Kinetics (MDY) Flashcards
What is meant by Activation Energy
The minimum amount of energy needed for a reaction to occur
What is a catalyst?
A substance that speeds up a chemical reaction by providing an alternative reaction pathway with lower activation energy, and remains chemically unchanged at the end
What is meant by enthalpy change?
Heat change at a constant pressure
What is meant by rate of reaction?
Change in concentration of a reactant per unit time
According to collision theory, what 3 things must happen in order for a chemical reaction to happen?
- Molecules must collide
- With sufficient energy
- And correct orientation in order to break bonds and form new ones
What is an ineffective collision?
When reactant molecules collide with insufficient energy or incorrect orientation, resulting in no reaction
What is a successful collision?
When particles collide with sufficient energy and the correct orientation to react
What is collision frequency?
The number of collisions per unit time
How can a high collision frequency affect rate?
Particles gain energy from ineffective collisions, so a higher collision frequency means the number of particles with sufficient energy increases and therefore the rate of reaction increases
What is meant by activation energy?
The minimum amount of energy particles need to have to react
What is an endothermic reaction?
One where the energy of the products is higher than the reactants
What is an exothermic reaction?
One where the reactants have more energy than the products
Do exothermic reactions have a positive or negative enthalpy change?
Negative
Do endothermic reactions have a positive or negative enthalpy change?
Positive
Name the 5 factors that affect rate of reaction
- Temperature
- Concentration
- Pressure
- Surface area
- Catalyst
What does the peak on a Maxwell-Boltzmann distribution represent?
The most probable energy of any particle
Where is the mean energy in relation to the peak of a Maxwell-Boltzmann distribution?
Slightly to the right
What does the area under the curve represent in a Maxwell-Boltzmann curve?
The total number of molecules
What happens to the peak of a Maxwell-Boltzmann curve if you increase the temperature of the reaction?
It moves down and to the right
Which factor is the only one that affects the general shape of the Maxwell-Boltzmann curve?
Temperature
Describe how a Maxwell-Boltzmann curve changes at a higher temperature
The curve flattens and the peak moves down and to the right. The area under the curve stays the same and shows more particles have energy higher than the activation energy.
Describe how the shape of a Maxwell-Boltzmann curve changes in a more concentrated reaction mixture
The curve keeps the same general shape, but the graph is taller at a higher concentration, so the area under the curve increases
Describe how the shape of a Maxwell-Boltzmann curve changes in the presence of a catalyst
The shape of the graph does not change at all, but the activation energy shifts to the left on the x axis
Why does a small increase in temperature lead to a large increase in rate of reaction?
- Particles have more kinetic energy
- Will move faster
- Greater proportion of molecules will have sufficient activation energy
- AND collision frequency increases.
- Increases the frequency of successful collisions and therefore rate
Why does increasing concentration increase rate?
There are more reactant particles so collision frequency increases. This leads to an increase in successful collisions and rate of reaction
Why does increasing pressure increase rate of reaction?
- Particles are forced closer together
- Collision frequency increases.
- Increase in successful collisions and rate of reaction
Why does increasing surface area increase rate?
- More surface for reactant particles to react with
- Collision frequency increases.
- Increase in successful collisions and rate of reaction
Why does the presence of a catalyst increase rate?
Catalysts provide an alternative reaction pathway with lower activation energy so more particles have sufficient energy and frequency of successful collisions increases.
What is the equation for rate?
change in amount of reactant or product / time
What is the unit for rate of reaction?
mol dm−³ s−¹
Why does a Maxwell-Boltzmann curve start at 0,0
Because all particles have energy
What is meant by experimental error?
The difference between measurement and the true value or between 2 measured values
What is meant by uncertainty?
An estimate attached to a measurement which characterises the range of values within which the true value lies
How do we generally estimate uncertainty?
Take it to be half a division of the smallest unit on the scale
Percentage uncertainty equation
(uncertainty / reading) x 100
What does a larger percentage error mean
A more uncertain result
Experimental error equation
100 x ((real answer - experiment answer) / real answer)
When working with two different pieces of equipment what do you do with the uncertainties?
add them together to get an apparatus error
