C6 - The Rate and Extent of Chemical Change Flashcards
What is the rate of a chemical reaction?
How fast the reactants are changed into products
Collision Theory
Explains how reactions happen at different rates (which are the different factors)
What are the 2 factors that rate of a chemical reaction are dependant on?
- Collision frequency (how often they collide). The more collisions, the faster the reaction is
- The energy transferred during a collision, particles have to collide with enough energy for the collision to be successful
What are the factors affect the rate of reaction?
- Temperature
- Surface area
- Concentration of the solution or pressure of a gas
- Presence of a catalyst
Why does temperature increase the reaction rate?
- Particles move faster
- Collide more frequently
- More collisions is more energy
Why does increasing the concentration or pressure increase the reaction rate?
- If the solution is more concentrated, it means there are more particles knocking about in the same volume of water
- In a gas, the same concept is applied
- This makes collisions between the reactant particles more frequent
Why does increasing the surface area increase the rate?
- Breaking a solid up will increase its surface area to volume ratio
- This means that for the same volume of the solid, the particles around it will have more area to work on
- This means there will be more collisions
Why does using a catalyst increase the rate?
- Catalysts work by decreasing the activation energy needed for the reaction to occur
- They do this by providing an alternative reaction pathway with a lower activation energy
What is activation energy?
Minimum amount of energy for chemical reactions to happen (through particles)
Advantages of Catalysts
Reduces necessary energy
Disadvantages of Catalysts
Many are toxic and contaminate ecosystems
What are catalysts
Increase the rate of reaction by providing an alternative pathway with a lower activation energy and does not get used in the reaction
What are the 3 ways to measure to measure the rate of reaction?
- Precipitation and colour change
- Change in mass
- Volume of a gas given off
What are the variables with time and reactants used up in a graph?
y-axis is reactants used up
x-axis is time
How do you find the mean reaction rate for a reaction?
Work out the overall change in the y-value and divide it by the total time taken
How to work find the reaction rate a certain point on a graph?
- Use a tangent to find out the gradient
- Change in y over change in x
- This gives the gradient (aka the rate in this case)
What does this ‘⇌’ symbol show?
That the reaction is reversible and the products can react for form the reactants again
What happens to energy at equilibrium?
It is conserved, no energy is gained or lost
What is meant by the system being at equilibrium?
- As the reactants react, their concentrations fall - so the forward reaction will slow down. But as more and more products are made and their concentration rise, the backward reaction will speed up
- After a while the forward reaction will be going at exactly the same rate as the backward one
What happens at equilibrium?
- Both reactions are still happening, but there’s no overall effect (dynamic equilibrium)
- This means the concetrations of reactants and products have reached a balance and won’t change
What is required for equilibrium to be reached?
- The reversible reaction needs to take place in a ‘closed system’
What is dynamic equilibrium?
- The rates of forwards and backwards reactions are equal
- Both the forward and backward reactions are still taking place
- The reactants and products do not change when a reaction is at dynamic equilibrium
What is a closed system?
None of the reactants or products can escape and nothing else can get in
Why is the position of equilibrium important?
- When the reaction is at equilibrium, it doesn’t mean the amount of reactants and products are the same
What conditions affect the position of equilibrium?
- Temperature
- Pressure
- Concentration
Equilibrium lies at the right (p vs r)
- Temperature decreases
- The concentration of products (exothermic) is greater than that of the reactants (endothermic)
Equilibrium lies at the left (p vs r)
- Temperature increases
- The concentration of the reactants (endothermic) is greater than that of the products (exothermic)
What happens to the position of equilibrium once you increase pressure?
Shift to the reaction that produces the fewest gas molecules
What happens to the position of equilibrium once you decrease pressure?
Shift towards the side of the reaction that produces the most gas molecules
How do endothermic and exothermic reactions affect reversible reactions?
- If the reaction is endothermic in one direction, it will be exothermic in the other direction
- The energy transferred from the surroundings by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction
- A good example is the thermal decomposition of hydrate copper sulphate
What is Le Chatelier’s Principle?
- The idea that if you change the conditions of a reversible reaction at equilibriumm the system will try to counteract that change
Why can Le Chatelier’s principle be useful?
- It can be used to predicts the effect of any changes you make to a reaction system
How Le Chantelier’s principle apply to increasing temperature?
- If you raise the temperature, the equilibrium will move in the endothermic direction to try to decrease it. You’ll now get more products for the endothermic reaction and fewer products for the exothermic reaction
How Le Chantelier’s principle apply to decreasing temperature?
- If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat. This means you’ll get more products for the exothermic reaction and fewer products for the endothermic reaction
How Le Chantelier’s principle apply to increasing pressure?
- If you increase the pressure, the equilibrium shifts to the side with the smaller number of molecules
How Le Chantelier’s principle apply to decreasing pressure?
- If you decrease the pressure, the equilibrium shifts to the side with the larger number of molecules
How Le Chantelier’s principle apply to decreasing concentration?
- If you increase the concentration of products, the system tries to increase it again by reducing the amount of products
How Le Chantelier’s principle apply to increasing concentration of reactants?
- If you increase the concentration of the reactants, the system tries to decrease it by making more products
