1. Controlling the Rate of Reaction Flashcards
Why would you increase the rate of a reaction?
in order to make more product in a quicker time so to make more money.
Why would you decrease the rate of a reaction?
if reactions go too fast, they can go out of control and cause an explosion. This is called a thermal runaway.
What are the 4 ways to increase the rate of reaction?
Increase temperature
Increase concentration
Decrease particle size
Add a catalyst
How can the rate of a reaction be measured experimentally?
Decrease in MASS of reaction flask.
Volume of GAS produced.
Change in CONCENTRATION of products or reactants.
What is the equation for calculating the rate of a reaction?
Rate = Delta Quantity over Delta Time
What does collision theory state?
For reactant particles to make product they must collide together
with enough force and with the correct geometry
If you were asked why a reaction was faster, considering its concentration, how would you answer?
The reaction with the HIGHER concentration is faster, because there are more particles present so you are more likely to get a successful collision.
OR
As there are more particles physically in the beaker more collisions must be taking place so there must be more successful collisions.
On a graph, how would you show a higher concentration?
A slightly higher curve, but same endpoint
What is the equation for relative rate?
1/t
How do reactions graph of rate against concentration look like?
It is a straight line, going up. no end point!
What happens to the rate of reaction, when you increase the temperature?
Increasing the temperature causes the reactant molecules to move FASTER.
This means that they collide MORE often and also that they collide with MORE kinetic energy.
What can temperature be regarded as?
Temperature can be regarded as the measure of the average kinetic energy in all the particles of a substance
What is a way to remember rate of reaction with temperature?
If there are 100 pupils in a games hall, they are not very likely to collide as much. However if you ask everyone to run around (ie increase the temperature), there will be a higher chance of successful collisions with one another.
How does the graph for temperature against rate look like?
A low curve, then high at the end. just going up!
What is the definition of Activation Energy?
The Activation Energy is the minimum amount of energy reactant particles must possess in order for a successful collision to take place.
What do points A, B and C represent on a kinetic energy graph?
Point A represents the lowest kinetic energy.
Point B. represents the average kinetic energy
Point C represents the fewest molecules (HIGH KINETIC ENERGY)
Where is Ea represented on a graph?
Towards the bottom, the shaded section comes AFTER the line.
How should the energy of collision be, in order for a pair of molecules to react?
greater than or equal to the activation energy (Ea)
Only particles in the shaded area can form product as they have enough energy.
How does increased temperature look on a graph?
The curve has moved the the RIGHT, and is slightly lower than the previous graph
However, the shaded area (Ea) remains the SAME.
Give the full description of temperature affecting rate of reaction.
When the temperature increases the particles will move faster so the graph will move to the right. The new graph is further to the right and is slightly lower than the graph for the colder temperature.
As the graph has now moved to the right there are now more particles that have energy greater than or equal to the activation energy (Ea) T
here is a bigger shaded area. As more particles have energy greater than or equal to the Ea, more successful collision will take place so you will get a faster reaction rate.
Though, Temperature does not affect the activation energy. It stays the same.
How does increased concentration look on a graph?
A HIGHER curve. still the same start and end point!
Give the full description of increasing concentration to the rate.
When the concentration increases the shaded area is bigger. in the graph as there is a higher curve. This means there are more particles with an energy greater than or equal to the Ea (the activation energy)
So you are more likely to have successful collisions occur, so a faster reaction rate.
How does increasing the pressure affect the rate?
Increasing the pressure of a reaction can also increase the rate of a reaction because the reactant particles are squeezed closer together so you are more likely to get successful collisions.
What is a way to remember increasing pressure vs rate or reaction?
If there are 100 pupils in a games hall, it is very spacious and easy to move around. However, if there are 100 pupils in a classroom instead (ie increasing pressure), there will be less space to move around and more successful collisions will occur.
What is collision geometry?
For a reaction to occur, the reactants must collide.
Reaction rate depends on collision GEOMETRY i.e. how the particles are lined up as they collide.
Particles must collide with sufficient energy (the ACTIVATION energy) but also the correct geometry. If the particles collide in the wrong way a reaction WON’T occur
Describe the diagram on how to describe collision geometry.
Correct way:
There is a particle, with lines in between. In order for the particle to collide, the small circle must collide with the lines in the centre. The correct geometry is that. Any other way is deemed as wrong!
What happens in a EXOTHERMIC reaction?
In an EXOTHERMIC reaction energy is LOST to the surroundings.
What happens in an ENDOTHERMIC reaction?
In an ENDOTHERMIC reaction energy is GAINED.
What does enthalpy (H) mean?
Enthalpy (H) is a measure of the energy stored in a chemical.
What are examples of EXOTHERMIC reactions, and why?
- Combustion
- Displacement
- Neutralisation
- Respiration.
This is because they release energy to the surroundings.
Explain the relation of the amount of products + reactants in an EXOTHERMIC reaction. Why is this?
LESS PRODUCT ENERGY, MORE REACTANT ENERGY.
As exothermic reactions LOSE energy (usually as heat) the energy of the products must be LESS than the reactants.
How do you show the Potential Energy Diagram for products vs reactants in an EXOTHERMIC reaction?
Reactants on top | | | \ Products on bottom
What can the enthalpy change, ΔH be defined as in an equation?
ΔH = Hproducts - Hreactants
What is the Enthalpy change (∆H) for an EXOTHERMIC reaction?
∆H (enthalpy change) is NEGATIVE for an exothermic reaction
What are the units of enthalpy?
The units of enthalpy = kJmol-1
What are examples of ENDOTHERMIC reactions, and why?
- Photosynthesis
- Reaction of Ethanoic acid with Sodium hydrogen carbonate.
This is because these reactions take in heat from the surroundings causing them to FEEL COLD. Some reactions may take in light. The energy stored in the chemical increases, so it is ENDOTHERMIC
How does the potential energy diagram look for an ENDOTHERMIC reaction?
Products on top ^ | | | Reactants on bottom
What is the enthalpy change for an ENDOTHERMIC reaction?
∆H is POSITIVE for an endothermic reaction.
Explain in detail the connection between Enthalpy change and ENDOTHERMIC reactions in detail!
The ∆H must be a POSITIVE value because the products have more energy than the reactants.
So ∆H = Hproducts – Hreactants must be a POSITIVE number as Hp is bigger than Hr.
The products have more energy because energy is taken in from the surroundings
What is an Activated Complex?
An ACTIVATED COMPLEX is the unstable arrangement of atoms found at the maximum on a potential energy diagram.
How do you figure out the Ea on a Potential Energy diagram?
Ea = Highest Point - Lowest Point
How do you figure out Enthalpy Change (ΔH) on a Potential energy diagram?
Where the reaction ends MINUS one level.
What is one thing to remember when calculating Enthalpy Change (ΔH) from a graph?
EXOTHERMIC MINUS ONE LEVEL IS NEGATIVE.
ENDOTHEMIC REMAINS THE SAME: POSITIVE.
What is the definition of a CATALYST?
A catalyst is a chemical that speeds up a reaction without being used up.
What are the two types of catalysts? What do they mean?
HOMOGENOUS: the catalyst is in a SAME state to the reactants.
HETEROGENOUS: the catalyst is in a DIFFERENT state to the reactants.
Where can catalysts be found? What do they do there?
Catalysts are found in the catalytic converter in car exhaust systems.
They turn harmful gases into less harmful ones
Where does Catalysis occur?
Catalysis occurs on the SURFACE of the catalyst.
What is the site name where reactions take place in Catalysis?
ACTIVE sites.
Explain the full process of how a Catalyst works
- Reactants ADSORB onto the surface of the catalyst
- Bonds break in the reactant molecules, and new bonds form to make new product molecules.
- The products DESORB off the surface and the process repeats!
What happens if the active sites are Blocked in a Catalyst?
If the active sites are blocked we say the catalyst has been POISONED.
Describe an experiment, that is an example of Homogenous Catalysis. Why was it Homogenous?
A mixture of hydrogen peroxide and Rochelle salt was warmed to 60 C and no bubbles of gas were observed.
A little PINK cobalt chloride solution was added to the flask.
The solution turned to a GREEN colour and BUBBLES of gas were QUICKLY given off.
As the reaction died down the colour of the solution changed back to PINK again.
The catalyst in this reaction was Cobalt Chloride, and we know it is unchanged at the end of the reaction because it is still a pink colour.
This is an example of homogeneous catalysis because the catalyst and reagents are in the same state - ie Both catalyst and reagents are in SOLUTION.
How does a Catalyst look on a potential energy diagram?
Curve is slightly lower than uncatalysed reaction
Still same start and end point!
What does a Catalyst provide?
The catalyst provides an ALTERNATIVE route for a reaction.
What do the Catalysts do?
A catalyst DECREASES the ACTIVATION energy. So it is easier for BONDS to break in the REACTANTS.
The catalyst does not affect the ENTHALPY CHANGE ( ΔH ).
How does the effect of a Catalyst look on an energy distribution graph?
The shaded area is much larger, with the activation energy (Ea) being much LOWER. it is moved to the LEFT.
Why is the Activation Energy (Ea) lower when a catalyst is used?
The activation energy is LOWER when a catalyst is used because it provides a different reaction PATHWAY.
Explain in detail, why using a catalyst increases the rate of reaction!
The graphs show that because of the LOWER activation energy more collisions have kinetic energy equal to or greater than the ACTIVATION energy. This leads to a BIG increase in the rate of reaction.
The graph clearly shows this as the activation energy (Ea) moves to the left … as the energy is being lowered .. the size of the shaded area gets much bigger.
So now more particles have energy greater than or equal to the activation energy (Ea) so more successful collisions so faster reaction.
What does the catalyst to do enthalpy (ΔH) ?
NOTHING !! :)
