The rate and extent of chemical change Flashcards
What is meant by rate of reaction
Measure of how quickly a reactant is used up or a product is formed in a reaction per unit of time
How do you determine the rate of reaction
Measuring the products made or the reactants consumed, and the time it takes
What are the two equations for rate of reaction; give the units
- Quantity of reactant used (g or cm^3)/time taken (seconds)
- quantity of product formed (g or cm^3)/time taken (seconds)
What are the units which can be used to measure rate of reaction
g/s or cm^3/s
What are limiting reactants
Chemical used up in a reaction that limits the amount of product formed
How can you measure the rate of reaction at a specific point on a graph
- Draw a tangent from the point
- Measure the rise and the run
- Do rise/run (quantity formed/time taken)
What factors affect the rate of reaction
- Concentration
- Pressure
- Surface area
- Temperature
- Presence of catalysts
How does temperature affect the rate of reaction
As temp increases, particles gain more energy therefore they move faster, so particles will collide more frequently and with more energy each time (so they will more likely exceed the activation energy), which increases the rate of reaction
How does concentration affect the rate of reaction
As concentration increases, there are more particles per unit of volume, therefore collisions are more frequent, which increases the rate of reaction
How does pressure affect the rate of reaction
As pressure increases, there are more particles per unit of volume, therefore collisions are more frequent, which increases the rate of reaction
How does surface area affect the rate of reaction
As surface area increases, frequency of collisions increases, therefore rate of reaction increases
How does the presence of a catalyst affect the rate of reaction
Increases the rate of reaction by lowering the activation energy, therefore less energy is needed for a reaction to occur as less successful collisions are required for a reaction to occur
What is collision frequency
Number of successful collisions between reactant particles that happen each second
At any point in a reaction, what will the rate of reaction depend on
- Amount of energy that the particles have; the more energy they have the more energy they transfer during collisions, making it more likely to surpass the activation energy
- Frequency of collisions; how often particles collide with each other. The more often they collide, the more successful collisions there will be over all
What does the collision theory state
For particles to react, they have to collide with sufficient energy (activation energy)
What is a hypothesis
A proposal that could explain a fact or an observation
Describe the practical to discover how concentration affects the rate of reaction
- Measure 50cm3 of 2mol/dm3 hydrochloric acid using a measuring cylinder and pour it into a 100cm3 conical flask
- Add a 3cm strip of magnesium ribbon to the flask and put the bung attached to a gas syringe into the flask
- Record the volume of hydrogen gas given off every 10 seconds up to 100 seconds
- Repeat this using 1mol/dm3 hydrochloric acid and 0.5mol/dm3
What is a catalyst
A substance which increases the rate of chemical reactions but are not used up during the reaction
How do catalysts work
They increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy
How does the presence of a catalyst affect how an energy profile graph looks
- The activation energy with the catalyst is lower than it would be without the catalyst, so the curve is lower on the diagram
What is a reversible reaction
A reaction where the products can react together to reform the original reactants
How can you change the direction of a reversible reaction
By changing the conditions
What is a reversible reaction represented by
A + B <–> C + D
Give 2 examples of reversible reactions
- Ammonium chloride -> ammonia + hydrogen chloride
- Thermal composition of hydrated copper sulphate; Hydrated copper sulphate (blue) -> anhydrous copper sulphate (white) + water
In a reversible reaction, what reaction will either the forward or backward reaction be
Exothermic or endothermic
If the forward reaction is exothermic, what will the backwards reaction be
endothermic
In the reversible reaction of ammonium chloride, which direction of reaction is heating or cooling and exothermic or endothermic
- Forwards reaction is heating and is endothermic
- Backwards reaction is cooling and is exothermic
In the thermal decomposition of hydrated copper sulphate, which direction of reaction is endothermic or exothermic
- The forwards reaction is endothermic
- The backwards reaction is exothermic
What is equilibrium
When the forward and reverse reactions occur at exactly the same rate
What is the concentration of reactants and products at equilibrium
They stay the same and are constant, but this does not mean they have the same concentration
What is meant by the position of equilibrium moving to the left
More reactant and fewer products
What is meant by the position of equilibrium moving to the right
More products and less reactants
What condition must there be for equilibrium to be reached
The reversible reaction must be done in a closed system, which means a sealed environment where none of the reactants or products can escape (if products escaped you would not reach equilibrium)
How can the position of equilibrium change
By changing the conditions; if a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change
What is Le Chatelier’s principle
The idea that if you change the conditions of a reversible reaction at equilibrium, then the position of equilibrium will shift to try and counteract the change
What changing conditions on a system at equilibrium can be applied to Le Chatelier’s principle
- Changing concentration
- Changing temperature
- Changing pressure
How does changing concentration effect equilibrium
- If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentration of all the substances will change until equilibrium is reached again
- If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again, so the position of equilibrium will move to the right
- If the concentration of products is increased, the system will counteract this until equilibrium is reached again, so the position of equilibrium will move to the left
- If the concentration of a product is decreased, more reactants will react until equilibrium is reached again, so the position of equilibrium will move to the right
How does increasing temperature effect equilibrium
- Equilibrium shifts to favour the endothermic reaction to reduce the temperature, as energy is taken in causing the temperature to fall
- The relative amount of products at equilibrium increases for an endothermic reaction
- The relative amount of products at equilibrium decreases for an exothermic reaction
How does decreasing temperature effect equilibrium
- Equilibrium shifts to favour the exothermic reaction to increase the temperature as energy is released, causing the temperature to increase
- Relative amount of products at equilibrium decreases for an endothermic reaction
- The relative amount of products at equilibrium increases for an exothermic reaction
How does increasing pressure effect equilibrium
The position of equilibrium shifts to the side with the smaller number of molecules in order to lower pressure (this is because pressure measures how many particles there are per unit of volume, so to lower the volume the position of equilibrium will shift to the side with the least number of molecules)
How does decreasing pressure effect equilibrium
The position of the equilibrium shifts to the side with the larger number of molecules in order to increase pressure
How does pressure effect a system where the number of molecules on both sides is the same
Changing pressure has no effect on the position of equilibrium
