Kinetics Flashcards
What does activation energy mean
The minimum energy needed for a reaction to occur
What is a catalyst
A substance that speeds up a chemical reaction be but is chemically unchanged at the end. They work by providing an alternate pathway with a lower activation energy
What does enthalpy change mean
A heat change at constant pressure
What does rate of reaction mean
The change in concentration of a reactant(product) per unit time
What 3 things must occur for a chemical reaction to take place
1.particles must collide with enough energy to start breaking bonds
2.particles must be in the correct orientation
3. Particles must collide
What is an ineffective collision
When particles collide in the wrong orientation or when they don’t have enough energy and bounce off each other without causing a chemical reaction. They give no reaction
What is a successful collision
Occurs when particles collide with sufficient energy to result in a reaction
How do collisions and reaction rate affect rate of reaction?
When more collisions per unit time take place, the number of particles with energy greater than the activation energy increases. These lead to more successful collisions. This causes an increase in the rate of reaction
What is an exothermic reaction
When the reactants are higher in energy than products
Enthalpy change is negative
Energy is transferred from the reacting system to the surroundings
What is an endothermic reaction
When the reactants are lower in energy than products
Enthalpy change is positive
Energy is transferred to the reacting system from the surrounding
What does the enthalpy profile look like for an exothermic reaction
What does the enthalpy profile look like for an exothermic reaction
What does the enthalpy profile look like for an endothermic reaction
How does increasing temperature, increase ROR
-Increases frequency of collisions
-Increases number of successful collisions
Why does increasing temperature increase the ROR
It increases the kinetic energy of particles
This increases the frequency of collisions and a greater proportion of collisions will have the energy required to react.This gives more successful collisions.
How does increasing the concentration of a solution increase the ROR?
Increases number of successful collisions
Why does increasing the concentration of a solution increase the ROR?
More particles of that reactant are present which leads to more successful collisions between particles in a given period of time
How does increasing the pressure of a gas increase the ROR
Increases number of successful collisions
Why does increasing the pressure of a gas increase the ROR
The particles are forced together which leads to more successful collisions between reactant particles in a given period of time
How does increasing the SA of a solid increase the ROR
Increases number of successful collisions
Why does increasing the SA of a solid increase the ROR
Increases the exposed surface of the reactants. This increases the number of successful collisions in a given period of time
How does using a catalyst increase the ROR
Increases number of successful collisions
Why does using a catalyst increase the ROR
Provides an alternative reaction pathway of a lower activation energy. By lowering the activation energy, more collisions are successful in a given period of time.
What is a Maxwell-Boltzmann distribution curve?
A graph that shows distribution of energies in a gas at a certain temperature.
What do you need to know about the Maxwell-Boltzmann distribution curve?
-Energy distribution should go through the origin as no particles have no energy
-The most probable energy is the peak of the curve
-the area under the curve represents the total number of particles present
-The end of the curve is where there are very high energy molecules
-The activation energy is where the particles can do a chemical reaction as they have the most sufficient energy
What happens to the energy of particles when temperature increases and decreases
Increases-particles gain kinetic energy
Decreases-particles lose kinetic energy
What happens to the shape of a curve when the temperature increases and decreases
Decrease-curve narrows, rises and shifts to the left
Increase-curve shifts to the right and the height of the peak falls
What happens to the area under the curve as temperature increases and decreases
Doesn’t change for both
Why does the curve shift to the right and flatten when temperature increases
When temp increases, the most probable energy increases(peak shifts to the right)and there is a greater spread of values(peak flattens)
What is the unit for ROR
Moldm-3 s-1
What happens to the shape of a curve when pressure or concentration changes?
Stays the same
What happens to the height of a curve when pressure or concentration increases?
It increases because there are more particles
What happens to the ROR when pressure or concentration increases? (reference maxwell boltzmann curve)
It increases because since there are more particles, so there are more frequent collisions and more successful collision.
what is a homogeneous catalyst
Homogeneous means that the catalyst is in the same phase as the reactants. For example, the reactants and the catalysts are all in solution
What is a heterogeneous catalyst
Heterogeneous means that the catalyst is in a different phase to the reactants. For example, the reactants are gases but the catalyst used is a solid
What is a heterogeneous catalyst
Heterogeneous means that the catalyst is in a different phase to the reactants. For example, the reactants are gases but the catalyst used is a solid
Why do we use catalysts?
- Allow reactions to take place at lower temperatures decreasing energy costs.
- Enable different reactions to be used with better atom economy reducing waste.
- Are often enzymes generating specific products.
- Can reduce pollution (e.g. use of catalytic converters)
What is the problem of measurement?
-No physical quantity can be measured with perfect certainty
-There are always errors in any measurement
-If we measure some quantity and repeat the measurement we will almost certainly measure a different value the second time
How can we reduce errors?
-By taking greater care in our measurements and and applying more refined experimental methods. This means we have more confidence in our measurements getting closer to a true value
What is an experimental error?
The difference between a measurement and the true value or between two measured values
What is an uncertainty?
An estimate attached to a measurement which characterises the range of values within which the true value lies. We use +/- to show uncertainty.
What is an uncertainty?
An estimate attached to a measurement which characterises the range of values within which the true value lies. We use +/- to show uncertainty.
What is accuracy
Accuracy measures how close a measured value is to the true value or accepted
Since a true or accepted value for a physical quantity may be unknown, it is sometimes not possible to determine the accuracy of a measurement.
What is precision?
Precision measures how closely two or more measurements agree with other.
Precision is sometimes referred to as repeatability or reproducibility.
A measurement which is highly reproducible tends to give values which are very close to each other.
What is percentage uncertainty?
Imagine a balance with an uncertainty of +0.1g.
If you measure 60g the uncertainty is small compared to the measurement.
If you measure 0.4g then the uncertainty is nearly as big as the measurement.
We use percentage uncertainty to express this idea: uncertainty/reading x100
In general, the larger the percentage error…
…the more uncertain the result is. We can reduce this uncertainty by selecting a piece of equipment to measure that will give a lower percentage uncertainty.
If we are using multiple pieces of equipment, what happens with the percentage errors?
They are added together to give a total apparatus error
What is the equation that contains moles, volume and concentration?
moles= conc x vol
