Rates Of Reactions Flashcards
Define rates of reaction
Rate of reaction is the change in the concentration per unit of time of any one reactant or product
How are rates of reaction measured?
the formation of a product per unit time can be measured
the loss of a reactant per unit time can be measured
How to calculate average rate of reaction?
Average rate of reaction = Amount of substance formed or lost/ time taken for that to happen
What is ment by instantaneous rate of reaction
the rate of a reaction at any one particular time during the reaction
how to find instantaneous rate of reaction from a graph
draw a tangent to the curve at time asked
choose two good points.
find slope using formula
What dertimes weather a reaction does or does not take place
Reactants will collide with each other when mixed.
These reactats have a certain amount of energy.
if the colliding reactants meet the required energy(acivation energy) it will be an effective collision
What is activation energy
The minium combined energy of colliding particles for effective collisions/ for an reaction to take place
What is mean by an effective collision
A collision that reached activation energy and results in the formation of products
Draw out an large activation energy reaction profile diagram
reactants
large activation energy = slow rate of reaction
activation energy (Eact)
products
delta H
Describe the energy of products vs reactants in an exothermic reaction
energy of products is less than energy of reactants
ΔH = Energy of products – Energy of reactants
(ΔH will be a negative value)
Describe the energy of products vs energy of reactants in an endothermic reaction
energy of products is greater than energy of reactants
ΔH = Energy of products – Energy of reactants
(ΔH will be a positive value
Name the 5 factsots affecting the rate of a reaction
1-Nature of reactants
2-Particle size
3-Concentration
4-Temperture
5-Catalyst
Describe the nature of the reactants (Type of bonding) that occours involving ionic compounds in solution
Reactions involving ionic compounds in solution are faster.
-In solution, ionic compounds are dissociated, preset as free ions.
No bond breaking is required, the activation energy is lower.
Describe the nature of the reactants (Type of bonding) that occours involving covalent compounds in solution
Reactions involving covalent compounds have slower reactions
-Covalent compounds are not dissociated.Their bonds need to be broken and new ones reformed, The activation energy is higher
How does particle size affect the rate of a reaction, and explain why
Smaller particle size increases rate of a reaction
Larger size decreases the rate of a reaction
Reason
Smaller particles have a greater surfance area, meaning greater frequency of collisions occour, means that there will be more effective collisions- more collisions reach activation energy
Dust explosions present a risk in industry, Give five conditions necessary for a dust explosion to occour
particles must be combustible
dust particles must be dry
a source of ignition
Oxygen must be present
must be in an enclosed space
How does concentration of reactants affect the rate of a reaction, explain the reason
Increasing concentration increases rate of reaction
Decreasing concentration decreases rate of a reaction
-More concentrated solution has a greater number of particles.
Greater number of particles means greater frequency of collisions will occur, meaning more effective collisions will occour. More collision reach the required activated energy
What is the relationship between Concentration and the rate of reaction
directly proportional
Why does the rate of a chemical reaction generally decrease with time
as reaction proceeds, more reactants are used up, concentration decreases. Less frequecy of collisions, less effective collisions, less collisions reach activation energy
In the expeirment showing the effect of particle size of calcium carbonate in HCl, describe the difference between whole marble chips vs ground marble
smaller particle size - had a greater rate of reaction, steeper slope. The smaller particle size levels off first( reaction finishes off first)
Both the reactions rise to the same height eventially- both produce the same amount of product.
In the expeirment showing the effect of concentration on rate of reaction between calcium carbonate and HCl, describe the difference between 2 M HCl and 1M HCl
The Higher conc of HCl - higher conc reaction has a greater rate of reaction. The higher concentration reaction levels off first (finishes first) Reactions with double concentration of reactant rise to twice the height - produce twice as much product
How does temperture affect the rate of a reaction and explain the reason.
Increasing temerture increases the rate of reaction.
Decreasing temperture decreases the rate of an reaction
1.Increasing temp increases the energy of the particles (More effective collisions - more collisions reach required activation energy)
2. Increasing temp also increases velocity of collisions (greater frequency of collisions will occour - more reach reuired activation energy)
Given the two reasons why temperture increases the rate of reaction, which is more significant? Explain
Increasing the energy of the particles.
The increase in velocity is small compared to the substantial increase in energy in the particles ( which cause more effective collisions)
What effect does increasing/decreasing temperature have on the activation energy of a reaction?
None
Describe the procedure to study the effect of concentration on reaction rate using sodium thiosulfate and hydrocloric acid
using a graduated cylinder, add a fixed volume of sodium thiosuldate to concial flask. The conical flask has a ** ‘X’ drawn on the bottom** with a marker. Then add dilute HCl quickly to the sodium thiosulfate. The conical flask is swirled and stopwatch is immediately started.
While looking down through the solution, time the time is takes for the yellow sulfur precipitate to obscure the cross. Expeirment repeated w/ different conc of sodium thiodulfate. Graph of rate vs conc is plotted
How is rate caluclated
rate of reaction is the reciprocal/ inverse of time
rate = 1/time
plot a graph for the reaction between sodium thiosulfate and HCl
best fit line through the origin
explain the relationship between rate of reaction and concentration of solution
rate of reaction is directly porportional to concentration of solution.
A more concentrated solution has a greater number of particles.
A greater number of particles mean greater frequency of collisions will occour.
so more effective collisions
Why is it essential the same volume and concentration of HCl are used during each reachtion
To study the effect of concentration on reaction rate using sodium
thiosulfate and hydrochloric acid
only one variable can be changed in order for the expeirment to be a faur test. All other vairables must be fixed
Identify the precipate in this reaction? Descibe its apperance
To study the effect of concentration on reaction rate using sodium
thiosulfate and hydrochloric acid
Sulfur - precipates as a fine pale yellow powder
describe how the reaction time in measued in this expeirment? describe how you could determine when the same mass of sulfur has been formed
To study the effect of concentration on reaction rate using sodium
thiosulfate and hydrochloric acid
The conical flask has a X drawn on bottom.
The sodium thiosulfate and HCl are mixed and looking down through the solution, using a stopwatch, the time for how long it takes the yellow sulfur precipate to obscure the cross is recorded
explain the signifinance of using the same shaped conica flask during each run
To study the effect of concentration on reaction rate using sodium
thiosulfate and hydrochloric acid
Only one vairable can be change to ensure a fair test.
other vairables must remain fixed
Describe this procedure
To study the effect of temperature on reaction rate using sodium
thiosulfate and hydrochloric acid
Using a graduated cylinder, add a fixed volume of sodium thiosulfate to a conical flask.
The conical flask has a cross drawn on the bottom
the conical flaks is placed in a water bath on a hot plate and use thermoeter set it to a specific temperture
Add a fixed volume of dultue hydrovloric acid.
The conical flask is swirled and stopwatch is immediately stated While looking down through the solution, time the time it takes for the yellow sulfur precipitate to obscure the cross is recorded.
using a hot plate, water bath and theromometer, the expeirment is repeated at different tempertures
results are entered in a table and a graph of rate vs temperture is plotted
Explain the relationship between rate of reaction and temperture
To study the effect of temperature on reaction rate using sodium
thiosulfate and hydrochloric acid
as temperture increases, the rate of reaction increases exponentially.
1. Increasing temp increases energy of particles.more effective collisions- more collision reach the required activation energy
2. increases velocity-greater frequency of collisions will occour-more effective collisions
Why is a water bath preferable to bunsen burner/directly heating with hot plate
To study the effect of temperature on reaction rate using sodium
thiosulfate and hydrochloric acid
it is easier to obtain a desired temperture using a water bath- allows for more gentle heating
What is a catalyst
a substance that alters the rate of a chemical reaction without being used up in the reaction
explain why the presence of catalyst can speed up the rate of reaction
a catalyst lowers the activation energy required for an effective collision providing an alternative pathway for a reaction to occour.
lower activation energy means more effective collisions- more collisions reach the required activation energy.
compared to an energy profile of an exothermic reaction,describe the changes that occour with a catalyst
the activation energy is lowered, same amount of peroducts, The ΔH value does not change. -ΔH
List the properties of catalysts
1.Chemically unchanged at the end of an reaction
2.Only required in small amounts
3.specific to a certain type of chemical reactions.
4.can be inactivated by catalytic poisons
5.Brings a reversible reaction to eqilibrium more quickly but does not affect the state of equilibrium
What is an enzyme
biological catalyst that is protein in nature
Name two examples of enzymes
Catalayse
Amylase
How could you reduce the rate of a chemical reaction that takes place in solution
lowering the temperture- cooling mixture
lowering conc of reactants- dilution the reaction micture
adding an inhibitor
Name the three types of catalysis that can occour
homogenous catalysis
heterogenous cataysis
Autocatalysis
What is homogenous catalysis and name an example
in which the catalyst is in the same phase as the reactants and products.
eg esterification
what is hetrogenous catalysis and name an example
in which the catalysis is in a different phase to the reactants and products
eg. haber process
What is auto catalysis and name an example
in which one of the products of the reaction acts as a catayst for the reaction.
eg. titrating potassium manganate against iron II solution
Breifly describe the procedure for
Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst
1.Heat sprial of platium wire in a numsem burner and heat the methanol to 60C in a fumehood.
2.Place a cardboard t-shape covered in aluminium foil in a conical flask. With a sprial of platinum wire above methanol
Describe the observations during the reaction
Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst
- The sprial of platinum goes through cycles of glowing red hot and then dimming
- Popping sounds are heard
- the smell of methanal (formaldehyde)
What is the purpose of the cardboard T piece and why is it wrapped in luminium foil
Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst
the cardboard T piece acts as a chimney allowing waste gasses formed to be swept away
its wrapped in aluminium foil to prevent the cardboard from being burned
explain why the sprial of platinum glows red hot
Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst
the reaction is exothermic
Explain why the sprial of platinum glows then ceass glowing for a while before becoming red hot again
Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst
Carbon Monoxide is a waste gas formed from the incomplete combustion of methanol- it poisons the platinum catalyst
when the carbon monoxide gas is swept away, the catalyst can begin working again
Explain the popping or mini explosions sounds that can be heard
Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst
the formation of hydrogen gas (hydrogen ignited with a pop)
Give three major products of this reactions
Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst
Methanal (formaldehyde)
Water
Hydrogen gas
What type of catalysis is involved in this reaction?Explain
Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst
Heterogenous catalysis- the solid platinum catalyst is in a different phase to the reactants and products
Explain one way in whcih the presence of the platinum catalyst speeds up the oxidation of the hot methanol
Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst
the catalyst lowers the activation energy required for an effective collision providing an alternative pathway for a reaction to occour.
Lower activation energy means more effective collisions- more collision reach the required activation energy
Name the two mechanisms of catalysis
- Internediate formation theory
- Surface adsorption theory
Describe the mechanism for intermediate formation theory
- One of the reactants reacts with the catalysts to form an intermediate compound
- the intermediate compound is not stable and tempoary- it reacts with the other reactant to form the final product with the catalysts regenerated
What does intermediate formation apply to
homogenous catalysis (reactions in solution)
brefiely explain the procedure to demostrate intermediate theory - oxidation of potassium sodium tartate by hydrogen peroxide catalysed by cobalt II ions
set up the conical flask with cobalt chloride crystas to potassium tartrate soltuion in it. plate on hot plate. note pink colour. Add some hydrogen peroxide to the beaker.
State obervations
Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions
- Pink colour turns green
- Effervesence/fizzing is observed
- the fizzing ceases and the green colour returns to pink
EXPLAIN how these obervations provide evidence for intermediate formation theory
Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions
initial pink caused by Co2+ ions
pink to green to pink again suggests an intermediate substance is formed which is tempoary.
The effervesence of the green solution suggests that the internediate substance is reacting and forming product
What product is cause this effervesence
Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions
CO2
What property of catalysts is shown by the solution returning to a pink colour?
Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions
Catalysis are chemically unchanged by the end of the reaction-Co2+ iosn still present
What type of catalysis is involved in this reaction?Explain
Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions
Homogenous catalysis- the catalyst is in solution ie. the same phase as the reactants and the products
What is adosrption
the attachment of liquid or gaseous molecules to a solid surface
Describe the mechanism of surface adsorption theory
- Adsorbtion-the reactants attach onto the surface of the catalysts, forming temporart bond
- Reaction- the reactants are now in high conc on the surface of the catalysts and are more likely to collide
the Bonds within the reactant molecules are weakened and the collision of reactants are more likely to be effective - Desorption- the product is formed and leave the surface of the catalysts
What type of catalysis is the mechanism surface adsorption
Heterogenous catalysis
Explain why the reaction rate is greater when powered platinum is used instead of larger pieces of platinum
Surface adsorption theory
A powdered platinum has a larger surface area exposed - greater frequency of collisions will occour- more effective collisions
Explain how a catalyst poison makes a catalyst inactive
a catalyst poison is preferentially adsorbed onto to catalyst surface and forms a permanent bond with it.
It is unable to desorb and blocks the reactants from adsorbing on the surface preventing their reaction
What is a catalytic converter
A catalytic converter is a devide in the exhause of a motor vehicle that uses catalysts to convert harmful gases in the exhausts into less harmful gases
Name the three elements used as catalysts in catalytic converters
Platinum (Pt)
Palladium (Pd)
Rhodium (Rh)
Comment on the shape of a catalytic converter
honeycomb shape for larger surface area
Give the name of three harmful substances that enter a car’s catalytic converter and name the substances that they are converted to by the catalysts mentioned
Carbon monoxide - Carbon Dioxide
NOx compounds (eg Nitrogen monoxide)- Nitrogen gas
Hydrocarbons (CxHy) - Carbon Dioxide (Co2) + water (H2O)
Give three environental benefits of a catalytic converter in cars
- carbon monoxide, nitrogen monoxide and nitrogen dioxide - poisonous
2.Nitrogen Monoxide contributed to acid rain
3.Hydrocarbons - cause smog and contribute to greenhouse effect
Name two substances that poisions the catalysis in a catalytic converter
Lead compounds
Sulfur compounds
What type of catalysis is involved in a catalytic converter? Explain
Heterogenous catalysis- the solid catalysis is in a different phase to the reactants and products
Breifly explain the procedure of monitoring the rate of production of oxygen from hydrogen peroxide using manganese dioxide as a catalyst
Magnesium Oxide added to Hydrogen peroxide from a small test tube in conical flask.
mixture is shaken and simultaneouly, a stopwatch is started. The Oxygen gas produced is collected over water using a beehive shelf.
vol of O2 measured at regular intervals
Explain the general trend in the curved obtained when a graph of volume of oxygen vs time is plotted
Beginning- Slope steepest – reaction occurring at its fastest rate, most volume of gas produced per time
Middle- Slope becomes less steep - reaction slows down, less volume of gas produced per time
End- Slope is 0 - reaction is finished, no more gas produced
Write a balanced equation for the catalysed decomposition of hydrogen peroxide using
manganese dioxide
To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst
H2O2 –(MnO2)–> H2O + 1/2O2
what is the apperance of the maganese dioxide catalyst
To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst
Black powder
How is it ensured the stopwatch is started at excatly the same time as the reaction starts
To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst
The Manganese dioxide is added to the hydrogen peroxide from a small test tube in the conical flask
The mixture is shaken, and the stopwatch is started at excatly the same time
What chemical hazard symbols are associated with i) hydrogen peroxide ii) manganese dioxide?
Explain their meaning and suggest how you could manage their risk
i) Oxidiser- keep away from other substances
ii) Irritant- wear gloves
What effect would increasing the mass of the catalyst have on the rate of reaction? Explain
no effect, they are only required in small amounts
Wat type of catalysis is involved in this reaction? Explain
To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst
Heterogenous catalysis- the solid magnesium dioxide catalyst is in a different phase to the reactants and products
Name and describe the mechanism for the action of the manganese dioxide in speeding up the
decomposition of hydrogen peroxide
To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst
Surface adsorbtion theory
Adsorption - The hydrogen peroxide reactants are adsorbed onto the surface, forming
temporary bonds
Reaction - The hydrogen peroxide reactants are now in high concentration on the
surface of the catalyst and are more likely to collide
The bonds within the hydrogen peroxide molecules are weakened and collisions of
reactants are more likely to be effective i.e. the activation energy is lowered
iiDesorption - The water and oxygen products are formed and leaves the surface of the
catalyst
