TOPIC 6 - THE RATE AND EXTENT OF CHEMICAL CHANGE Flashcards
What is the rate of a reaction?
How quickly the reactants are changed into products.
What does the rate of a reaction depend on?
The collision frequency of reacting particles. The more collisions there are the faster the reaction is. Eg doubling the frequency of collisions doubles the rate.
The energy transferred during during a collision. Particles have to collide with enough energy for the collision to be successful.
What is the activation energy?
The minimum amount of energy that particles need to react.
Particles need this much energy to break the bonds in the reactants and start the reaction.
What four factors determine the rate of a reaction?
Temperature
The concentration of a solution or the pressure of gas
Surface area
The presence of a catalyst.
These four things can be explained by increasing the number of successful collisions between the reacting particles.
How does increasing the temperature increase the rate?
When the temperature is increased, the particles all move faster.
If they are moving faster they are going to collide more often.
Also, the faster they move the more energy they have, so more of the collisions will have enough energy to make the reaction happen.
How does increasing the concentration or pressure increase the rate?
If a solution is more concentrated, it means that there are more particles in the same volume of water.
When the pressure of a gas is increased, it means that the same number of particles occupies a smaller space.
This makes collisions between the reactant particles more frequent.
How does increasing the surface area increase the rate?
If one of the reactants is a solid, then breaking it up into smaller pieces 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 - so there will be collisions more frequently.
How does using a catalyst increase the rate of a reaction?
A catalyst is a substance that speeds up a reaction, without being used up in the reaction itself. This means it is not part of the overall reaction equation.
They work by decreasing the activation energy needed for all reactions to occur. They do this by providing an alternative reaction pathway with a lower activation energy.
Enzymes are biological catalysts - they catalyse reactions in living things.
Explain how reversible reactions will reach 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 concentrations rise, the backwards reaction will speed up.
After a while the forward reaction will be going at exactly the same rate as the backward one - the system is at equilibrium.
What happens when a reaction is at equilibrium?
At equilibrium, both reactions are still happening, but there is no overall effect. This is called dynamic equilibrium. This means the concentrations of reactants and products have reached a balance and won’t change.
What conditions are needed for equilibrium to be reached?
Equilibrium is only reached if the reversible reaction takes place in a ‘closed system’. This is because non of the reactants or products can escape and nothing else can get in.
What does it mean if the equilibrium lies to the right?
The concentration of the products is greater than that of the reactants.
What does it mean if the equilibrium lies to the left?
The concentration of reactants is greater than that of the products.
What does the position of equilibrium depend on?
The temperature
The pressure (only affects equilibria involving gases)
The concentration of the reactants and products.
How can reversible reactions be endothermic and exothermic?
In a reversible reaction, if the reaction is endothermic in one direction, it will be exothermic in the other. The energy transferred from the surroundings is equal to the energy transferred to the surroundings during the exothermic reaction. Eg the thermal decomposition of hydrated copper sulfate.
