C6- The Rate and Extent of Chemical Change Flashcards
How is the rate of reaction determined
How fast the reactants are changed into products
How can you find the speed of a reaction
By recording the amount of product formed, or the amount of reactant used up over time
Particles must _ with enough energy in order to react
Collide
The rate of reaction depend on what 4 thing
- Temperature
- The concentration of a solution or the pressure of ags
- Surface area
- The pressure of a catalyst
What does more collision do
Increase the rate of reaction
What does increasing the concentration or pressure do to reaction rate
Increase
What does increasing the surface area do to a reaction
Increases the rate
Rate of reaction =
Amount of reactant used or amount of products formed/ Time
What are the 3 ways to measure the rate of reaction
- Precipitation and colour change
- Change in mass
- The volume of gas given off
How can you use precipitation ad colour change to measure the rate of reaction
- You can record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution (it becomes opaque)
- You can observe a mark through the solution and measure how long it takes for it for it to disappear- the faster the mark disappears, the quicker the reaction
- If the reactants are coloured and the products are colourless (or vise versa) you can time how long it takes for the solution to lose (or gain) its colour
- The results are very subjective-different people might not agree over the extract point when the mark ‘disappears’ or the solution changes colour. Also, if you use this method, you can’t can’t plot a rate of reaction graph from the results
How can you use a change in mass to measure the rate of reaction
- Measuring the speed of a reaction that produces a gas can be carried out using a mass balance
- As the gas is released, the mass disappearing is measured on the balance
- The quicker the reading on the balance drops, the faster the reaction
- If you take measurements at regular intervals, you can plot a rate of reaction graph and find the rate quite easily
- This is the most accurate of the three methods described on this page because the mass balance is very accurate. But it has the disadvantage of releasing the gas straight into the room
How can you use the volume of gas given off to measure the rate of reaction
- This involves the use of a gas syringe to measure the volume of gas given off
- The more gas given off during a given time interval, the faster the reaction
- Gas syringes usually give volumes accurate to the nearest cm^3, so they’re quite accurate. You can take measurements at regular intervals and plot a rate of reaction graph using this method too. You have to be quite careful though-if the reaction is too vigorous, you can easily blow the plunger out of the end of the syringe
What will reversible reactions reach
An equilibrium
What is the equation for a reversible reaction. A+ B ⇌
C+D
Does an equilibrium mean mean the amount of reactants and products are equal
No
What does it mean if in an equation the equilibrium lies on the right
The concentration of products is greater than that of the reactants
What does it mean if in an equation the equilibrium lies on the left
The concentration of reactants is greater than that of the products
What does the position of an equilibrium (in a chemical equation) depend on
- Temperature
- Pressure
- Concentration
Hydrated copper sulfate ⇌
Anhydrous copper sulfate + water
How is rate of reaction calculated
Rate of reaction = amount (e.g. grams, cm3
) of reactant used or product formed
time
Rate of reaction (mol/s) = Moles of reactant used or product formed
time
Name three common ways of
measuring rate of reaction
- Loss in mass of reactants
- Volume of gas produced
- Time for a solution to become opaque
Describe measuring the rate by monitoring mass
loss
Place the reaction flask on a balance. In these reactions (e.g. metal carbonate +
acid) a gas is given off, so record the decrease in mass in time intervals (note
hydrogen is too light). Plot a graph of mass vs time.
Describe measuring the rate
by monitoring the volume of a
gas
Connect a gas syringe to a reaction flask and measure the volume of a gas
formed in time intervals. Plot a graph of volume vs time.
Describe measuring the rate
by monitoring the
disappearance of a cross
Take a piece of paper and mark a cross (X) on it. Put the reaction flask on this
cross. Mix the reagents, and measure how long it takes for a cloudy mixture to
conceal a cross.
How to find a rate of reaction at
some time, t, from a graph of
amount of reactant vs time?
- Pick a point corresponding to the time t, and find the tangent to the curve at
this point. - The tangent is the gradient of this graph - it tells you how fast the reaction
proceeds at this point. The steeper the tangent line, the faster the rate.
Gradient of tangent can be expressed in change in y values over change in x
values.
State five factors affecting the
rate of a chemical reaction
- Concentration of reactants
- Pressure of gases (volume)
- Surface area
- Temperature
- Catalysts
What is the collision theory?
Chemical reactions can occur only when reacting particles collide with each other
with sufficient energy (more than or equal to activation energy).
Describe and explain the
effect of increasing
temperature on the rate of
reaction
T increases = faster reaction
As T increases, kinetic energy of particles increases, i.e. more energetic
collisions
Also, they move faster, so they collide more frequently
However, there is no straight line relationship between rate and temperature,
i.e. they are not directly proportional to each other
Describe and explain the effect
of increasing concentration on
the rate of reaction
Conc. increases = faster reaction,
More reactants = more frequent collisions
Describe and explain the effect
of increasing pressure of a gas
on the rate of reaction
Increasing the pressure of reacting gases, is the same as increasing
concentration. It increases the number of gas molecules in the same volume and
so increases the frequency of collisions and therefore increases the rate of
reaction.
Note that volume and pressure are inversely proportional to each other.
Increasing the volume retards the reaction.
Describe and explain the
effect of increasing surface
area
If solid reactants are in smaller pieces, they have a greater surface area.
Increasing the surface area of solid reactants increases the frequency of collisions
and so increases the rate of reaction, e.g. block of magnesium reacts slower with
acid then magnesium powder.
What is a catalyst and how
does it work? How does it
affect the reaction profile?
A catalyst changes the rate of reaction but is not used up. It increases rate of
reaction by providing a different pathway for the reaction that has a lower
activation energy. The reaction profile for a catalysed reaction will have a lower
maximum of the curve (lower activation energy)
What is an enzyme?
An enzyme is a molecule that acts as a catalyst in a biological system
What is a reversible reaction?
A reversible reaction occurs when the products of a reaction can react backwards
to produce the original reactants
When is dynamic
equilibrium reached?
In a closed system, when the forward and reverse reactions occur at the same rate
and the concentrations of reactants and products remain constant.
Describe Le Chatelier’s
Principle
If a system is at equilibrium and a change is made to any of the conditions, then
the system responds to counteract change and restore the equilibrium.
Describe the effect of
changing the concentration of
reactant and product on the
position of the equilibrium
If the concentration of one of the reactants or products is changed, the system is
no longer at equilibrium and the concentrations 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. If the
concentration of a product is decreased, more reactants will react until equilibrium
is reached again.
Describe the effect of
changing temperature on the
position of the equilibrium
If the temperature of a system at equilibrium is increased:
• the relative amount of products at equilibrium increases for an endothermic
reaction
• the relative amount of products at equilibrium decreases for an exothermic
reaction.
Describe the effect of
changing pressure on the
position of the equilibrium
This applies to equilibria that involve gases
An increase in pressure causes the equilibrium position to shift towards the side
with the smaller number of molecules as shown by the symbol equation for that
reaction. A decrease in pressure causes the equilibrium position to shift towards
the side with the larger number of molecules as shown by the symbol equation
for that reaction. Pressure has no effect on the reactions where the numbers of
gas molecules are equal on both sides of the equation.
Describe the effect of a catalyst on the position of
the equilibrium
No effect.
It just speeds up both forward and backward reactions equally.
i.e. equilibrium is achieved faster.
How do Pressure and Concentration impact the rate of reaction
- Increasing either of these means there are more particles in the same volume
- This means the particles will collide more frequently
- So the rate of reaction increases
How does Surface area impact the rate of reaction
- Increasing surface area means more particles are exposed to the surface
- This means the particles will collide more frequently
- So the rate of reaction increases
How does Temperature area impact the rate of reaction
- Increasing temperature means the particles have more energy
- This means the particles will collide more frequently and with more energy
- So the rate of reaction increases
- (Temperature is the only thing the energy of the particles changes)
How does Catalyst area impact the rate of reaction
This lower the activation energy so that more collisions of are successful
The rate of reaction always starts _ and then _ _
Fast
Slows down
Why does the rate of reaction always start fast and then slow down
- This is because there are lots of reactants at the start and very little/none at the end
- The graphs for rates of reactants are usually curved to show this change in rate
Define rate of reaction
The change in concentration of a reactant or product PER UNIT TIME
What is the Collison Theory
- For a reaction to happen, particles have to collide
- They must collide with enough energy (the activation energy of the reaction) for a reaction to happen
- The more collision there are the faster the rate of reaction will be
How do catalysts speed up a reaction
They lower the activation energy
Define dynamic equilibrium
Where the rates of backwards and forwards reaction are the same in a closed system (sealed container) so the amounts don’t change
At the start of the reaction, the rate of the forward reaction is _ but as the reactants are used up it _
Fast
Decreases
The backwards reaction starts _ but speeds up as more of the _ are made
Slow
Products
At a certain point, the speed of the _ and _ reaction become the same so the amounts of reactants and products are _
Forwards
Backwards
Constant
Changing the conditions will make the equilibrium shift left (to the _) or right (to the _)
Reactants
Products
If an equilibrium shifts to the right the yield _
Increases
If an equilibrium shift to the left the yield _
Decreases
Increasing the pressure shifts the equilibrium to the side with _ moles of _ on it
Fewer
Gas
Increasing the temperature shifts the equilibrium to the _ side
Endothermic
These changes happen to _ the equilibrium
Restore
Increasing the concentration of a chemical will shift the equilibrium _ from this chemical reaction
away
Decreasing the concentration shifts the equilibrium goes to the _
Right (products)
ΔH=
-ve (this is an endothermic reaction)
