Catalysts
- A catalyst that is in the same phase as the reactants Heterogenous Catalysts
- A catalyst that is in a different phase from the reactants