Reaction rates Flashcards
Rate
Change in concentration / time
How concentration effects the rate of reaction
An increase in concentration increases the number of particles in the same volume. The particles are closer together and collide more frequently. In a given period of time there will be more effective collisions and the rate of reaction will increase.
Why are some reactions not effective
The particles do not collide at the right orientation. The particles do not have sufficient energy to overcome the activation energy.
Why does increasing the pressure of a gas affect the rate of a reaction
The concentration of the gas molecules increases as the gas molecules are now occupying a smaller volume of space. The gas molecules are closer together and collide more frequently leading to more effective collisions.
How can you measure the rate of a reaction
Measuring the mass loss of reactants. Monitoring the volume of gas produced at regular intervals.
How do you measure initial rate from graphs
You draw a tangent at 0. The gradient of the tangent gives the reaction rate.
Catalyst
It increases the reaction rate without being used up in the overall reaction. It allows a reaction to proceed via a different route with a lower activation energy.
Homogenous catalyst
Has the same physical state as the reactants. The catalyst reacts with the reactant to form an intermediate. The intermediate then breaks down to give the product and regenerate the catalyst.
Heterogynous catalyst
Has a different physical state from the reactants. Reactant molecules are adsorbed on to the surface of the catalyst, where the reaction takes place. After the reaction the product molecules leave the surface of the molecules by desorption. Heterogynous catalysts are normally solids reacting with gaseous reactants.
Catalysts - sustainability and economic importance
Increase the rate of many industrial chemical reactions by lowering the activation energy which reduces the temperature needed for the reaction. This means that less electricity and fossil fuels are used. Making the product faster and using less energy cuts cost. Using less fossil fuels will cut emissions of carbon dioxide, a gas linked to global warming.
Features of the Boltzmann distribution
No molecules have zero energy, the curve starts at the origin. The area under the curve is equal to the total number of molecules. There is no maximum energy for a molecules, the curve does not met the X-axis.
The Boltzmann distribution and temperature
The average energy of the molecules will increase. There will be more molecules with an energy greater then or equal to the activation energy. The graph is stretched over a greater range of energy values. The peak of the graph is lower on the Y-axis and further along the x-axis.
How does temperature affect reaction rate
At a higher temperature, more molecules will have an energy greater then or equal to the activation energy. Therefore, a greater proportion of collisions will lead to a reaction, increasing the rate of reaction. Collisions will also be moving faster meaning that there will be more collisions.
The Boltzmann distribution and catalysts
Compared to the normal activation energy a greater proportion of molecules will have the energy equal to or greater then the activation energy with a catalyst. On collision more molecules will react to form products
Overall order
The overall effect of the concentrations of all reactants on the rate of reaction.
Overall order = Sum of the orders
