3.2.2 Flashcards
How does concentration or pressure effect the rate of reactions?
If the concentration or pressure is increased, so is the rate of reaction. This is because there will be more particles per unit volume therefore the particles are closer together so there will be more collisions resulting in more reactions per second.
What is the role of a catalyst?
Something which increases the rate of reaction by providing an alternative pathway with a lower activation energy without being used up in the overall reaction.
What do the types of catalysts mean?
Homogeneous: where the catalyst and reactants are the same phase/ state.
Heterogeneous: where the catalyst and reactants are in different phases/ states.
An example of a reaction with a homogeneous catalyst and their advantages/ disadvantages:
Methanol + ethanoic acid —H+ (e.g. sulphuric acid)—> methyl ethanoate + water
Homogeneous catalysts allow for good contact with the reactants, but will need to be separated from them which may be difficult e.g. the temperatures needed for distillation may destroy them.
An example of a reaction with a heterogeneous catalyst and it’s disadvantages:
Nitrogen + hydrogen —iron—> ammonia
Heterogeneous catalysts need to be very finely divided to provide a big enough surface area.
Why are catalysts used?
Catalysts have great economic importance and increase sustainability by lowering temperatures thereby reducing the cost and energy demand from combusting fossil fuels which result in carbon dioxide emissions.
Catalysed reactions are usually more specific so reduce side reactions thereby increasing the yield and reducing waste which might otherwise be toxic. This also improves the atom economy.
How do catalytic converters function in cars?
Platinum, palladium, and rhodium catalysts are used to convert carbon monoxide which is poisonous into carbon dioxide, nitrogen dioxide which causes low level ozone damage into nitrogen and oxygen, and remove unburnt hydrocarbons which irritate the throat and lungs. They work in three stages:
1. Adsorption: one of the reactants (oxygen) binds to the surface of the catalyst. This weakens it’s bonds thereby lowering the activation energy.
2. The bonds of the reactants break and react to form new ones.
3. Desorption of the products from the catalyst surface.
What are some examples of catalysts?
Fertilisers contain minerals needed for plant growth. The ammonia is industrially manufactured using the Haber process. Nitrogen and hydrogen react in a reversible reaction in the presence of an iron catalyst at high temperatures ~450°C, and high pressures ~200 atm. N2 + 3H2 ⇌ 2NH3. The ammonia is cooled, liquefied, and removed. Any remaining gases can be recycled so it has a 100% atom economy. The catalyst means the reaction can undergo at lower temperatures which reduces costs and makes the wanted reaction more favourable.
Vegetable oil + hydrogen —nickel—> margarine, which can be kept for longer periods of time: Oil seeds are crushed and refined, and then it goes through hydrogenation. The nickel catalyst is regenerated in the reaction. There are further processes like deodorising and packaging to make it more desirable for the customer.
etc.
What is a Boltzmann distribution?
A graph with energy on the x-axis and number of molecules on the y-axis. The area under the graph represents the total number of molecules.
What happens to a Boltzmann distribution if the temperature increases?
The peak moves right and down. You will see that a greater number of particles have energies greater than or equal to the activation energy.
What happens to a Boltzmann distribution if a catalyst is used?
The graph remains the same, but a new Eact can be added further to the left. A greater proportion of particles with have energies greater than or equal to the new activation energy.
How would you measure the rate of a gas producing reaction?
Using a gas syringe. Or you can collect it in a measuring cylinder over water, however the scale is harder to use upside down and if the gas is water soluble like carbon dioxide the results will be incorrect.
You can also measure the loss in mass if gas escapes, the cotton wool allows the gas to leave but keeps any drops of liquid inside the flask.
How would you measure the production of a solid producing reaction?
You could draw an X on a piece of paper below the flask and record the time taken for the opacity of the reactants to reach a point where you can no longer see it.
Or you could shine a light source over it and use a colorimeter.
How could you measure the rate of a neutralisation reaction?
You can add universal indicator and monitor the pH of the solution until it reaches the correct colour. You can also use this method if a reaction produces and acidic or alkaline product.
