4.3 Rates of reaction Flashcards
Define activation energy
The amount of energy the particles need to react
What does the overall rate of a reaction involving millions of particles depend on?
- Collision frequency: how often do the particles collide with each other?
- Percentage success: what percentage of the collisions have enough energy
A chemical reaction can only happen if…?
If the reactant particles collide with enough energy.
What happens when particles collide with less energy than the activation energy?
They rebound and remain unchanged
Higher solid surface area increases rate because:
-More of the solid is exposed to the other particles
-So the collision frequency is higher
-So there are more successful collisions per unit time
Higher solution concentration / gas pressure increases rate because:
-There are more particles per unit volume
-So the collision frequency is higher
-So there are more successful collisions per unit time
Higher temperature increases rate because:
-The particles have more kinetic energy
-So the percentage of collisions with enough energy is higher
-So there are more successful collisions per unit time
Catalysts increase rate because:
-They provide an alternative route with a lower activation energy
-So the percentage of collisions with enough energy is higher
-So there are more successful collisions per unit time
How can increased concentration be accomplished?
-by dissolving more solute particles
How can increased pressure be accomplished?
-by making the reaction vessel smaller
What variables affect the rate of reaction between sodium thiosulphate and hydrochloric acid? (disappearing cross)
IV: Concentration of Na2S2O3(aq)
DV: Time taken for the cross to disappear
Control variables:
-Total volume of solution
-Concentration of HCl(aq)
-Temperature
How is the rate of reaction calculated in this experiment? (disappearing cross/magnesium and acid)
Rate (s^-1) = 1 / Time taken (s)
How is the effect of concentration on rate demonstrated in this experiment? (disappearing cross)
-A solid (Sulphur) is slowly produced
-After a while, you can’t see through the mixture any longer.
-Measured by observing a black cross under the mixture.
What do the results show about the relationship between concentration and rate? (disappearing cross)
-The graph shows a straight-line through the origin.
-This is because rate is proportional to concentration:
* Double the concentration = double the rate
* Triple the concentration = triple the rate
* Etc
How is increased surface area accomplished?
By cutting or grinding up the solid
What variables affect the rate of reaction between calcium carbonate (marble chips) and hydrochloric acid?
IV: the sizes of the CaCO3(s) chips
DV: the volume of CO2(g)
Control:
-Mass of CaCO3(s)
-Volume and concentration of HCl (aq)
-Temperature
How is volume of CO2 measured in the marble chip experiment?
-using a gas syringe or inverted measuring cylinder in water
What changes occur when comparing the higher surface area line (red) to the original (black) line?
-Steeper start:
Due to increased rate, CO2 volume increases faster.
-Same final volume:
As reactant amounts remain unchanged, the reaction eventually produces the same amount of CO2.
What variables affect the rate of reaction between magnesium and hydrochloric acid?
IV: the temperature of the HCl (aq)
DV: the time taken for Mg (s) to disappear
Control:
-Mass and surface area of Mg (s)
-Volume and concentration of HCl (aq)
What do the results show about the relationship between temperature and rate? (magnesium and acid)
-Graph is not a straight line (because rate is not proportional to temp)
-Graph doesn’t go through the origin (because rate isn’t zero at 0*c)
What are catalysts?
They are substances (usually solids) which:
* Increase rate
* Remain chemically unchanged
What variables affect the rate of reaction demonstrated using the decomposition of hydrogen peroxide?
IV: which catalyst is being used
DV: volume of O2(g)
Control:
-Volume and concentration of H2O2(aq)
-Temperature
-Mass and surface area of catalyst
What catalyst accelerates the reaction that is normally slow and what colour is it? (hydrogen peroxide)
Manganese oxide (MnO2)- black solid
How long does the reaction take without and with the catalyst?
Without- days
With- only a few seconds
What effect do more effective catalysts have on reaction rate?
More effective catalysts increase the reaction rate more effectively.
What do the results show about the relationship between catalysts and rate?(hydrogen
peroxide)
- Effective catalysts increase reaction rate more efficiently.
- Steeper graph line indicates enhanced rate with catalysts.
- Initial reactant amounts are same, leading to eventual flattening of lines at same O2 volume.
- MnO2 catalysed decomposition of H202 used to produce O2 in the lab
