Topic 9 Kinetics I Flashcards
What is the rate of reaction?
It is the change of concentration/amount of a reactant produced per unit time
Rate = Amount of reactant used or product made / time
What is collision theory?
Particles move all the time and collide into each other too. But most collisions don’t lead to a reaction
- For a reaction to occur the particles must collide in the right direction
- They must also have a minimum amount of KE.
What is Activation energy?
The minimum amount of energy required for a reaction to occur is called the activation energy.
An energy profile diagram can show the energy changes in a reaction. Reactions that have a low activation energy need less energy to break the bonds and vice versa.
What is a Maxwell Boltzmann distribution?
It shows the energy in gas particles
Particles of gas in a sample move at different speeds. Some move slowly, some move quickly. They have different amounts of KE
The area under the curve = the total number of molecules
Particles right at the start of the model have very little energy and move slowly. The peak is the most likely energy of a particle in a sample. Particles with energy greater than activation energy can react
What affects rate?
Temperature affects the rate of reaction.
When we increase temp:
1. Curve sifts right
2. Peak is lower
3. The area under curve is the same
4. Area under curve beyond AE increases
Particles on average have more KE when they are heated. A larger proportion of the molecules will have energy greater than the AE. Larger area under the curve beyond the AE
When we decrease temp:
1. Curve sifts the left
2. Peak is higher
3. Area under curve is the same
4. Area under curve beyond AE decreases
When we decrease temperature, a smaller proportion of the molecules will have energy greater than the AE. Smaller area under the curve beyond the AE
Why do we get a faster rate of reaction when temp is increased?
Particles move around more at higher temperatures. They collide more often and hence the reason why reactions happen faster at higher temps.
The combination of more collisions and more energetic collisions means that small increases in temperatures leads to a large increase in rate
Do concentration and pressure also affect rate?
Increasing pressure will increase the rate of reaction. Particles are closer together and collide more often. There are more frequent collisions and a higher chance of a reaction.
Increasing concentration will increase the rate of reaction. Particles are closer together and collide more often. There are more frequent collisions and a hugher chance of a reaction
What are catalysts?
A substance that increases the rate of reaction by providing an alternative pathway that has a lower AE. The catalyst is chemically unchanged at the end of the reaction.
Catalysts are generally used to speed up a specific reaction. Different catalysts are used for different reactions. They are used to make products faster and can be used to lower the temp required for a reaction. This saves energy and money
What are the two types of catalysts?
Heterogenous:
This is a catalyst that is in a different phase from the reactants. The catalyst is in a different state to the reactants. For example, the Haber process. A solid iron catalyst is used which is a different state to the gaseous reactants. Increasing the SA of the heterogenous catalyst will increase the rate of reaction. More particles can react with the particle at the same time
Homogenous:
This is a catalyst that is in the same phase as the reactants. Generally, homogenous catalysts are aqueous in aqueous reactants. Homogenous catalysts form intermediate species by reactants combining with the catalyst which react to form products. The catalyst is reformed again.
Heterogenous catalyst
Substances adsorb to the surface of solid heterogeneous catalysts
Reactions occur on solid heterogeneous catalysts. They bond with the surface of the catalyst. This is called aDsorption
The bonds in the reactants weaken and break to form radicals. The radicals react with each other to make new substances.
The new molecules are then released from the surface of the catalyst in a process called desorption
Homogenous catalysts energy profile
They have different energy profile diagrams. They have 2 AE. The catalyst lowers the AE so more particles now have enough energy to react
Intermediate is formed at the dip between the 2 AE’s. 2nd AE is required to break up the intermediate
Why use catalysts?
Catalysts are used in industry for the following reasons
1. Lower the temp needed for a reaction to proceed. This means less money spent and less CO2 produced
2. Speed up the reaction by providing an alternative pathway for the reaction to proceed.
3. Change the properties of the product.
There are environmental benefits using a catalyst
1. Lower temperatures and pressures are required. This reduces energy and reduces CO2 produced.
2. Less waste is produced as catalysts can allow scientists to use reactions with better atom economies
Catalytic converters in cars reduce levels of pollution. They are made from rhodium, platinum and palladium alloy.
The reaction is 2CO + 2NO —–> 2CO2+N2
How can rate be measured in experiments?
How long it takes precipitates to form:
Place a cross on paper and time how long it takes for the cross to disappear
The problem is that it is difficult to know when, exactly the cross disappears. Try to use the same observer to reduce errors
Amount of mass lost:
For reactions that produce a gas - place reaction on balance and measure the mass lost as gas is lost.
Fairly accurate method however use a fume cupboard if gas is harmful or toxic
The volume of gas produced:
Another way of measuring the rate of a gas produced. Measure the amount of gas produced using a gas syringe. Measure this over a specific time
Calculating rate from a graph (straight line)
Rate can be found from the gradient
change in y / change in x
The bigger the section of graph you use to work out the gradient better.
Calculating rate from graph: (curve)
Rates can be found from the gradient of a tangent on a curved line of best fit
- With curves you just draw a tangent
- Extend the line right across the graph and work out the gradient as mentioned before.
- The gradient is the rate at the specific point on the curve
