C8 Flashcards
What is rate of reaction?
Rate means speed. We can increase the rate of a reaction to make a product faster. Scientists want to increase the speed of a reaction but also reduce the energy needed (£££)
The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed.
For a reaction to take place particles must:
1:Collide in the right orientation (direction) to react
2: Collide with enough energy to react
Frequency of collisions
How often particles bump into eachother.
Higher frequency is likely to lead to a faster reaction so long the particles have enough energy when they collide
Activation energy (EA)
EA: The minimum amount of energy required for a chemical reaction to occur
EFfect on rate when increasing a variable:
SUrface area
Concentration
Pressure
Temperature
Using a catalyst
Surface area- more frequent collisions, no change in energy.
COncentration - more freuqnet collisions, no change in energy
Pressure - more frequent collisions, no change in energy
Temperature more freuqent collisions,increases energy
USing a catalyst - no change in collisons, reduces EA
Reaction profiles
SHow the relative differemce in the energy level of the reactants compared to the products.
Measuring rate as concentration changes (Required practical)
As the concentration of a reagent is increased, the reaction rate increases. This can be measured in two ways:
Measure volume of gas produced
or
Measure the turbidity of solution (how
quickly the X disappears):
They will both have a different conentration of solution
Calculating the rate of a reaction
Mean rate of reaction = quantity of reactants used (g) / time taken (s) = rate (g/s)
or
volume of gas produced (cm3( / timme taken (s) = rate (cm3/s) - (mol/s for higher)
ALWAYS CALCULATE RATE PER SECOND
Catalysts
A catalyst is a chemical added to a reaction which increases the rate.
Catalysts are not used up and can be used over and over many times.
Enzymes are biological catalysts.
Catalysts are not reactants, so they are not written in the chemical equation.
They increase the rate by providing an alternative reaction pathway with lower activation energy.
Different reactions need different catalysts.
Reversible reactions ⇌
SOme reactions are reversible: the products can react and turn back into the starting reactants.
The direction of reversible reactions can be changed by changing the conditions.
Drawing a tangent
Tangent: a straight line that touches the curve. The slope of the
tangent is the rate of reaction at that point.
E.g. “draw a tangent at 10 seconds” (bold line)
Closed systems
Closed system: WHen reactants abd products cant escape (like a flask with a lid on)
IN a closed system reversible will reach equilibrium.
At this point the yield of the product cannot increwase because the forward reaction is happening at the same rate as the backward reaction.
Note: this does not mean taht there is 50% product and 50% reactants. Just that the quantities of reactants and products doesn’t change overall.
Le Chatelier principle
If a change is made to the conditions of a system that is at equilibrium, then the system will respond to counteract a change.
System: the reaction
Counteract: reverse the change
equilibrium: equal in both directions
Le Chatelier principal to predict the effect of changing conditions in a sstem such as:
We can use Le Chatelier’s Principle to predict the effect of changing conditions in a system such as:
If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.
If the concentration of a product is decreased, more reactants will react until equilibrium is reached again
heating or cooling a reaction
adding more of some of the chemicals involved.
removing some of the chemicals involved
Examples of Le chateliers principle
Increasing temperature
Increasing pressure
In this example, if you increase the temperature of the equilibrium, it will move in the endothermic direction as if to try to reduce the temperature.
For reactions between gases, if you increase pressure, the equilibrium position will shift to favour the side with fewer moles of gas (as shown by the symbol equation) because every gas occupies the same volume of space. In the example above, there is 1 mole of N2O4 for every N2 and 2O2 . So, increasing pressure will favour N2O4 (fewer moles), whereas decreasing pressure will favour N2 and 2O2 (more moles).
