Reaction Rates Flashcards
What is the definition of rate of reaction?
-the change in concentration of reactant or product per unit time. The standard units of rate are moldm-3s-1.
-also express rate as gs-1or cm3s-1
Which factors may affect the rate of a chemical reaction?
-Temp
-Presence of a catalyst
-Pressure
-SA
-Conc
What is collision theory used to do?
explain how some of the factors can change the rate of a chemical reaction
what does the collision theory state?
for a chemical reaction to occur, particles must collide in the correct orientation, with sufficient energy (the activation energy).
when does concentration affect the rate of reaction?
for an aqueous solution reacting with another reactant
what happens at a higher concentration?
-there are more reactant particles in a given volume
-means that there will be more frequent collisions and therefore more successful collisions per unit time; the reaction rate increases.
what happens when the pressure of a gas is increased (by decreasing volume of a container)?
molecules are pushed closer together, number of molecules remains the same but less is space occupied meaning that there are more frequent collisions and therefore successful collisions per unit; resulting in an increase in the rate of reaction.
how can the pressure of gas also be increased?
by having more molecules/particles in the same space
what is the experiment of monitoring the production of a gas using gas collection? (rate of reaction for decomposition of hydrogen peroxide)
- Hydrogen peroxide is added to the conical flask and the bung is replaced.
- The initial volume of gas in the measuring cylinder is recorded.
- Manganese dioxide, MnO, catalyst is then quickly added to the conical flask and the bung is replaced. A stop clock is started.
- The volume of gas produced in the measuring cylinder is recorded at regular intervals until the reaction is complete.
- The reaction is complete when no more gas is evolved.
what is activation energy (Ea)?
Activation energy (Ea) is the minimum energy required to break bonds, thus allowing a reaction to start.
What do reactions with a large activation energy have?
very slow rate, or do not take place at all.
When does the decomposition of calcium carbonate take place?
-not at RTP
-Requires large amount of energy input in order to occur
What happens to reactions with very small activation energy?
Take place very rapidly
why can’t the increase in the rate of reaction with increased temperature be fully explained by collision theory?
If we increase the temperature of a gas phase reaction by 10 K the rate approximately doubles. The increase in frequency of collisions would be relatively small and would not double with this energy increase, therefore there must be another explanation.
what is the maxwell Boltman distribution?
A plot of the number of particles having each particular energy; vanity as well as kinetic energy although particles can also vibrate and rotate
what are the rules of the Boltman distribution curve?
-The line must meet the origin
-The line must not touch that X axis
what is the area under the curve on a boltman distribution curve?
A measure of the total number of particles present
what happens when the majority of the particles don’t have enough energy to react when they collide?
we have to change the shape of the curve or move activation energy further to the left
What happens a high temperature?
Particles have activation energy or greater
How do catalysts affect the rate of reaction?
-increase the reaction rate without being used up by the overall reaction
-speeds up the rate of reaction by providing a lower energy pathway; decreases activation energy for a certain reaction
what is the addition of a catalyst mean?
more particles have the activation energy or greater
what doesn’t happen to catalysts?
they are not consumed by the overall reaction
What do catalysts affect?
-The conditions needed, often requiring lower temperatures and reducing energy demand and therefore CO2 emissions from burning fossil fuels
-This leads to reduced waste and more economic reactions (including better atom economy)
What are the 2 types of catalyst?
homogeneous and heterogeneous
What happens in homogeneous catalysis?
-the catalyst is in the same state (phase) as the reactants.
-A homogeneous catalyst forms an intermediate with the reactants.
-The intermediate then breaks down to give the product and regenerates the catalyst.
What happens in heterogenous catalysis?
The catalyst is in a different phase from the reactants
what is an example of homogeneous catalysis?
Depletion of ozone-
The chlorine radical (CI) and ozone are in the same (gas) phase. Cl reacts with ozone to make an intermediate ClO which then is reformed afterwards.
What is an example is Heterogeneous Catalysis?
Haber process-
The iron catalyst is in a different phase (solid to nitrogen and hydrogen.
Why are catalysts used in industrial process
-have great economic importance.
-used in many industrial processes because they speed up processes which would be too slow without them. They increase sustainability by allowing lower operating temperatures to be used. This then reduces energy demand needed, from the combustion of fossil fuels. Thus reducing COz emissions.
What are examples of catalysts used in industrial process?
-Petroleum processing
-Margarine Manufacture
-Ammonia Manufacture
