Rates of Reaction Flashcards
(31 cards)
Rate of reaction
The rate of a reaction is the change in concentration of either the reactants or products per unit time.
Average rate
- Taken over a set period of time.
- Rate of reaction changes with time.
- Usually fastest at the start but slows down over time as reactants are used up.
- Average rate = Amount produced/ Time (seconds).
Instantaneous rate
- Taken at a particular time.
- Changes as time passes.
How to calculate the instantaneous rate
Draw graph, draw tangent to the curve at the 5 minute mark, measure the slope.
Interpretation of reaction rate graphs
- Reaction is finished when the line becomes horizontal.
- Rate is proportional to the slope of the line = the steeper the slope the faster the reaction.
- Rate is normally fastest at the start and slows down because the concentration od the reactants drops as they are used up.
Slope formula
RISE/RUN or m= y2-y1/x2-x1
Activation energy
Minimum energy two colliding particles must have in order to have an effective collision.
Collision theory
- Particles must collide.
- To form products particles must reach a minimum energy called activation energy.
Effective collision
Effective collision is one that results in the formation of products.
Maxwell-Boltmann distribution
The spread of energies in a sample of gas molecules is given by the Maxwell-Boltzmann distribution curve.
Factors that effect the rate of reaction
- Concentration
- Temperature
- Catalysts
- Particle Size
- Nature of Reactants
Temperature
- The higher the temperature the faster the rate:
1. Particles are moving faster and collide more frequently.
2. When they collide more of them will have activation energy. - 10C rise in temperature doubles the rate of reaction.
High body temperature (bad)
Denatures enzymes and upsets biochemical reactions.
Concentration
- Doubling the concentration of one of the reactants doubles the rate of reaction. Directly proportional.
E.g. straight line through origin. - Doubling the concentration of two of the reactants quadruples the rat of the reaction.
- The higher the concentration the more often particles collide and so can react. The higher the reactions the more collisions the more effective collisions.
High body temperature (good)
Increases the production defence chemicals.
Catalysts
- Catalysts alter the rate of reaction but are not used up in the experiment.
E.g. Pt, Fe. - Lower activation energy.
- does not form more product.
- if the reaction is reversible, they speed up the reaction in both directions equally.
- specific catalyst for specific reaction.
Enzyme
Enzymes are produced in living cells and act as biological enzymes.
E.g. Amylase, Yeast, Zymase.
Example of particle size
Explosions in flour mills.
factors necessary for this to occur:
- combustible dust
- oxidising agent
- ignition source
- dryness
- confined area
Particle size
- The smaller the particle the faster the rate.
- The larger the surface area the more reactants come into contact.
- Powder > Granules > Lumps
Nature of reactants
Ionic - tend to have very fast reactions. Almost instantaneous.
Covalent- slow to react in general.
Theories of Catalysis
- Heterogeneous
- Homogeneous
- Autocatalysis
Heterogeneous
Heterogeneous catalysis is catalysis in which the reactants and the catalyst are in different phases.
E.g. Fe in the Haber Process for the industrial production of ammonia.
The decomposition of hydrogen peroxide and manganese dioxide.
Homogeneous
Homeogeneous catalysis is catalysis in which both the reactants and the catalyst are in the same phase.
E.g. Esterification reactions.
Autocatalysis
Autocatalysis is catalysis in which one of the products of the reaction acts as a catalyst for the reaction.
This reaction starts slowly but speeds up as the catalyst product starts to form.
