Unit 2.2 Rate of reaction Flashcards
How is the rate of reaction usually determined/calculated?
Measure the amount of reactant used up per unit time
Or
Measuring the amount of product made per unit time
What are the mains ways to increase rate/speed of a chemical reaction?
(A level 6 ways)
- Temperature
- SA or particle size
- Catalysts
- Concentrations
- Pressure
- Light
(last 2 are the new ones)
What is the collision theory?
- 2 reactant particles must collide successfully
- To form new product
- An effective collision
- To increase rate of reaction, u must increase number of effective collisions
How may many collisions between reactant particles not form new product?
- Reactant particles didn’t have sufficient energy
to overcome activation energy - Particles collide in wrong orientation
State how increase in concentration will affect the rate of reaction
Increasing the conc. of a solution, or the pressure of a gas, increases the rate of reaction. This is because more reactant particles present in the same volume ∴ there is an increase in number of collision per unit time. There will ∴ be a greater chance of successful collisions
State how increase in temperature will affect the rate of reaction
Increasing temperature increases rate of reaction. This is because the reactant particles have greater KE at higher temps., so they move a lot faster. More of the particles have energy that is greater than the activation energy, so more successful collisions occur per unit time.
How do they increase surface area of a solid reaction?
Cut up or ground into power
State how increase in surface area will affect the rate of reaction
- More of the reactant is exposed
- Larger area for the reaction to take place
- Increases rate of reaction
- More chance of an effective collision
What is a catalyst?
Provides an alternate route with a lower activation energy
Without undergoing permanent change (can be re-used)
State the 4 catalysts used for these 4 reactions:
- Haber process
- Catalytic convertors in cars
- Contact process
- Decomposition of hydrogen peroxide
- Iron
- Platinum
- Vanadium oxide
- Manganese oxide
All transition metals
State how increase in pressure will affect the rate of reaction
- Decreases volume of gas
- While maintaining number of particles
- As they are closer together
- More likely collisions
State how increase in light will affect the rate of reaction
E.g. chlorination of methane or photosynthesis
- Some reactions driven by light
- Light providing energy to either break bonds or produce more of a product
- It’s all the same tbh
Do u know how to draw the changes of product if u change any of the factors within the reaction?
… yes
How do u calculate the rate of a reaction?
Change in concentration/time
moldm-3/s = moldm-32-1
How to calculate rate of reaction in terms of mass?
g/s = gs-1
How to calculate rate of reaction in terms of volume?
cm3/s = cm3s-1
How to establish relationship between reactant concentrations and rate?
(graph)
Well on a real:
At steepest - rate is fastest at start of reaction cuz concentration of reactants are the greatest (not much has been used up)
Line become less steep - rate slows down as reaction proceeds since the concentration of reactants decreases
Horizontal - rate becomes zero when reaction stops (reactants used up)
Define activation energy Ea
Minimum energy required to start a reaction by the breaking of bonds
Explain exothermic reaction
Reaction gives out heat and therefore the energy of the products is less than that of reactants - they have lost energy
Explain endothermic reaction
Reaction takes in heat and therefore the energy of the products is higher than that of the reactants - they have gained energy
Describe exothermic energy profile
Reactants = high
Curve (Ea) = high to low
Therefore △H = negative
Products = low
Ya gotta have known how to visualise this
Describe endothermic energy profile
Reactants = low
Curve (Ea) = low to high
Therefore △H = positive
Products = high
Describe reversible reaction energy profile
If one side is exo, the other side is endo
It’s a reversible reaction after all
Reactants is high (Vertical arrow with Ea forward)
Curve is high to low
Products is low (Vertical arrow on whole thing with Ea reverse)
How to calculate enthalpy change of a reversible reaction energy profile?
△H = Eaf - Eab
(forward reaction - backward reaction)
Exothermic reaction △H = negative
Endothermic reaction △H = positive
How does temperature affect the amount of energy the reactants have?
- Increasing temp = reactants more energy, move faster, ∴ more collisions, statistically more product, produced in a set time, ∴ rate of reaction increases
- More particles have activation energy
Explain the Boltzmann distribution
- It’s a graph.
- Y-axis = number of particles with energy, E
- X-axis = kinetic energy
- Near the ends there’s a boundary:
Ea aka the activation energy
Only the number of particles represented under this section of the graph have sufficient energy to react
Explain the difference of low temperature and high temperature in a Boltzmann distribution
At Low T:
- N° of molecules at it’s highest
- The activation energy is very small near the ends (the area)
At high T:
- The whole line looks like optimum pH level (if u know u know)
- Therefore technically n° not at its highest
- Activation energy is relatively bigger than of low T
Describe the relation at high T within the Boltzmann distribution
As the temperature increases then the proportion of molecules with greater than activation energy also increase, therefore the rate increases
Broken english? wat??
and apparently 2 parts to the answer:
1. The increase in temperature, increases the energy (speed) of the molecules, therefore more collisions, therefore rate increases.
2. A greater proportion of molecules has greater than the required activation energy, therefore more product produced in the same time, therefore rate increases
What are the 2 types of catalysts?
- Homogeneous catalyst
- Heterogeneous catalyst
Explain homogeneous catalysts
The catalyst and reactants are in the same phase:
usually liquid mixtures or substances in solution
An example of a homogenous catalyst?
Enzymes
Benefits of enzymes?
- Lower temp and pressure so lower costs
- Operate in mild conditions so will not harm food or fabrics
- Biodegradable - easy to dispose of
- Often the products are pure with no side-products so no need to separate the unwanted products
1 downside of enzymes?
- Homogeneous catalyst = same phase
- Therefore difficult to separate from the product
(Though in some and in biological reactions, not necessary to remove em)
Difference between enzymes and chemical catalysts?
Enzymes = react under extremely mild conditions (eg body temperature)
Chemical catalysts = often need severe conditions (high temp & pressure)
errr extremely effective at increasing the reaction rate - much more so than industrial catalysts
Explain heterogeneous catalysts?
The catalysts and reactants are in different phases
(the trick = reactants are liquid/gas/aq but the catalyst is a solid soo voila)
- Many are d-block transition metals
- Gases absorbed on surface
- React and then desorb from the surface
- Larger surface area = better catalyst works
Why do industries rely on catalysts?
- Reduces costs
- Lowers activation energy
- Less energy required for molecules to react
- Less electricity needed
- LOWER COSTS
How can colorimetry be used in studies of some reaction rates?
Wait…. I never know I was missing one more topic D:
I’d like to hope that’s it. No way this topic gets any complicated
…. right?
