1.5 Kinetics Flashcards
The reaction of magnesium w/ dilute hydrochloric acid is exothermic
Use your understanding of collision theory to explain why the student did not obtain a straight line.
In an investigation of the rate of reaction between hydrochloric acid and pure magnesium, a
student obtained the following curve
- As concentration increases the amount of heat given out increases / temperature increases
- More successful collisions or reactions in a given time OR more particles have the activation energy (M2)
- (An increase in temperature or more heat given out) increases the rate of a reaction
Magnesium ribbon reacts with hot water. Heated magnesium ribbon reacts with steam. State two differences between these reactions
- Slower with hot water or faster with steam
- The hot water produces Mg(OH)2 / the hydroxide OR steam produces MgO / the oxide
State what is meant by the term activation energy (2)
- The activation energy is the minimum / least / lowest
- Energy for a reaction to occur / to go / to start
- For a successful / effective collision
In terms of particles, explain why, at a fixed temperature, you might expect the rate of this reaction to double when the concentration of sodium thiosulfate is doubled and the concentration of hydrochloric acid remains the same (2)
- Twice as many / double number of particles
- More / twice / double (effective) collisions (in a given time) / Double / greater / increased collision frequency
State what is meant by the term rate of reaction (1)
- (Measured) change in concentration (of a substance) in unit time / given time
- axes labelled:- y: number (or fraction or %) of molecules (or particles) x: energy (or KE)
- curve starts at origin
- skewed to right
- approaches x axis as an asymptote
- second curve displaced to the left (and does not cross T1 curve for a second time)
- and peak higher
- many fewer molecules
- fewer molecules have E > Ea
- molecules (or particles or collisions) do not have enough energy
- increase the pressure / concentration
- increases the collision frequency
- add a catalyst
- lowers activation energy
State and explain what has happened to the rate of this reaction at point W in Figure 1 (2)
- At W The rate / it is zero
- The magnesium has all reacted / has been used up
In terms of collision theory explain why, at a fixed temperature, the rate of this reaction doubles when the concentration of acid doubles (2)
- Twice / double as many particles (In a given volume)
- Twice / double as many effective / successful collisions (in a given time) / with either sufficient energy / frequency increases
The figure below shows the Maxwell–Boltzmann distribution of molecular energies in a sample of gas
Label the y-axis on the figure above (1)
State why the curve starts at the origin (1)
State what X indicates on the figure above (1)
- Number of molecules (with a particular energy)
- There are no molecules with no energy
- Most probable / common / modal energy
This gas decomposes on heating
Explain why an increase in temperature increases the rate at which this gas decomposes (2)
- an increase in the number/amount/proportion/fraction of molecules with
E ≥ Ea / with activation energy - more molecules have E ≥ Ea / with activation energy
Explain the effect that lowering the temperature would have on the rate of reaction (2)
- Rate of reaction decreases
- Fewer particles will have energy greater than or equal to the activation energy
- Fewer successful collisions in a given time
- Deduce the change that was made to the reaction conditions
- Explain the effect that this change has on the rate of reaction (3)
- The amount of gas present (or number of molecules) has been reduced / or the pressure has been reduced
- Rate of reaction decreases (no mark)
- Particles are spread further apart
- Fewer collisions between gas particles so fewer successful collisions
Maxwell.Boltzmann
State why the curve starts at the origin (1)
- There are no molecules / particles with zero energy
With reference to the Maxwell–Boltzmann distribution, explain why a decrease in temperature decreases the rate of decomposition of this gas (2)
- The rate of reaction decreases as the temperature decreases because
- A decrease in the number / proportion of molecules with E ≥ Ea
- Fewer effective / productive / successful collisions in a given time / given period
Explain with reference to both curves on the graph how a small change in temperature leads to a large change in the rate of reaction (2)
- (Relative areas under curves indicate) many (owtte) more molecules with E greater than or equal to Ea (at higher T)
- (Large) increase in (number of) successful (owtte) collisions per unit time
State and explain the effect of a higher pressure on the equilibrium yield of methanol (3)
- Yield increases
- More moles/molecules (of gas) on left/fewer on right
- Equilibrium shifts/moves (to right) to reduce pressure/oppose higher pressure
Explain what must happen for a reaction to occur between molecules of two different gases (2)
- collision OR collide
- with the activation energy
- with the correct orientation
Explain why a small increase in temperature has a large effect on the initial rate of a reaction (1)
- much higher proportion of / a lot more / significantly more molecules / particles / collisions with E ≥ Eact/ energy
With reference to the Maxwell–Boltzmann distribution, explain why an increase in temperature increases the rate of a chemical reaction (2)
- Increase in the number/proportion of molecules with E ≥ Ea
- More effective/productive/successful collisions
Explain how a catalyst increases the rate of a reaction (2)
- Catalysts provide an alternative route/pathway/mechanism
- that has a lower activation energy
Explain the process that causes some molecules in this sample to have very low energies (2)
- Collisions
- Cause some molecules to slow down or lose energy
State which one of X, Y or Z best represents the mean energy of the molecules (1)
Z
Explain why, even in a fast reaction, a very small percentage of collisions leads to a reaction (1)
- Only a small percentage/very few collisions have E >Ea
Other than by changing the temperature, state how the proportion of successful collisions between molecules can be increased. Explain why this method causes an increase in the proportion of successful collisions (1)
- Add a catalyst
- Lowers Ea
- More collisions/molecules have energy > Ea
Explain, in terms of collision theory, the effect of increasing the concentration of hydrogen peroxide on the rate of reaction (2)
- more particles/molecules/reactants/H2O2/moles in given/same volume/space
- successful collisions are more frequent
Give the meaning of the term activation energy (2)
- The activation energy is the minimum / least / lowest energy
- for a reaction to occur / to go / to start
- for a successful / effective collision
Suggest one reason why a solid catalyst for a gas phase reaction is often a powder (1)
- Increases the surface area
