Chapter 10 - Reaction rates and equilibrium Flashcards
What is the rate of a reaction?
How fast a reactant is being used up, or how quickly a product is made
What is the equation for the rate of a reaction? (mol dm^-3 sec^-1)
Change in concentration / time
Why is the rate fastest at the start of a reaction?
Each reactant is at its highest concentration
Why does the rate of reaction decrease as a reaction progresses?
The concentrations of the reactants decrease as they are used up
What are the four factors that can affect the rate of a chemical reaction?
Concentration, temperature, catalyst and surface area
What are the two characteristics of a successful collision?
The particles collide with the correct orientation
They collide with sufficient energy
What is the activation energy?
The minimum amount of kinetic energy particles need to break the bonds in the reactants and start a reaction
How does increasing the concentration of particles increase the rate of reaction?
Increasing the concentration increases the number of particles in the same volume. This means that they are closer together and so will collide much more frequently, so there are more chances to react
How does increasing the pressure of a gas increase the rate of reaction?
An increase in pressure means that volume decreases. There is, therefore, the same number of particles but in a smaller area, which means that they are closer together and successful collisions are more common
Two ways of measuring the progress of a reaction
Monitor the decrease in concentration of a reactant
Monitor the increase in the concentration of a product
How can gases be used to find the rate of reaction?
Gas collection - monitor the volume of gas produced at regular intervals
Conduct the reaction on a balance and measure the loss of mass of the reactants
How would you measure the rate of reaction of the decomposition of hydrogen peroxide?
Place the hydrogen peroxide in a conical flask attached to a gas syringe
Add some manganese dioxide, which acts as a catalyst
Record the volume of gas in the cylinder at regular intervals
When no more gas is produced, the reaction has finished
How would you work out the rate of reaction from a graph?
Draw a tangent against the curve and find the gradient
Gradient = dy / dx
This is the rate of reaction at that time
How would you measure the rate of reaction between calcium carbonate and hydrochloric acid?
Add the reactants to a conical flask, which is upon a balance
Record the mass of the flask initially and at regular intervals
The reaction is complete when no more mass is lost
What is a catalyst?
A substance that speeds up a reaction by providing an alternative route with a lower activation energy. At the end of the reaction, the catalyst is regenerated
What catalyst is used in the Haber process?
Iron
What is a homogeneous catalyst?
The catalyst and reactants are in the same states, e.g. both being aqueous
What happens when the catalyst and reactants are in the same states?
The catalyst forms an intermediary , which breaks down to form the products of the reaction
What is a heterogeneous catalyst?
The catalyst and reactants are in different states, e.g. a solid catalyst and gaseous reactants
What happens when the catalyst and the reactants are in different states?
Reactant molecules are weakly bound to the surface of the catalyst and held in the correct orientation for a reaction to take place
The catalyst in the Haber process
Iron
The catalyst in the hydrogenation of alkanes
Nickel
What is autocatalysis?
A reaction where one of the products acts as a catalyst
The reaction starts slowly and speeds up as the catalyst is formed
Why is using a catalyst beneficial for the environment?
The temperature would otherwise have to be raised a lot to make the reaction occur fast enough. The fact that less energy is used means that fewer fossil fuels are burnt and less electricity is used
What can catalysts do to the products of reactions?
They can change their properties, e.g. make them denser and more rigid, with a higher melting point
Why is using a catalyst beneficial for the producer?
It is cheaper as less energy is used up and the product is created faster, increasing profitability. Catalysts can also provide an alternative route which has a higher atom economy
What is the Boltzmann distribution?
The spread of molecular energies in gases
Why does the curve of the Boltzmann distribution graph start at the origin?
No molecules have no energy
What does the area under a Boltzmann distribution curve equal?
The number of molecules
Why does a Boltzmann distribution curve not meet the x-axis?
There is no maximum energy for a molecule
How does an increase in temperature affect a Boltzmann distribution curve?
If you increase the temperature, the particles will have more kinetic energy and will move faster. More particles will have energy that is higher than the activation energy and so will be able to react. The shape of the curve shifts to the right
How does a catalyst affect a Boltzmann distribution curve?
It provides an alternative route with a lower activation energy, so more molecules have enough energy to overcome the new, lower activation energy and react
What is a reversible reaction?
The reaction can take place in the forward and reverse directions
What is the symbol for a reversible reaction?
Two half arrows
What are two important features of a dynamic equilibrium system?
The rate of the forward and reverse reactions are equal
The concentration of reactants and products won’t change
What is a closed system?
The reaction is isolated from its surroundings so temperature, pressure and concentrations don’t change
What is Le Chatelier’s principle?
When a system in equilibrium is subject to an external change, the equilibrium shifts to minimise the effect of that change
How and why will the equilibrium shift if the concentration of products in a reaction increases?
The equilibrium tries to counteract the change and use up the extra so favours the reverse reaction and shits to the left
How and why will the equilibrium shift if the concentration of reactants in a reaction increases?
The equilibrium tries to counteract the change and use up the extra so favours the forward reaction and shifts to the right
Example of a reaction which illustrates the equilibrium shifting to counteract changes in concentration
The equilibrium between chromate ions and dichromate ions
What colour are chromate ions?
Yellow
What colour are dichromate ions?
Orange
When you add sulfuric acid to an equilibrium, what happens?
You are increasing the concentration of H+ ions, so the equilibrium will shift to the right and favour the forward reaction to minimise the number of these ions
When you add sulfuric acid to the equilibrium, what happens to the colour and why?
It turns orange as the forward reaction is favoured and more dichromate ions are formed
When you add sodium hydroxide to an equilibrium, what happens?
The OH- ions react with the H+ ions decreasing the concentration of them. This makes the equilibrium shift to the side favouring the H+ ions to counteract the change
When you add sodium hydroxide to the equilibrium, what happens to the colour and why?
The equilibrium shifts to the left to make more H+ ions and so makes more yellow chromate ions
What happens to the equilibrium when you increase the temperature and why?
It shifts to the endothermic direction to use up the heat
What happens to the equilibrium when you decrease the temperature and why?
It shifts in the exothermic direction to try and replace the heat
Why does changing the temperature of a solution of two cobalt complexes change the colour?
Co(H2O)6 is pink and CoCl4 is blue
When will changing the pressure of a reaction have an effect?
When all reactants are gases and there are an uneven amount of moles
What will increasing the pressure of a closed gaseous system do?
It shifts the equilibrium to the side with the fewest gaseous moles, reducing the pressure
What effect does a catalyst have on equilibrium?
No effect on the position of equilibrium but they decrease the time it takes to establish an equilibrium
Why is ethanol production a good example of compromising between yield and time?
The forward reaction is exothermic, so to make the maximum amount of ethanol the temperatures should be low. But, this means a slower rate of reaction, so a compromise of 300c is used
Higher pressures favour the forward reaction and make the maximum amount of ethanol, but these conditions are expensive and dangerous to produce so a compromise of 300 atm is used
What is the equilibrium constant?
A measure of the actual position of the equilibrium
How to work out the equilibrium constant?
Kc = [C]^c x [D]^d
[A]^a x [B]^b
What are the square brackets for?
To show the concentration of
What are a, b, c and d?
The balancing numbers in the equation
What are [A]. [B], [C] and [D}?
The equilibrium concentrations of the substances
How to work out the units for the equilibrium constant?
Put the units into the equation and see which cancel out. There may well be no unit
What does a value of Kc greater than 1 show us?
The position of equilibrium is towards the products
What does a value of Kc less than 1 show us?
The position of equilibrium is towards the reactants
What does a value of Kc equal to 1 show us?
The position of equilibrium is exactly half way
Why don’t catalysts affect the position of equilibrium?
They alter the forward and reverse reactions by the same amount
