9: Kinetics I Flashcards
What factors do the rate of a chemical reaction depend upon
- Collision frequency
- Collision energy
- Activation energy
What does it mean by successful collisions
- Particles collide with the correct orientation
- Particles collide with sufficient energy for the reaction to occur
What is collision frequency
The number of collisions between particles per unit of time in a system in a system
How can collision frequency be altered
- Changing concentration of reactants
- Changing total pressure
- Changing temperature
- Changing size of reacting particles
How does changing the concentration of the reactants affect collision frequency
Increasing the concentration means more particles available to react in the same volume/space leading to more frequent, successful collisions
How does changing the total pressure affect collision frequency
Increasing the pressure means that there will be the same number of particles but in a smaller volume leading to more frequent, successful collisions
How does changing the temperature affect collision frequency
Increased temperature increases the kinetic energy of the reacting particles resulting in more frequent, successful collisions
How does changing the size of the reacting particles affect collision frequency
Decrease in particle size increases the surface area of the reactants meaning there are more particles available to react in the same volume/space leading to more frequent, successful collisions
What are unsuccessful collisions
Collisions that don’t result in a reaction (instead the particles bounce off each other)
What causes unsuccessful collisions
- Colliding species don’t have enough energy to break the necessary bonds
- Not sufficient energy to result in a chemical reaction
Define activation energy
The minimum amount of energy needed to be overcome in order for a reaction to take place
What is the activation energy profile for exothermic reactions
What is the activation energy profile of endothermic reactions
How can the success of a reaction be measured
By measuring the rate of reaction via:
- The amount of reactant lost
- The amount of product formed
- Time taken for precipitate to form/colour change
What are common techniques to measure the rate of reaction
- Mass loss
- Gas production
- Concentration changes
Describe how to measure rate of reaction by mass loss
- If gas is produced, the mass of the reaction will decrease
- Mass is measured every few seconds, and mass over time is plotted
- Mass can also be measured by difference
- Mass loss provides a measure of the amount of reactant
- Limitation is mass must be sufficiently dense, otherwise change in mass is too small
Describe how to measure rate of reaction by volume of gas produced
- Gas produced can be trapped and volume produced over time can measure the rate of reaction
- Gas collection can be done over water or with a gas syringe
- Volume measured every few seconds
- Limitations are gas may be soluble in water, or the gas isn’t dense enough to move the syringe
Describe how to measure the rate of reaction by concentration changes
- Titrations (when quenching reaction) can be used
- More efficient is stopping timer when a visible point is reached (e.g. colour change)
- Sodium thiosulphate and HCl form a yellow precipitate when reacted
- Limitation is the human error of observing when the change has happened
What is a Maxwell-Boltzmann distribution curve
A graph that shows the distribution of energies at a certain temperature
What does this distribution curve show
- A sample of gas showing that some particles have low energy, some have high energy, and most have an energy in between
- The activation energy is also drawn
- Only the particles that meet the activation energy are able to have successful collisions and react
How is the Maxwells-Boltzmann curve changed in an increase of temperature
- Increased temperature means particles gain more energy, and have more frequent successful collisions
- As the proportion of successful collisions increases, it results in a higher proportion of particles with the activation energy to cause the reaction
- This causes the curve to flatten and the peak shifts to the right
Define a catalyst
A catalyst is a substance that increases the rate of reaction by providing the reactants with an alternative reaction pathway which has a lower activation energy than the usual reaction pathway, and the catalyst isn’t included/used up in the reaction
Define a homogenous catalyst
A catalyst in the same phase as the reactants (e.g. reactants and catalysts all in solution)
Define a heterogenous catalyst
A catalyst that is in a different phase to the reactants (e.g. reactants are gases but the catalyst is solid)
Describe the process of production of ammonia by the Haber Process (and equation)
- N2(g) + 3H2(g) -> 2NH3(g)
- Hydrogen and nitrogen obtained from natural gas and air pumped into the compressor
- The gases are compressed to 200 atm
- The pressurised gases reach 450 degrees C, and are pumped into containers filled with solid iron (heterogenous catalyst) where some hydrogen and nitrogen react forming ammonia
- Unreacted hydrogen, nitrogen and ammonia product pass into the cooling tank where ammonia is liquified and removed
- The unreacted H and N are recycled and used again
Why is the temperature 450 degrees C in the production of ammonia
- Higher temperature favours the reverse reaction as its endothermic, so a higher yield of reactants made
- Lower temperature favours the forward reaction as its exothermic, so a higher yield of products made
- BUT this is very slow, so the compromise is 459 degrees C
Why is the pressure 200 atm in the production of ammonia
- Lower pressure favours the reverse reaction, so a higher yield of reactants made
- Higher pressure favours the the forward reaction, so a higher yield of products made
- Higher pressure is dangerous and expensive, so 200 atm is a compromise
Why is a solid iron catalyst used in the production of ammonia
Without a catalyst the required temperature and pressure for the Haber Process would be much higher, which is more dangerous, expensive, and less environmentally friendly
How does the solid iron catalyst work in the production of ammonia
- Diffusion of nitrogen and and hydrogen gas to the iron surface
- Adsorption of the reactant molecules onto iron surface by forming bonds between iron and reactant atoms
- The reaction takes place between the adsorbed nitrogen and hydrogen atoms reacting on the iron surface to form NH3
- Desorption occurs when bonds in NH3 break and then diffuse from the iron surface
What are the benefits of catalysts
- Speed up rate of reaction, so lower temperatures and pressures can be used to
- Saves energy costs due to reduced energy demand
- Less electrical pumping costs due to reduced pressure demands
- Eco friendly as less CO2 emissions
- Increase atom economy by increasing product yield and decreasing waste yield
How do impurities affect the effectiveness of heterogenous catalysts
- Absorb onto catalyst surface and occupy active sites
- Prevent bond weakening in reactants
- Take up surface area on catalyst by forming strong bonds
