- Rate -delta[R]/delta t = +delta[P]/delta t mol L-1 s-1 fast explosion slow rusting milk spoils fast summer slow fridge average long time instantaneous tangent speedometer.
- Rate law rate=k[A]^m[B]^n experimental m order A n order B overall m+n can be 0 zero independent 1 2 fractional negative taste test experimental molecularity theoretical elementary step colliding molecules 1 unimolecular 2 bimolecular 3 trimolecular integer only.
- Zero order rate=k constant [A]=-kt+[A]0 straight [A] vs t slope -k t1/2=[A]0/2k depends [A]0 leaking tap constant first order rate=k[A] ln[A]=-kt+ln[A]0 [A]=[A]0 e^-kt ln[A] vs t straight slope -k t1/2=0.693/k constant independent radioactive decay crowd leaving proportional more people faster.
- Pseudo first order second order but one excess water 55.5 M constant appears first order true 2 apparent 1 ester hydrolysis water excess cane sugar inversion C12H22O11+H2O glucose fructose teacher infinite question papers.
- Temperature rate doubles per 10C Arrhenius k=A e^-Ea/RT ln k=ln A -Ea/RT log k=log A -Ea/2.303RT graph ln k vs 1/T slope -Ea/R intercept ln A Ea barrier mountain activated complex high unstable higher mountain slower cooking faster high flame.
- Collision theory effective collisions need energy >=Ea and proper orientation p steric 0-1 Z frequency e^-Ea/RT fraction energy effective=p Z e^-Ea/RT k=p Z e^-Ea/RT A=pZ orientation head-on not side cars collide need speed angle handshake right hand.
- Catalyst speeds up not consumed lowers Ea alternative path shortcut tunnel guide returns homogeneous same phase acid H+ heterogeneous different phase solid Fe gas N2 H2 Haber enzyme biological protein specific 37C lock key does not change equilibrium only faster both forward reverse yeast bread MnO2 H2O2 Pt catalytic converter CO to CO2.
- Applications food fridge low T rate slow milk slow fridge fast outside freezer slower medicine expiry first order half-life cool dry Haber N2+3H2 2NH3 high P high T Fe catalyst Mo promoter optimum rate vs equilibrium combustion fast high T exothermic.
Chapter 3 - Chemical Kinetics
Beginner
Topper
1. Rate of Reaction - How Fast?
Some reactions fast like explosion firecracker in second, some slow like rusting iron months, some very slow like diamond to graphite years. Rate tells how fast concentration changes.
Rate = - change in reactant concentration / time = + change in product / time. Negative for reactant because decreases, positive for product increases. Unit mol L-1 s-1.
Average rate over long time, instantaneous rate at particular moment slope of concentration vs time graph tangent.
Daily: milk spoils faster in summer heat slow in fridge, food cooking faster on high flame, iron rusts faster near sea salty water.
Visual Learning Method: Fast explosion firecracker, slow rusting iron, graph concentration vs time decreasing reactant increasing product tangent slope instantaneous.
Daily Analogy: Rate like speed of car. Fast car explosion covers distance quickly, slow car rusting takes months. Speedometer shows instantaneous speed at moment tangent.
Rate of Reaction - How Fast?
Revision Points - Easy:
- Rate = change in concentration / time.
- Reactant negative because decreases, product positive increases.
- Fast explosion, slow rusting, very slow diamond to graphite.
- Instantaneous rate slope at that moment.
2. Rate Law & Order - Who Controls Speed?
Rate law experimental expression rate = k [A]^m [B]^n where k rate constant depends on temperature, m order with respect to A, n order with respect to B, overall order = m+n. Order found by experiment not from balanced equation.
Example rate = k [A]^1 [B]^2 overall 3 third order. Order can be 0 zero rate independent of concentration, 1 first, 2 second, even fractional 1.5 or negative means rate decreases with concentration.
Molecularity is theoretical number of molecules colliding in elementary step single step mechanism always small integer 1 unimolecular 2 bimolecular 3 trimolecular rare. Only for elementary steps.
Key difference: order experimental for overall reaction can be 0,1,2,fractional,negative, molecularity theoretical for elementary step always integer 1,2,3.
Visual Learning Method: Rate law formula rate=k[A]^m[B]^n, order m+n experimental, molecularity theoretical elementary step A+B product bimolecular.
Daily Analogy: Order like recipe taste test experimental you taste to know how much salt matters. Molecularity like number of people actually colliding to cook theoretical. Taste test experimental vs actual people colliding theoretical.
Rate Law & Order
Revision Points - Easy:
- Rate law rate=k[A]^m[B]^n found by experiment.
- Order m+n sum of exponents can be 0,1,2,fractional,negative.
- Molecularity molecules colliding in elementary step 1,2,3 integer only.
- Order experimental overall, molecularity theoretical elementary.
3. Integrated Rate Laws - Zero, First Order - Half-life
Rate law gives rate at moment, integrated law gives concentration after time.
Zero order: rate = k constant independent of concentration. Integrated [A]= -k t + [A]0 straight line concentration vs time slope -k. Half-life time for half reactant t1/2=[A]0/2k depends on initial concentration more initial more half-life. Example photochemical reactions, enzyme saturated.
First order: rate = k[A] proportional to concentration. Integrated ln[A]= -k t + ln[A]0 or [A]=[A]0 e^-kt. Graph ln[A] vs t straight line slope -k. Half-life constant t1/2=0.693/k independent of initial concentration same always. Like radioactive decay half-life constant, 50% decays each half-life.
Daily: zero order like leaking tap constant rate independent of water level, first order like crowd leaving hall more people faster leaving rate proportional to number.
Visual Learning Method: Zero order [A] vs t straight line slope -k half-life [A]0/2k depends on initial, first order ln[A] vs t straight line slope -k half-life 0.693/k constant independent.
Daily Analogy: Zero order like fixed salary spending constant rate independent of bank balance. First order like interest proportional to balance more balance more interest rate.
Integrated Rate Laws - Zero First Order
Revision Points - Easy:
- Zero order rate constant independent of concentration [A]=-kt+[A]0 t1/2=[A]0/2k depends on initial.
- First order rate proportional concentration ln[A]=-kt+ln[A]0 t1/2=0.693/k constant independent.
- Graph zero [A] vs t straight line, first ln[A] vs t straight line slope -k.
4. Pseudo First Order - When One is in Excess
Sometimes reaction is actually second order but one reactant in huge excess its concentration almost constant so appears first order called pseudo first order.
Examples: hydrolysis of ester CH3COOCH3 + H2O -> CH3COOH + CH3OH water excess 55.5 M almost constant rate = k' [ester][water] but water constant so rate = k[ester] pseudo first order. Similarly inversion of cane sugar C12H22O11 + H2O -> glucose+fructose water excess, hydrolysis of methyl acetate.
So order appears 1 but true order 2 if both considered. Experimental rate law uses excess condition simplifies.
Visual Learning Method: Ester hydrolysis water excess 55.5 M constant rate=k[ester], cane sugar inversion glucose fructose, true second apparent first.
Daily Analogy: Pseudo first order like exam where one teacher has infinite question papers excess, rate depends only on students ester not teacher water because teacher infinite.
Pseudo First Order
Revision Points - Easy:
- Second order but one reactant huge excess concentration constant appears first.
- Examples ester hydrolysis, cane sugar inversion water excess 55.5 M.
- True order 2 apparent order 1 pseudo first order.
5. Temperature Dependence & Arrhenius Equation - Heat Speeds Up
Food cooks faster on high flame milk spoils faster summer. Rate increases with temperature generally doubles every 10 C rise.
Arrhenius equation k = A e^-Ea/RT where k rate constant A pre-exponential frequency factor Ea activation energy barrier energy needed to cross R gas constant T Kelvin. Ea in J/mol.
Take log ln k = ln A - Ea/RT or log k = log A - Ea/2.303RT. Graph ln k vs 1/T straight line slope -Ea/R intercept ln A. So from slope find Ea.
Activation energy is minimum energy reactants must have to form activated complex high energy unstable state before product. Like mountain to climb before sliding down. Higher mountain higher Ea slower reaction.
Visual Learning Method: Cooking faster high flame, Arrhenius formula k=A e^-Ea/RT, graph ln k vs 1/T slope -Ea/R, activation energy mountain barrier activated complex.
Daily Analogy: Activation energy like mountain between two valleys reactants and products. Need to climb mountain barrier energy before sliding down to product valley. Higher mountain harder slower.
Arrhenius Equation - Temperature Dependence
Revision Points - Easy:
- Rate increases with temperature generally double per 10C.
- Arrhenius k=A e^-Ea/RT.
- ln k vs 1/T straight line slope -Ea/R gives activation energy.
- Ea barrier energy minimum to cross activated complex mountain.
6. Collision Theory - Molecules Must Collide Right
For reaction molecules must collide. Not all collisions give product only effective collisions with enough energy >=Ea and proper orientation.
Collision frequency Z number of collisions per second. Effective collisions = p Z e^-Ea/RT where p steric factor orientation factor 0 to 1 many collisions wrong orientation no reaction, e^-Ea/RT fraction with enough energy Boltzmann factor.
So k = p Z e^-Ea/RT compared to Arrhenius A = p Z.
Daily: two cars collide only effective if enough speed energy and proper angle orientation head-on not side touch. Orientation matters.
Visual Learning Method: Molecules colliding wrong orientation no reaction, proper orientation with energy >=Ea product formed, formula effective = p Z e^-Ea/RT.
Daily Analogy: Collision theory like two people shaking hands need right speed energy and right hand orientation proper not backside. Wrong orientation no handshake reaction.
Collision Theory
Revision Points - Easy:
- Only collisions with energy >=Ea and proper orientation give product.
- Collision frequency Z collisions per second.
- p steric factor orientation 0 to 1.
- Effective collisions = p Z e^-Ea/RT.
7. Catalyst - Helper Without Being Used
Catalyst speeds up reaction without being consumed provides alternative path with lower activation energy Ea. Like shortcut tunnel through mountain lower height to climb.
Homogeneous catalyst same phase as reactants like acid catalysis ester hydrolysis H+ same solution. Heterogeneous different phase like solid catalyst gas reaction Fe in Haber ammonia N2+3H2 Fe catalyst solid gas.
Enzyme biological catalyst proteins in body works at 37 C specific for each reaction. Catalyst does not change equilibrium position or enthalpy only speeds up both forward reverse equally reaches equilibrium faster.
Daily: yeast catalyst in bread fermentation, MnO2 in H2O2 decomposition, Pt in catalytic converter car converts CO to CO2.
Visual Learning Method: Energy diagram without catalyst high mountain Ea, with catalyst lower mountain alternative path, homogeneous same phase, heterogeneous solid Fe gas, enzyme lock key.
Daily Analogy: Catalyst like guide showing shortcut tunnel through mountain lower climb faster but guide returns not consumed. Without guide long climb high Ea.
Catalyst - Helper
Revision Points - Easy:
- Catalyst speeds up without being consumed lowers Ea provides alternative path.
- Homogeneous same phase acid, heterogeneous different phase solid Fe gas.
- Enzyme biological catalyst specific proteins body 37C.
- Does not change equilibrium position only reaches faster.
8. Applications & Final Revision - Daily Kinetics
Food preservation: refrigeration lowers temperature rate slows milk spoils slow in fridge fast outside, freezer even slower. Preservatives slow rate.
Medicine expiry date based on rate of decomposition first order half-life. Must store cool dry.
Industrial: Haber process N2+3H2 2NH3 high pressure high temperature Fe catalyst Mo promoter optimum temperature balances rate vs equilibrium.
Combustion fuel burns fast high temperature high Ea but once ignited exothermic keeps going.
Visual Learning Method: Fridge food slow spoilage, medicine expiry half-life, Haber Fe catalyst high P high T, combustion fast, summary formulas chart.
Daily Analogy: Fridge like slowing time for food molecules move slow collisions less rate slow. Medicine expiry like half-life ticking clock decomposition rate decides shelf life.
Applications & Final Revision
Revision Points - Easy:
- Fridge slows rate food spoilage slow low temperature.
- Medicine expiry first order half-life cool dry storage.
- Haber ammonia Fe catalyst high pressure high temperature optimum rate vs equilibrium.
- Combustion fast high temperature.
Gold Reference Table - All Formulas + Daily Life Visual
| Concept | Formula / Easy Fact | Daily Life Visual / Uses |
|---|---|---|
| Rate | -delta[R]/delta t = +delta[P]/delta t mol L-1 s-1 | Fast explosion slow rusting milk spoils |
| Average vs Instantaneous | Average long time Instantaneous tangent slope | Average speed vs speedometer |
| Rate law | rate=k[A]^m[B]^n experimental | Taste test experimental |
| Overall order | m+n can be 0 1 2 fractional negative | 0 independent 1 proportional |
| Molecularity | Elementary step molecules colliding 1 unimolecular 2 bimolecular 3 trimolecular integer only | Theoretical colliding count |
| Zero order integrated | [A]=-kt+[A]0 [A] vs t straight slope -k t1/2=[A]0/2k depends [A]0 | Leaking tap constant |
| First order integrated | ln[A]=-kt+ln[A]0 [A]=[A]0 e^-kt ln[A] vs t straight slope -k t1/2=0.693/k constant independent | Radioactive decay crowd leaving proportional |
| Pseudo first order | Second order but one excess water 55.5 M constant appears first | Ester hydrolysis water excess cane sugar inversion |
| Arrhenius | k=A e^-Ea/RT ln k=ln A -Ea/RT log k=log A -Ea/2.303RT | Cooking faster high flame mountain barrier |
| Graph ln k vs 1/T | Slope -Ea/R intercept ln A | Find Ea |
| Activation energy | Minimum energy to form activated complex high energy unstable | Mountain climb barrier |
| Collision theory | Effective = p Z e^-Ea/RT p steric orientation 0-1 Z frequency e^-Ea/RT fraction energy | Cars collide need speed and angle |
| k = p Z e^-Ea/RT | A=pZ | Orientation matters |
| Catalyst | Lowers Ea alternative path not consumed homogeneous same phase heterogeneous different phase enzyme biological specific | Guide shortcut tunnel yeast bread MnO2 H2O2 Pt catalytic converter |
| Applications | Fridge low T rate slow Haber Fe high P high T medicine expiry half-life | Food preservation medicine expiry industrial ammonia |
Final Exam Ready Revision - Easy Day-to-Day Hooks: