Reaction Rates (kinetics) Flashcards
Reaction rate
Number of moles of product formed per second
OR
Numberof moles of reactant used per second
What are the methods of measuring the rate of a reaction?
- Mass change
- Volume of gas
- Solid disappearing
- Color change
How doe you measure the rate of a reaction using mass change?
How do you measure the rate of a reaction using volume of gas?
By using two different methods. Either the dispalcement of water, or a gas syringe.
How do you measure the rate of a reaction using solid disappearing?
- Measure time until solid disappears
- Do this several times
- Calulate average
How doe you measure the rate of a reaction using color change
- Time until color change is observed
- Calculate the rate
What is the problem of measuring the rate of reactions uing solid disappearing?
- There is no graph and so, you can only calculate the average rate.
What is the problem of using color change to measure the rate of reactions?
- There is no graph
- It is subjective
How do you calculate the rate of a reaction at a specific point?
By using the graph and finding the tangent and calculating the gradient of that tangent.
What is the trends with the rate of the reaction?
The rate of a reaction ALWAYS decreases from the beginging to the end. The initial rate is always the highest.
Why does the rate of a reaction decrease as time passes?
Because the frequency of collisions decreases (collisions of particles colliding with eachother with eachother per second).
Why does the frequency of the collisions between particles in a reaction decrease as the time increases?
This is because the reactants are used up during the reaction causing fewer to be availabe to react. At the end of the reaction there are no more reactants available. Therefore, no collisions can occur.
What needs to occure for a reaction to happen?
During a reaction, particles need to collide. These collisions require 2 properties in order to be successful.
What properties do the collision of particles require in order to be successful?
- Sufficient energy (activation energy)
- Correct orientation
What factors can affect the rate of a reaction?
- Surface area
- Temperature
- Catalyst
- Concentration/preassure
When do you use mass change to measure the rate of a reaction?
When a reaction produces gas or makes gas
When do you use the volume of gas to measure the rate of a reaction?
when a reaction produces gas and there was no gas initially.
When do you use solid disappearing to measure the rate of a reaction?
When an (aq) is produced from a solid and liquid.
When do you use color change to measure the rate of a reaction?
When 2 (aq) are forming new chemicals and none of the other methods apply. (Displacement)
When hydrogen is produced in a raction, should you use volume of gas produced or mass change to measure the rate of the reaction?
Usually, mass change would be used. However, hudrogen is very light and therefore, the mass change is hard to detect.
What is the advantage of using vol. of gas to measure the rate of a reaction?
You can measure the rate of the reaction at any point of the reaction.
What must occure for the rate of a reaction to increase?
- The frequency of collisions must increase
- The proportion of successful collisions must increase by:
-changing the energy of particles
-changing the orientation of particles
Method to measure the effect of SA on the rate of a reaction.
- Measure 25cm³ of HCl into a chonical flask
- Record the mass of the apparatus
- Record the mass of CaCO₃ (calk form)
- Add CaCO₃ to HCl (on the balance)
- Start timing
- Record the mass every 20 seconds until reaction ceases
- Repeate with an equal mass of CaCO₃ powder
Explain why there is a difference in reaction rate when using powdered solid instead of large pieces.
The pwdered solid has a higher SA, therefore, more particles are available to react at the SA of the solid. So, there is a higher frequency of collisionsbetween the paricles.
What happens to the rate of a reaction when you increase the temperature?
Increasing the temp will increase the rate of a reaction through 2 factors:
* Particles gain energy, move faster and collide more frequently. (This affect is minor, there is only a slight change on the frequency)
* The proportion of particles with Ea is increased
Use a graph to compare what occures in a reaction when the themp is increased.
Why does the rate of a reaction increase with a higher temperature?
At a higher temp. a greater proportion of particles have the required Ea. It is more likely that each collision will be succesful. The frequency for successful collisions will increase and therefore, the rate shall as well. additionally, the particles will be moving faster as they have more energy. Therefeore, the collisions between reactants will be more frequent.
What is the difference between energy and temperature?
Temp: the average energy/movement of particles (does not depend on no. of particles)
Energy: the total movement of particles in a subsatnce (depends on no. of particles)
How does changing the concentration effect the rate of a reaction?
- Only affects solutions
- If there are more particles in the same volume then they will collide more frequently
- Increases the no. of reactant particles
- Increasing the concentration increases the number of reactant particles in a specific volume causes there to be more particles in the same volume. This results in an increased collision frequency.
How does changing the surface area affect the rate of a reaction?
- Only affects solids
- Increases frequency of collisions
- Increases proportion of partic;es available to react
How does changing the temperature affect the rate of a reaction?
- Increases collision frquency
- Increases proportion of successful collisions
- Increases proportion particles with Ea
- Affcts movement of particles
How does adding a catalyst affect the rate of a reaction?
- Increases collision frequency
- Increases proportion of successful collisions
- Affects the position of Ea
- Affects orientation of particles
Catalyst
A substance which increases the rate of a reaction without being used up in the reaction.
Why do catalysts affect the frequency of collisions with the right orientation?
Because one of the reactants ususally adsorbs to the catalyst due to the size of the catalyst particles. The frequency is increased since the reactant molecules are fixed in plave. Collisions are more likely to have the correct orentation due to the position of adsorbtion.
Draw 2 graphs to show hoe catalysts affect the rate of a reaction
Why would the rate in a reaction increase with time?
Because the reaction is exothermic. If the reaction releases sufficient energy and the temp. increases a lot, than the rate of the reaction will increase (freq. collsions + greater proportion of particles with sufficient energy)
What is a reversible reaction?
A reaction in which the reactants combine to form products, and the products can also react to form the original products.
In a reversible reaction, what is the reaction forming the products called?
Fprward reaction
In a reversible reaction, what is the reaction forming the reactants called?
The reverse OR backwards reaction
Are reverse reactions exo. or endo?
If the forward reaction is endo, than the reverse reaction will be exo (and vise versa). Note that the same amount of heat is transferred in both directions.
What is the word equation for the test for water?
hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water
What is the symbol eqyuation for the test for water and what color are the chemicals?
CuSO4*5H2O ⇌ CuSO4 + 5H2O
CuSO4 = blue (hydrated)
CuSO4 = white anydrous
What does it mean for a salt to be hydrated?
salts that contain water of crystallisation which affects their molecular shape and colour.
What is needed for the test for water?
- The chemicals
- HEAT (to dehydrate the hydrated copper (III) sulfate.
What happens during the forward reaction of the test for water, is it endo. or exo? Why?
- The Hydrated copper (III) sulfate is seen as blue crystals
- When it is heated, it forms a anydrous copper (III) sulfate (white crystals)
- This reacytion is endo as energy is taken in to remove the water
What happens during the backwards reaction of the test for water, is it endo. or exo? Why?
- The anhydrous salt is copper(II) sulfate is usually seen as white crystals / powde
- The released water from the forward reaction causes the anhydrous salt to form CuSO4*5H2O (original)
- This reaction is highly exothermic
In the test for water which reaction is endo. and which one is exo?
(The forward reaction or backward reaction)
The forward reaction is exothermic and the reverse reaction is endothermic
How does the enthalpy change vary in a reverse reaction?
- The enthalpy change is diff. for the forward and backwards reactions.
- The enthalpy change is the equal but one is + (endo) and the other is - (exo)
- This depends on which way the equation is written
What is the symbol equation for hydrated hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water
CoCl2*6H2O ⇌ CoCl2 + 6H2O
What are the word equations of 2 reverse reactions for hydrated and anydrous salts?
hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water
AND
hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water
What is the color change observed in the following reaction?
CoCl2*6H2O ⇌ CoCl2 + 6H2O
pink => blue
What happens during the forward reaction of the following equation, is it endo. or exo? Why?
CoCl2*6H2O ⇌ CoCl2 + 6H2O
- The hydrated salt is heatedto form anhydrous cobalt(II) chloride, CoCl2
- This reaction is endothermic as energy is taken in to remove the water
- Concentrated HCl is added
What happens during the backwards reaction of the following equation, is it endo. or exo? Why?
CoCl2*6H2O ⇌ CoCl2 + 6H2O
- Adding water to the anhydrous salt forms the hydrated cobalt(II) chloride hexahydrate, CoCl2*6H2O
- This reaction is highly exothermic
Additional water must be added
What are the observations during the following reaction?
CoCl2*6H2O ⇌ CoCl2 + 6H2O
When the solid is heated, it formas a region of colorless (g). As it begins to cool down the solid reforms
What is an important rule to remember for the rate of reversible reactions>
The rate for forward/backwards reactions are independant and can be affected by temp/SA/conc/catalysts.
In what conditions can a reversible reaction reach an equilibrium and why are these conditions required?
What: A reversible reaction can reach equilibrium in a closed system
Why: This is so none of the reactants/products don’t leave the reaction vessel and nothing else can enter
What occures when a reversible reaction reaches equilibrium?
- The rate of the forward reaction is equal to the rate of the reverse reaction
- The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)
Equilibrium?
The state in a reversible reaction where the conc of reactants and products do not change. The rate of the forwards and backwards reactions are equal but not zero. The conc of the reactans and products however, do not need to be the same
Describe the process by which a reversible reaction reaches equilibrium
- Conc of reactants decreases until a constant level is reached
conc. of products increases until a constant level is reached - The time that both reactions reach a constant level is the same
- The conc reached by each substance is different (except in some cases.
Do the concentartions of the reactants and products need to be the same in an equilibrium?
No, they just must have a constant rate.
Draw a graph which represents an equilibrium and explain what is occuring
At the beginning, only the forward reaction happens. A soon as the product (B) starts to be made, the backwards reaction changes some of the B back to the original reactants (A). The rate of the forward reaction gradually decreases as the rate of the backwards reaction increases.
What determines the relative amounts of reactants and products at equilibrium?
The relative amounts of reactants and products at equilibrium depend on the conditions of the reaction. This balance is explained by Le Chatelier’s Principle
Le Chatelier’s principle states that:
When a change made to a system in equilibrium, the system moves to oppose the change.
Position
(in terms of equilibrium)
Measure of the % of total chemicals present as products
What cnditions affect the position of equilibrium?
- Temperature
- Pressure
- Concentration
- catalyst-ish
What does it mean when the equilibrium shiftes to the right?
It means the forward reaction is favoured
This means that there is an increase in the amount of products formed
What does it mean when the equilibrium shifts to the left?
It means the reverse reaction is favoured
So, there is an increase in the amount of reactants formed
What is necessary to predict the effect of temperature changes on systems in equilibrium?
To predict the effect of temperature changes on systems in equilibrium, it is necessary to know whether the reaction is exothermic or endothermic.
If the temp. is raised what happens to the yield of both reaction in a reversible reaction?
- The yield from the endothermic reaction increases
- The yield from the exothermic reaction decreases
If the temperature is lowerd, what happens to the yield of both reaction in a reversible reaction?
- The yield from the endothermic reaction decreases
- The yield from the exothermic reaction increases
What will increasing the temp do to the rate of a revresible reaction?
Increasing the temp will increase the rates of both reactions, but it will affect the endo. reaction more.
Draw a table to represent the affect of temp on equilibrium.
How does pressure affect equilibrium?
What type of reactions are affected by changes in pressure?
Changes in pressure only affect reactions where the reactants or products are gases.
Draw a table to represent the effects of pressure on equilibrium.
What happens when the pressure is increased in a reversible reaction?
It favours the side with fewer gas molecules to reduce the pressure to oppose the change.
What happens to the equilibrium if the concentration of a reactant increases.
If the concentration of a reactant increases, then the equilbrium shifts to the right to balance this balance
Drwa a table to represent the effects of concentration on equilibrium.
What is the effect of a catalyst on the position of equilibrium?
A catalyst does not affect the position of equilibrium.
How does the precense of a catalyst affect an equilibrium?
What: The presence of a catalyst increases the rate at which equilibrium is reached.
Why:
* This is because the catalyst increases the rate of both the forward and backward reactions by the same amount by providing an alternative pathway requiring lower activation energy.
* As a result, the equilibrium concentration of reactants and products is the same as it would be without the catalyst
Draw a graph to represent the affect of catalyst on equilibrium.
What is the contact process?
Sulfuric acid is made in the contact process.
What is the main stage of the contact process?
The main stage in thr contact process converts sulfur dioxide to sulfur trioxide.
What is concentrated sulfuric acid used for?
Concentrated sulfuric acid is used in car batteries, making fertilisers, soaps and detergents
What are the raw materials involved in the contact process?
- Sulfur (S) => crude oil, which is a yellow solid, extracted from dead animals
- Oxygen (O2) => air, no need to seperate it
- Water (H2O)
Describe the 1st stage of the contact process?
The production of sulfur dioxide, which can be obtaind by two ways:
- Burininf sulfur: sulfur + oxygen → sulfur dioxide
S + O2 → SO2 - Roasting sulfur dioxide
What flame does the first stage of the contact process produce?
Blue
Describe the 2nd stage of the contact process.
the oxidation of sulfur dioxide to sulfur trioxide using a vanadium(V) oxide, V2O5, catalyst:
Describe the 3rd stage of the contact process.
What temp is used during this reaction and why.
Forward reaction = endo
Backwards reaction = exo
-
High temperature favours the reverse reaction as it is endothermic
So, a higher yield of reactants will be made -
Low temperature favours the forward reaction as it is exothermic
So, a higher yield of products will be made - However, at low temperature the rate of reaction is very slow
- So, **450 ºC **is a compromise temperature between having a lower yield of products but being made more quickly
What pressure is used in the following reaction and why?
Reactants: more gaseous moles
Products: less gaseous moles
-
Low pressure favours reverse reaction bcs more moles of gas in reactants
So, a lower yield of products will be made -
High pressure favours the forward reaction bcs fewer moles of gas in product
So, a higher yield of products will be made. - However, the position of equilibrium lies far to the right
The equilibrium mixture contains about 96% sulfur trioxide - So, the reaction is carried out at just above atmospheric pressure because:
High pressures can be dangerous and very expensive equipment is needed
A higher pressure causes the sulfur dioxide to liquify
What is the symbol of ammonia?
NH3
What is ammonia used for?
- To maufacture fertilizers
- To make explosives
What is produced in the harber process?
Ammonia (NH3)
Is the harber process endo or exo?
The harber process is exothermic.
What is ammonia made from in the harber process?
- Nitrogen, obtaind from the air
- Hydrogen, natural gas
How is hydrogen obtaind for the harber process?
- from natural gas => methane non-rewnewable :(
OR - from electroloysis of water
Where is nitrogen obtined from for the harver process?
It is obtained from the air (the air is 78% nitrogen)
Write a symbol equation of the harber process
What is the process by which ammonia is maufactured?
The harber process
What are the steps to the harber process
- Obtain N2 and H2 using suitible methods
- Pump them into a compressor through a pipe
- Gases are compressed to 200 atm
- Pressurised gases are pumped into tank containing layers of an iron catalyst at temp of 450 °C
- Some H2 and N2 react to form ammonia
- Unreacted H2 and N2 and ammonia product pass through coolin tank
- Ammonia is liquified
- Unreacted H2 and N2 are recycled back into system to start over again
What are the conditions during the harber process?
- Pressure = 200 atm
- Temp = 450 degrees celcius
What is the optimum and actual pressure used during the harber process? Why?
- Optimum pressure: high pressure, fewer mols of gas on the RHS
- Actual pressure: 200 atm because it lowers opperating costs and improves safety
- Optimum temp: Forward reaction is exo, therefore a low temp = high yield
- Actual temp: 450°C (mid temp) because:
low temp = high yield, slow rate hight
temp = low yield, fast rate
NOTE: ammonia must be removed quickly to avoid backwards reaction
Oxidation
A reaction where a substance gains oxygen
The loss of electrons
Reduction
A reaction where a substance loses oxygen
The gain of electrons
Why are salts better fertilizers than NH3?
Because salts are…
* ionic, therefore they dissolve in water
* neutral (NH3 is a base)
* contain additional elements
Also, NH3 decomposes to form H2 and N2 (with heat)
Redox
A reaction where a substance loses oxygen
The transfer of electrons
What are the different types of reactions which are examples of redox reactions?
- The extraction of metals
- alkane + oxygen (combustion of alkane)
Oxidation state
An indication of the no. of electrons an element has gained/lost compared to its elemental form (shown with sighn followed by the number).
What are the rules to work out the O.S of a compound?
- group 1 compounds are always = +1
- group 2 compounds are always = +2
- oxygen = -2 (except in peroxide => -1)
- halogen = -1 (except when boned with other halogens)
- hydrogen = +1 (except in metal hydroxides => -1)
- when there are 2 halogens the more reactive one is -1, the less reactive one is calculated.
