Chapter 10 - Reaction Rates & Equilibria Flashcards
What is the rate of reaction?
Measures how fast a reactant is used up or how fast a product is formed
What is the formula for rate? Units?
Change in concentration/ time
moldm^-3s^-1
What happens to the rate as a reaction progresses?
It decreases as reactants are used up so their concentration decreases
When is the rate the highest?
At the start of the reaction as each reactant is in its highest concentration
What is the collision theory?
Particles need to collide with:
- sufficient energy (to overcome the activation energy barrier)
AND
- with the correct orientation (ensuring the correct part of the molecule collides with the correct part of another molecule)
For successful collisions
What factors cause more successful collisions per unit of time?
- higher concentration
- higher gas pressure
- higher surface area (powder)
- higher temperature
- catalyst
How does higher concentration/gas pressure affect a reaction?
- more particles per unit volume
- particle more crowded
- higher frequency of collisions
- more successful collisions per unit time
How does higher surface area of reactants affect a reaction?
- more reactant particles are exposed and available for collision
- higher frequency of collisions
- more successful collisions per unit time
How does higher temperature affect a reaction?
- particles have higher kinetic energy
- more particles have energy above the activation energy
- particles move faster
- higher frequency of collisions
- more successful collisions per unit time
How does a catalyst affect a reaction?
- provides an alternate route with a lower activation energy
- more particles have energy above the activation energy
- more successful collisions per unit time
What are 2 ways to monitor the rate of a reaction?
- monitor the volume of gas produced at regular intervals - using a gas syringe/upturned measuring cyclic re submerged in water
- monitor the loss in mass over time as a gas released
What is a catalyst?
Speeds up the rate of reaction by providing an alternative reaction pathway of lower activation energy without being used up in the reaction
It may react with the reactant to form an intermediate in a multi-step reaction but will be regenerated by the end of the reaction
What is a homogenous catalyst?
Same physical state as the reactants
The catalyst reacts with the reactants to form an intermediate which breaks down to give the product and the catalyst is regenerated
What are 2 examples of homogenous catalysts?
- Making esters - concentrated sulfuric acid, all reactants and catalyst in liquid state
- Ozone depletion - Cl.(radical), reactant and catalyst both gases
What is the difference between absorption and adsorption?
Absorption = particles are taken in by material
Adsorption = particles are held on the surface
What are heterogenous catalysts?
Have a different physical state from the reactants
How do heterogenous catalysts work?
- tend to be solid in contacts with gaseous or solutions of reactants
- reactant molecules are adsorbed onto the surface of the catalyst where the reaction takes place
- after the reaction, the product molecules leave the surface of the catalyst by desorption
What are examples of heterogenous catalysts in use?
- Making ammonia (haber process): N2 (g) + 3H2 (g) <> 2NH3 (g) with a Fe (s) catalyst
- Hydrogenation of alkenes: C2H4 (g) + H2 (g) -> C2H6 (g) with a Ni (s) catalyst
Why are catalysts good industrially?
Products are made faster and at lower cost from lower use of energy, which increases profitability
Lower energy = less fossil fuels = less greenhouse gas emissions
What speed do molecules generally move with?
Average speed/energy
Some move fast, some move slow
What is a Boltzmann distribution?
The spread of molecular energies in gases expressed as a graph
Where does the Boltzmann distribution start from?
From the origin as no molecules have zero energy
What is the x and y asis?
x axis = energy
y axis = number of molecules
What is the area under the Boltzmann distribution?
The total number of molecules
