ROR notes Flashcards
When describing the ROR from a graph what do we always state?
- The speed of the ROR
- How is this ^ shown? + meaning (more product being formed in a given amount of time)
- What causes this? - ie lots of reactant molecules so more frequent successful collisions so more product being formed etc.
^Always mention the frequency of collisions!
State collision theory:
Chemical reactions can only take place when the reacting particles collide with each other. The collisions must have sufficient energy.
^The sufficient energy is known as the activation energy: The minimum amount of energy that particles must have to react.
How do we measure the rate of reaction?
Amount of product formed or Amount of reactant used up/Time (s)
Product formed/Reactant used up can be measured in:
- g (mass)
- cm^3 (volume)
- Moles
So rate of reaction = g/s mol/s or cm^3/s
What is the rate of a chemical reaction determined by?
The frequency of collisions of reactant molecules.
General rate of a chemical reaction:
Fast, then slows down, then stops.
This can be explain with collision theory.
Describe the relationship between the concentration of particles in a solution/gas and the rate of reaction.
Concentration is directly proportional to the rate of reaction.
If we increase the concentration of particles in a solution or in a gas, we increase the rate of reaction.
- As the conc. increases ROR increases.
- Because there are more reactant particles so more successful collisions/higher collision frequency.
- So amount of reactant particles used up/product formed increases.
^This applies to gas and pressure.
RS of surface area and rate of reaction:
- As SA increases rate increases so SA and rate are directly proportional.
- This is bc a larger surface area means reactant particles have a higher chance of colliding with the surface of the solid reactant.
**The smaller the block of reactant, the larger the SA:Volume ratio.
How does temperature affect the rate of reaction?
- Temperature is directly proportional to the rate of reaction.
- As temp increases ROR increases more energy is being supplied to the reactant particles.
- So there is an increase in the movement of particles and therefore more frequent collisions per second.
- As the number of successful collisions per second increases, the transfer of energy between the reactant particles increases.
- Reactant particles now have more energy to overcome the activation energy barrier.
- So even more FQ and more reactant formed.
- So ROR increases.
What are catalysts?
Substances that increases the rate of a chemical reaction but are not used up in them.
How do catalysts affect the reaction profile?
The reaction profile for catalysed reactions will have a lower activation energy (shown by a lower curve).
Why are catalysts useful?
- They can be reused - because they don’t get used up in a chemical reaction.
- They allow use to save money - because they allow use to increase the rate of the chemical reaction without increasing the temperature; this saves energy and money.
How do catalysts increase the rate of a chemical reaction?
- They provide an alternative pathway for the reaction to occur, this pathway has a lower activation energy.
- This means more particles, even those with low energy can cross the activation energy barrier and collide, so there are more frequent successful collisions per second.
- So the ROR increases.
True or false - different chemical reactions require different catalysts?
True or false enzymes are biological catalysts?
True
True
What is meant by a reversible reaction?
A reaction where products can react together to produce the original reactants.
- This is shown by the double arrow.
Factors which affect the rate of a reaction:
- Concentration of reactions in solution.
- The pressure of reacting gases.
- The surface area of solid reactants.
- The temperature of the reaction.
- The presence of catalysts.
Draw a reaction profile for catalysts.
An example of a reversible reaction:
A+B double arrow C+D
Ammonia chloride double arrow ammonia and hydrogen chloride
- In this reaction heating causes the forward reaction and cooling causes the backward reaction.
If a reversible reaction is endothermic in 1 direction, then in the other direction the reaction is?
Exothermic.
^However for each reaction the same amount of energy is transferred.
Endothermic reaction:
Exothermic reaction:
Energy ENTERS - so reaction temp decreases
Energy EXITS - so reaction temp increases.
Hydrated copper sulfate (blue)
RRS:
Forward reaction is:
Backward reaction is:
Anyhydrous copper sulfate (white) + Water
FORWARD = ENDO; heat
BACKWARD = EXO; cool
So if we wna get Anhydrous copper sulfate we heat and to get hydrated copper sulfate we cool.
Anhydrous copper sulfate is:
Hydrated copper sulfate is:
Anhydrous copper sulfate is white
Hydrated copper sulfate is blue
When does equilibrium occur?
Equilibrium occurs when a reversible reaction takes place in a sealed container which prevents any reactant and products from escaping and the forward and reverse reactions occur at the same rate.
What is Le Chatalier’s Principle?
If a system is at equilibrium and a change is made to the conditions, the reaction responds to counteract the change.
If the concentration of a reactant is increased:
- The reaction is no longer at equilibrium.
- Rate of forward reaction increases, equilibrium moves to the left.
- So more product will be formed until equilibrium is reached again.
If the concentration of a react is decreased:
- The reaction is no longer at equilibrium
- The rate of the backward reaction increases and the reaction moves to the left.
- So more product will turn into reactant until equilibrium is reached again.
If the concentration of a product is decreased:
- The reaction is no longer at equilibrium
- The rate of the forward reaction increases and equilibrium shifts to the left.
- More reactant will react to produce the product until equilibrium is reached again.
If the concentration of a product is increased
- The reaction is no longer at equilibrium
- The rate of the backward reaction increases, equilibrium shifts to the left
- So more product is going to be turned into reactant until equilibrium is reached again.
The format to answer an equilibrium question:
- What is the resultant change?
- What is the counterchange to restore equilibrium?
- So where will equilibrium move to for this to happen and why?
Endothermic reaction:
Exothermic reaction:
- Takes in heat energy from the surroundings and the environment so the system temp decreases.
- Releases heat energy to the surroundings and the environment so the system temp increases.
A reaction is taking place and the forward reaction is exothermic and the backward reaction is endothermic.
Describe what will happen if the temp of the reaction is increased and decreased.
INCREASED TEMP:
- We need to decrease the temp
- Reaction is no longer at equilibrium
- Equilibrium will shift to the right so more product is formed.
- Because the forward reaction is exothermic so energy will be released to the surroundings until equilibrium is restored..
DECREASED TEMP:
- We need to increase the temp.
- The reaction is no longer at equilibrium
- So equilibrium will shift to the left because that reaction is endothermic so energy will be taken in from the surroundings and the system temp increases until equilibrium is restored.
- Less product is produced and more reactant is formed.
Pressure relates to what to what substance state?
A gas.
N2 + 3H2 -><(RRS) 2NH3
What happens if we increase the pressure.
- The reaction will no longer be at equilibrium.
- So to restore equilibrium, equilibrium will shift to the right
- BC there are fewer molecules in the forward direction than the backward direction
N2 + 3H2 -><(RRS) 2NH3
What happens if we decrease the pressure.
- The reaction will no longer be at equilibrium
- So equilibrium will shift to the left because there are more molecules in the backward direction than the forward direction.
When the number of reactant molecules and the number of product molecules is the same, what happens to the position of equilibrium if we change the pressure?
Changing the pressure will have no effect on the position of equilibrium, when the number of reactant molecules is equal to the number of product molecules.
