Reaction Rates (10) Flashcards
Reaction Rates
How fast a reactant is used up or how fast a product is formed.
Concentration/Time
What a concentration-time graph looks like
At the beginning of the reaction, the rate will be the fastest as there’s the most substrate to react to. Then as time goes on, the curve will become less steep as the rate is slowing down because there are fewer substrates to react with. At a certain point, the graph will level out and be horizontal because the substrates have all been used up so the rate has reached its maximum.
Factors Affecting The Rate
Concentration (pressure), Temperature, Use of a catalyst, surface area of solvents
The collision theory
To reactant particles must collide successfully at the correct orientation and have enough energy above the activation energy for the reaction to occur. This is why all those factors increase the rate because it’s increasing the chance of successful collisions.
How to monitor the rate of a reaction
Monitor the removal of a reactant in a certain time or monitor the formation of a product in a certain time.
How to monitor the rate of reaction in a gas
Either measure the amount of gas formed with a gas syringe or monitor the amount of mass lost from the reactants by doing final-initial mass
How to find the initial rate of the reaction from the graph
Draw a tangent from t=0 and then find the gradient of it to get the initial rate
Graph of gas produced against time
Will go from left to right and increase. Starts off at zero, but increases as you go on.
Graph of mass lost against time
Will from left to right and decrease. Starts off high at zero time and low as the reaction goes on. More mass will be lost as time goes.
How to calculate the rate at a specific time
Find the point in time and look up to find the point on the curve. Draw a tangent there and find the gradient.
Catalyst
Can change the rate of the reaction without being used up or changed itself. It can react with the product to form an intermediate and at the end of the reaction, the catalyst will be regenerated.
How do catalysts speed up a reaction
They provide an alternate reaction pathway and will decrease the activation energy.
Homogenous catalyst
This is a catalyst that has the same state as the reactants in the reaction. It will react with it to form an intermediate which will break down to give the reactant. The homogenous catalyst is then regenertaed
Examples of a homogenous catalyst reaction
Ethanoic acid (l) + Ethanol (l) --> Ester (l) + H20 (l) Catalyst is sulphuric acid and it's reversible
Ozone (g) —> Oxygen (g)
Catalyst is chlorine radical and it’s reversible
Heterogenous catalyst
This is a catalyst that has a different state than the reactants. The catalyst is usually solid and the reactants are gases that will be absorbed onto the surface of the catalyst until the product has been formed and deabsorbed.
Examples of heterogeneous catalysts
Making ammonia
Hydrogenation of alkenes
Making Sulphur trioxide
Why catalysts are so good for reactions
They speed up the process for 90% of reactions. Thye are cheaper and more sustainable than fossil fuels and will help reactions go a lot faster so there are many benefits to catalysts
Boltzmann Distribution
The spread of energy in a reaction with gaseous molecules that have a given energy. No molecules have zero energy, so the curve starts at the origin. The area under the curve is equal to the total number of molecules in the reaction. When the energy is high, the number of molecules will never be zero. Most molecules have energy slightly in the middle. The activation energy is also drawn on.
Boltzmann Distribution and Temperature
The rise in temperature will mean more particles will have a greater energy, even higher than the Ea, so the peak won’t be as high on the y axis, and the graph will be stretched out more on the x-axis as more particles will have a greater spread of energy instead of all being low. The activation energy stays the same.
Boltzmann Distribution and Catalyst
This graph will be the same as the original, just that it will have a lowered activation energy because that’s what a catalyst will do, it doesn’t;t give the particles more energy it just provides them with an alternative reaction pathway.
Non-reversible Reactions
This is when the reactants are both used up and the reaction has gone to completion, it is not reversible so has a single-headed arrow
Reversible Reactions
This is when reactants haven’t all been used up or the reaction hasn’t gone to completion, so there is a forward and backward reaction. A good example is the Haber Process which makes ammonia from nitrogen and hydrogen. It has a half arrow going in both directions as its symbol
Dynamic Equilibrium
in an equilibrium system: the rate of the forward reaction is the same as the backward reaction and there is always the same concentration of reactants and products. The forward and backward reactions are always taking place at the same time in dynamic equilibrium.
What does a system have to be so it’s in equilibrium?
It has to be a closed system so it should be isolated and none of the factors should be affected by the exterior environment.
What is the position of equilibrium?
It will always be in the middle, but if some of the factors change, the position of equilibrium will change.
Le Chateliers Principle
When a system in equilibrium is subjected to an external change, the system will readjust itself to minimise the effects.
Effect of concentration on the equilibrium
If the concentration of reactants or products change, then the rate of the reactions will also change and so will the position of equilibrium
Equilibrium change when the reactants increase or products decrease
The equilibrium will shift to the right so more products are formed until a new equilibrium is found. So if there are more products formed, the equilibrium has shifted to the right.
Equilibrium change when the reactants decrease or products increase
The equilibrium will shift to the left so more reactants are formed until a new equilibrium is found
Investigating the concentration changes on equilibrium
You can look at the difference between chromate ions and dichromate ions. These have different concentrations of acids and are different colours so you can easily see the shift in equilibrium.
Effect of temperature on the equilibrium
If the temperature of the system changes, the equilibrium will shift and the rates will change too. Normally, the forward and backward reactions have the same amount of enthalpy change, just different signs because one is exo and one in endo
Equilibrium change when the temperature increases
If the temperature increases, the equilibrium will shift in the endothermic direction where the enthalpy change is positive because the surroundings are transferring the energy to the system so it’s endothermic. You have to figure out which way is endothermic
Equilibrium change when the temperature decreases
If the temperature decreases the equilibrium will shift to the exothermic direction where the enthalpy change is negative and this is because energy is transferred from the system into the surrounding so energy decreases in the system.
Investigating the temperature changes on equilibrium
Look at how cobalt chloride dissolves in water to form a complex that is pink, and if its cobalt chloride and water separately it’s blue. This is used to see the shift in equilibrium.
Effect of pressure on the equilibrium
Changing the pressure in a gaseous system will only result in an equilibrium shift if there are more moles of gas on one side than the other. 2 mol of the reactants would have twice the concentration and pressure of 1 mol of products so equilibrium has to shift
Equilibrium change when the pressure increases
If the pressure increases, the equilibrium will shift to the side of the equation where there are fewer moles. If you use No2 which is brown and N2O4 which is colourless and you increase the pressure, you will see the colour becomes more translucent. If you decrease the pressure the solution will become a deeper brown.
Effect of catalyst on equilibrium
The catalyst will not affect the position of the equilibrium as it increases the forward and backward reactions at the same rate.
Equilibrium Law (Kc)
Consider a reversible reaction:
aA+bB cC + dD
Kc = [C]c x [D]d
[A]a x [B]b
Kc = 1
This indicates the equilibrium position is halfway between reactants and products
Kc > 1
This indicates the position of equilibrium is more towards products
Kc < 1
This indicates the position of equilibrium is more towards reactants
