Chemistry - Rates of Reactions Flashcards

Rate of reaction is the change in the concentration per unit of time of any one reactant or product

the formation of a product per unit time can be measured
the loss of a reactant per unit time can be measured

Average rate of reaction = Amount of substance formed or lost/ time taken for that to happen

the rate of a reaction at any one particular time during the reaction

draw a tangent to the curve at time asked
choose two good points.
find slope using formula

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

The minium combined energy of colliding particles for effective collisions/ for an reaction to take place

A collision that reached activation energy and results in the formation of products

reactants
large activation energy = slow rate of reaction
activation energy (Eact)
products
delta H

energy of products is less than energy of reactants
ΔH = Energy of products – Energy of reactants
(ΔH will be a negative value)

energy of products is greater than energy of reactants
ΔH = Energy of products – Energy of reactants
(ΔH will be a positive value

1-Nature of reactants
2-Particle size
3-Concentration
4-Temperture
5-Catalyst

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.

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

particles must be combustible
dust particles must be dry
a source of ignition
Oxygen must be present
must be in an enclosed space

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

directly proportional

as reaction proceeds, more reactants are used up, concentration decreases. Less frequecy of collisions, less effective collisions, less collisions reach activation energy

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.

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

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)

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)

None

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

rate of reaction is the reciprocal/ inverse of time
rate = 1/time

best fit line through the origin

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

only one variable can be changed in order for the expeirment to be a faur test. All other vairables must be fixed

Sulfur - precipates as a fine pale yellow powder

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

Only one vairable can be change to ensure a fair test.
other vairables must remain fixed

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

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

it is easier to obtain a desired temperture using a water bath- allows for more gentle heating

a substance that alters the rate of a chemical reaction without being used up in the 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.

the activation energy is lowered, same amount of peroducts, The ΔH value does not change. -ΔH

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

biological catalyst that is protein in nature

Catalayse

Amylase

lowering the temperture- cooling mixture

lowering conc of reactants- dilution the reaction micture

adding an inhibitor

homogenous catalysis

heterogenous cataysis

Autocatalysis

in which the catalyst is in the same phase as the reactants and products.

eg esterification

in which the catalysis is in a different phase to the reactants and products

eg. haber process

in which one of the products of the reaction acts as a catayst for the reaction.

eg. titrating potassium manganate against iron II solution

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

The sprial of platinum goes through cycles of glowing red hot and then dimming

Popping sounds are heard

the smell of methanal (formaldehyde)

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

the reaction is exothermic

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

the formation of hydrogen gas (hydrogen ignited with a pop)

Methanal (formaldehyde)

Water

Hydrogen gas

Heterogenous catalysis- the solid platinum catalyst is in a different phase to the reactants and products

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

Internediate formation theory

Surface adsorption 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

homogenous catalysis (reactions in solution)

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.

Pink colour turns green

Effervesence/fizzing is observed

the fizzing ceases and the green colour returns to pink

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

CO2

Catalysis are chemically unchanged by the end of the reaction-Co2+ iosn still present

Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions

the attachment of liquid or gaseous molecules to a solid surface

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

Heterogenous catalysis

A powdered platinum has a larger surface area exposed - greater frequency of collisions will occour- more effective collisions

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

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

Platinum (Pt)

Palladium (Pd)

Rhodium (Rh)

honeycomb shape for larger surface area

Carbon monoxide - Carbon Dioxide

NOx compounds (eg Nitrogen monoxide)- Nitrogen gas

Hydrocarbons (CxHy) - Carbon Dioxide (Co2) + water (H2O)

carbon monoxide, nitrogen monoxide and nitrogen dioxide - poisonous

2.Nitrogen Monoxide contributed to acid rain

3.Hydrocarbons - cause smog and contribute to greenhouse effect

Lead compounds

Sulfur compounds

Heterogenous catalysis- the solid catalysis is in a different phase to the reactants and products

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

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

H2O2 --(MnO2)--> H2O + 1/2O2

Black powder

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

i) Oxidiser- keep away from other substances

ii) Irritant- wear gloves

no effect, they are only required in small amounts

Heterogenous catalysis- the solid magnesium dioxide catalyst is in a different phase to the reactants and products

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