Collision theory, activation energy, Maxwell-Boltzmann distributions β master the science of how fast reactions go! π₯
The rate of reaction is the change in quantity of a reactant or product per unit time.
rate = change in quantity / time taken| Method | What is Measured | Example Reaction |
|---|---|---|
| Gas syringe | Volume of gas produced over time | Mg + 2HCl β MgClβ + Hββ |
| Balance (mass loss) | Decrease in mass as gas escapes | CaCOβ + 2HCl β COββ + ... |
| Colorimetry | Change in colour intensity | Bromine water decolourising |
| Turbidity (cross test) | Time for cross to disappear | NaβSβOβ + HCl β sulfur precipitate |
For a reaction to occur, reacting particles must:
| Factor β | Effect on Rate | Collision Theory Explanation |
|---|---|---|
| Temperature | β Significantly | More KE β faster particles β more frequent collisions AND much greater proportion have energy β₯ Eβ |
| Concentration (aq) | β | More particles per volume β more frequent collisions per second |
| Pressure (gases) | β | Particles closer together β more frequent collisions |
| Surface area (solid) | β | More exposed solid particles β more collision sites |
| Catalyst | β | Provides alternative pathway with LOWER Eβ β greater fraction of particles can react |
Shows the spread of kinetic energies among particles at a given temperature. The area under the curve to the right of Eβ = fraction of particles that can react.
| Calculation | Formula | Example |
|---|---|---|
| Average rate | rate = Ξvolume / time (or Ξmass / time) | 48 cmΒ³ in 24 s β 2.0 cmΒ³/s |
| Initial rate | Gradient of tangent at t = 0 | Draw tangent to curve at origin; rise Γ· run |
Adjust temperature and concentration. Watch particles speed up and collide. Flashing collisions are successful (energy β₯ Eβ)!
Click to flip!
Problem: Excess marble chips react with 50 cmΒ³ of 2 mol/dmΒ³ HCl. In the first 10 seconds, 24 cmΒ³ of COβ is collected. After 60 seconds, total volume = 96 cmΒ³ and the reaction stops.
Rate = volume / time = 24 cmΒ³ / 10 s = ?
The steepest part of the volume-time graph is where rate is highest. Is this at the beginning or end? Type "beginning".
Marble is in excess. Which reactant ran out? Type its formula.
Higher temp β faster rate. But the amount of HCl is unchanged. Will the TOTAL volume of COβ produced be more, less, or the same? Type one word.
Click a term, then its match. Green = correct!
1 Method 1: Collect COβ in a gas syringe β record volume at regular intervals β; plot volume vs time β steep initial gradient that becomes less steep and eventually flat β
2 Method 2: Place flask on a balance β record mass at regular intervals β; mass decreases rapidly at first, then more slowly, then remains constant β
3 In both cases: reaction is fastest at the start (highest [HCl]) and slows as HCl is used up. β
1 Higher temperature β particles have greater kinetic energy and move faster. β
2 Collisions are more frequent. β
3 More importantly, a much greater proportion of collisions have energy β₯ activation energy (Eβ). β
4 Therefore more successful collisions per second β rate increases significantly. β
1 A catalyst provides an alternative reaction pathway with a lower activation energy (Eβ). β
2 On the Maxwell-Boltzmann curve, the distribution stays the same shape (temperature is unchanged). β
3 The Eβ line moves to the LEFT (lower value). β
4 A larger area under the curve now lies to the right of the new Eβ β more particles have sufficient energy β more successful collisions β faster rate. β
1 (a) A yellow/white sulfur precipitate forms, making the solution cloudy β so a cross drawn under the flask can no longer be seen through the solution. β
2 (b) The time taken for the cross to disappear β shorter time = faster rate. β
3 (c) The time decreases (gets shorter) as temperature increases β because higher temperature gives particles more kinetic energy β more collisions with energy β₯ Eβ β sulfur forms more quickly β cross disappears sooner. β
rate = Ξquantity / time
Fastest at t=0; zero when flat
Collide + E β₯ Eβ + orientation
Temperature (biggest effect)
Concentration / Pressure
Surface area / Catalyst
Tempβ: curve shifts right
Catalyst: Eβ shifts left
Both: bigger reactive fraction
NaβSβOβ + 2HCl β Sβ (cloudy)
Shorter time = faster rate
Temp β changes rate only
Total yield set by limiting reactant