The Rate and Extent of Chemical Changes Flashcards
How do we calculate the overall rate of reaction from a graph?
Total quantity of reactant used/Total quantity of product formed ÷ total time taken. ( △y ÷ △x )
How do we calculate the initial rate of reaction from a graph?
Draw a tangent to the line of best fit, starting at (0,0).
Then △y ÷ △x.
How do we calculate the rate of reaction at a specific point on a graph? (e.g. at 210 seconds)
Draw a tangent to the line of best fit at that point.
Then △y ÷ △x.
What units do we use to measure the quantity of a reactant used or product produced?
-Mass in grams
-Volume in cm³
What are the units for a rate of reaction?
- g/s
- cm³/s
-mol/s
How do you convert from grams to moles?
-Find the molar mass of the substance you are analysing
-Mol = mass ÷ Mr
In graphs showing the quantity of product formed/ the quantity of reactant used up, against time, what goes on the x-axis?
Time (s)
In graphs showing the quantity of product formed/ the quantity of reactant used up, against time, what goes on the y-axis?
The name of the product formed/ reactant used (cm³ or g)
What is the slope of a linear graph?
A measure of its steepness/The gradient.
What can we use on a graph to measure the rate of change?
The slope (gradient) of the tangent to a curve.
What does the collision theory state?
Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
Activation energy
The minimum amount of energy that particles must have to react.
Give four ways in which we can measure the rate of a chemical reaction.
-Measure the loss in mass, per minute
-Count the number of bubbles, per minute
-Measure the volume of gas collected in a syringe after 1 minute
-Measure the time it takes for an ‘x’ placed under a flask to disappear when viewed through the rection mixture
What is a posh way of saying that the cloudiness of a solution changes?
Its ‘turbidity’ changes.
Why might the mass of a reaction mixture decrease?
Due to gas given off.
The rate of reaction depends on (particle model)
- Frequency of collisions between reacting particles
- Energy transferred during collisions
- Activation energy - the minimum energy that particles must have to successfully collide and form bonds
Mean rate of reaction
Quantity of reactant used/time taken
or
Quantity of product formed/time taken
Factors affecting the rate of chemical reactions
- Concentrations of reactants in solution
- Pressure of reacting gases
- Surface area of solid reactants
- Temperature of the reactants
- Presence of catalysts
Rate of reaction
Mass/volume of product formed or reactant used up per unit time.
How does increasing temperature affect the rate of a chemical reaction?
-Particles will gain kinetic energy
-So the particles will move faster
-So they collide more frequently and with more energy
-So, there will be more collisions that surpass the activation energy
-So the rate of reaction will increase
How does increasing concentration affect the rate of a chemical reaction?
-There will be more particles per unit volume of solvent
-So, there will be more frequent collisions between reacting particles
-So, there will be more collisions that surpass the activation energy
-So the rate of reaction will increase
How does increasing pressure affect the rate of a chemical reaction?
-The same number of particles occupies a smaller space
-So, there will be more frequent collisions between reacting particles
-So, there will be more collisions that surpass the activation energy
-So the rate of reaction will increase
How does increasing surface area affect the rate of a chemical reaction?
-For the same volume of solid, the reacting particles around it will have more area to work on
-So, there will be more frequent collisions between reacting particles
-So, there will be more collisions that surpass the activation energy
-So the rate of reaction will increase
How can we increase the surface area (to volume ratio) of a solid?
Break it up into smaller pieces/a powder using a pestle and mortar.
How does using a catalyst affect the rate of a chemical reaction?
-They decrease the activation energy needed for a reaction to occur
-They do this by providing an alternative reaction pathway (with a lower activation energy)
-This increases the rate of reaction
Why do we not include catalysts in the overall reaction equation?
Catalysts are substances which speed up chemical reactions without being used up themselves.
Give an example of a good catalyst
-Transition metals like cobalt and nickel
-Enzymes
Enzymes act as catalysts in what?
Biological systems.
Draw a reaction profile for a catalysed reaction
When you get home xaxaxaxaxaa.
How can we identify catalysts in reactions?
-From their effect on the rate of reaction
-Because they are not included in the chemical equation for the reaction
Which theory explains how various factors affect rates of reactions?
Collision theory.
Explain the effects of changes in the size of pieces of a reacting solid in terms of surface area to volume ratio.
The larger the piece of solid, the smaller the surface area to volume ratio.
Therefore, there will be less area for reacting particles surrounding the solid to collide.
So, collisions will be less frequent leading to a slower rate of reaction.
The more collisions there are…
…the faster the reaction is.
e.g. doubling the frequency of collisions doubles the rate.
Particles need the amount of energy equivalent to the activation energy to do what?
Break the bonds in the reactants and start a reaction.
What is a successful collision?
A collision that ends in the particles reacting to form products.
What does a gentler gradient mean?
A slower rate of reaction.
What does it mean when a line plateaus at a higher mass/volume of product but the same reactants were used?
That more reactants were used.
Give an example of a slow reaction.
-Rusting
-Chemical weathering
Give an example of a moderate speed reaction.
Magnesium reacting with an acid to produce a gentle stream of bubbles.
Give an example of a fast reaction.
-Burning
-Explosions (over in a fraction of a second)
How to interpret rate of reaction graphs:
-The steeper the line, the faster the rate of reaction
-Over time, the line becomes less steep as the reactants are used up
-The quickest reactions have the steepest lines and become flat in the least time
Measuring the rate of reaction - precipitation and colour change
-Measure how long it takes for a cross to disappear when viewed through the reaction mixture
-Measure how long it takes for the solution to lose or gain its colour
-The faster this happens, the faster the reaction
When will a solution turn cloudy from transparent?
If the product is a precipitate which clouds the solution.
Measuring the rate of reaction - change in mass (for gaseous products)
-Place a flask containing your solution onto a mass balance
-As gas is released, the reading on the balance will drop
-The faster it does this, the faster the reaction
Measuring the rate of reaction - change in mass (for gaseous products)
#EVALUATION
-Most accurate of the three methods as a mass balance is very accurate
-You can take measurements at regular intervals and plot a rate of reaction graph
-Releases gas straight into the room
Measuring the rate of reaction - precipitation and colour change
#EVALUATION
-Results are subjective (people may disagree about the exact point when the mark ‘disappears’ or the solution changes colour)
-You can’t plot a rate of reaction graph
Measuring the rate of reaction - volume of gas given off with syringe
#EVALUATION
-Gas syringes are quite accurate (with volumes usually given to the nearest cm³)
-You can take measurements at regular intervals and plot a rate of reaction graph
-If the reaction is too vigorous, you can easily blow the plunger out of the end of the syringe
Give an example of two solutions that react together to produce a cloudy precipitate.
Sodium Thiosulfate and Hydrochloric acid.
What are the drawbacks of fast chemical reactions?
-It may be expensive to generate the necessary conditions
-Safety concerns
Why are faster reactions better most of the time?
They produce more product in a given time.
How can we increase the rate of a chemical reaction?
-By increasing the frequency (number) of collisions and/or the energy of reactant particles.
-This can be achieved by changing lots of different factors.
What is a catalyst?
A substance that increases the rate of a chemical reaction without being changed or used up in the process.
What is a reversible reaction?
A chemical reaction where the products can react to produce the original reactants.
How do we represent reversible reactions?
A+B ⇌ C+D
How can the conditions of a reversible reaction be changed?
By changing the conditions.
If a reversible reaction is exothermic in one direction, it is ___________ in the opposite direction.
endothermic
The same amount of energy is transferred in each case.
What is apparatus which prevents the escape of reactants and products called/an example of?
A closed system.
When a reversible reaction occurs in a closed system, when is equilibrium reached?
When the forward and backwards reactions occur at exactly the same rate.
What’s the difference between equilibrium and dynamic equilibrium?
Nothing they’re the same hahahaaa -trick question.
The relative amounts of all the reactants and products at equilibrium depends on what?
The conditions of the reaction.
What is 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 the change.
How can the effects of changing conditions on a system at equilibrium can be predicted?
using Le Chatelier’s Principle.
If the concentration of one of the reactants or products is changed, the system is no longer at ___________ and the ____________ of all the substances will…
equilibrium
concentrations
change until equilibrium is reached again.
What happens if the concentration of a reactant is increased?
More products will be formed until equilibrium is reached again.
What happens if the concentration of a reactant is decreased?
More reactants will be formed until equilibrium is reached again.
What happens if the concentration of a product is decreased?
More reactants will react until equilibrium is reached again.
What happens if the temperature of a system at equilibrium is increased?
For an endothermic forwards reaction, the relative amount of products at equilibrium increases.
&vice versa
What happens if the temperature of a system at equilibrium is decreased?
For an endothermic reaction, the relative amount of products at equilibrium decreases
& vice versa
For gaseous reactions at equilibrium, what does an increase in pressure cause?
The equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction. (Fewer moles)
For gaseous reactions at equilibrium, what does a decrease in pressure cause?
The equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction. (more moles)
What happens in a reversible reaction?
As the reactants react, their concentrations fall hence the forward reaction slows down.
More products are made simultaneously and as their concentrations rise, the backward reaction will speed up.
After time, the forward reaction and backward reaction will occur at exactly the same rate (the system is at equilibrium).
Equilibrium
At equilibrium, both the forward and backward reactions are happening but there’s no overall effect. (it’s a dynamic equilibrium).
This means that the concentrations of of reactants and products have reached a balance and won’t change.
Why can equilibrium only be reached if the reversible reaction takes place in a closed system?
Because none of the reactants or products can escape/come in.
When a reaction’s in equilibrium, what does this not mean ?
It DOESN’T mean the concentrations of reactants and products are equal.
What does it mean if the position of equilibrium lies to the right?
The concentration of products is greater than that of the reactants.
What does it mean if the position of equilibrium lies to the left?
The concentration of reactants is greater than that of the products.
ammonium chloride ⇌ ammonia + hydrogen chloride
What does heating/cooling this reaction do?
Heating shifts the equilibrium to the left and cooling shifts it to the right.
What does shifting equilibrium to the right mean?
An increased yield.
What is an example showing that the energy transferred from the surroundings by the endothermic reaction is equal the energy transferred to the surroundings during the exothermic reaction are equal?
The thermal decomposition of hydrated copper sulfate:
hydrated copper sulfate ⇌anhydrous copper sulfate +water
forward reaction - endothermic
backward reaction- exothermic
CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O
What colour are hydrated copper (II) sulfate crystals?
(CuSO₄·5H₂O)
Blue
What colour is anhydrous copper (II) sulfate powder?
White
How can Le Chatelier’s principle be used?
To predict the effect of any changes you make to a reaction system?
What is the symbol equation for the formation of ammonia?
N₂ + 3H₂ ⇌ 2NH₃
Sample structure for Le Chatelier’s Principle questions
Chatelier’s principle says the equilibrium will shift to try and ________e.g. Nitrogen dioxide.
This means the ______ reaction will be favoured.
Equilibrium will shift _____ and the colour will turn ______.
(Yield will ________).
Why is the rate of reaction usually fastest at the start?
Because there’s the highest concentration of reactants.
