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chemistry > rates of reaction > Flashcards

rates of reaction Flashcards

1
Q

how to calculate the mean rate of a chemical reaction

A

mean rate of reaction = quantity of product formed ( or quantity of reactant used) / time taken

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2
Q

why is the rate of reaction the fastest at the start of the reaction

A

as we have a large number of reactant molecules so lots of them are reacting to form the product, this means that we have a large number of collisions per second

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3
Q

why is the rate of reaction the slowest at the end of the reaction

A

as a lot of the reactant molecules have already reacted and turned into the product, so there are fewer molecules available to react, this means that we have a small number of collisions per second

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4
Q

why do reactions eventually stop

A

as all of the reactant molecules have already reacted so the number of collisions per second is 0

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5
Q

what is mean rate of reaction measured in

A

g/s

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6
Q

what does the collision theory state

A

chemical reactions can only take place when the reacting particles collide with each other with sufficient energy

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7
Q

how do you measure the time it take for a reaction to become opaque or change colour

A
  • time how long it takes for the mixture to change colour
  • you can work out the rate of reaction by 1/ time
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8
Q

what do you measure gas in

A

cm3

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9
Q

how to measure the rate of reaction of the amount of reactants used

A
  • measure the mass of the reaction mixture (g) before and after the reaction takes place and time how long it takes for the reaction to happen
  • the mass of the mixture should decrease
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10
Q

how to measure the rate of reaction for the amount of product formed of a gas

A

if one of the products formed is a gas, measure the total volume of gas with a gas syringe and the time it takes for the reaction to happen

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11
Q

what is the rate of a chemical reaction determined by

A

the frequency (number) of successful collisions per second

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12
Q

how does concentration affect rate of reaction

A
  • a higher concentration means that the particles are closer together so they collide more and so there is more successful collisions per unit time , increasing the rate of reaction
  • this means that rate of reaction is proportional to the concentration
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13
Q

REQUIRED PRACTICAL - affect of concentration of rates of reaction - disappearing cross

A

1) use a measuring cylinder to put 10cm3 of sodium thiosulfate solution in a conical flask
2) place the conical flask onto a printed black cross
3) add 10cm3 of HCL into the conical flask
4) swirl the solution and start a stopwatch
5) after a certain time the reaction should go turbid (cloudy), once you can no longer see the cross we stop the timer
6) repeat the experiment using lower concentrations of sodium thiosulfate solution and calculate the mean values for each concentration of sodium thiosulfate

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14
Q

what is a hypothesis

A

a proposal that could explain a fact or an observation

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15
Q

what makes a measurement reproducible

A

if it can be repeated by another person or using a different technique or equipment and still get the same result

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16
Q

ANOMALY OF REQUIRED PRACTICAL - disappearing cross

A

different people have different eye sights, so some people can see the cross for longer than others so the may not get the same results

17
Q

REQUIRED PRACTICAL - volume of gas affecting rate of reaction

A

1) use a measuring cylinder to place 50 cm3 of HCL into a conical flask
2) attach the conical flask to a bung and delivery tube
3) place the delivery tube into a container filled with water
4) placed an upturned measuring cylinder also filled with water over the delivery tube
5) add a 3cm strip of magnesium to HCL and start a stopwatch
6) the reaction produces hydrogen gas which is trapped under the measuring cylinder
7) every 10 seconds we measure the volume of hydrogen gas in the measuring cylinder and we continue until no more hydrogen is giving off
8) repeat the experiment using different concentrations of HCL

18
Q

how does surface area affect the rate of reaction

A

the larger the surface area of a solid the more particles its exposed to, so there is a higher number of collisions causing there to be a higher frequency of successful collisions per unit time increasing the rate of reaction

19
Q

what factors affect rate of reaction

A
  • temperature
  • surface area
  • concentration
  • pressure
20
Q

practical showing the affect of surface area on rate of reaction

A

1) react marble chips with HCL
2) this will produce CO2, we can use the CO2 to determine the rate of reaction, by either using a gas syringe or finding the mass lost, by placing the solution on a balance and a cotton wool onto of the beaker, the cotton wool allows the gas to escape and prevents acid from splashing out of the flask
3) we can change the surface area of the marble chips

21
Q

what is the activation energy

A

the minimum amount of energy that the particles must have in order to react

22
Q

how does temperature affect the rate of reaction

A

in a hot reaction the particles kinetic energy increases, causing them to move and collide more often with greater energy, so there are more successful collisions per unit time and therefore a faster reaction

23
Q

why does no reaction occur when particles with only a small amount of energy collide

A

as they have a low energy so they cannot overcome the activation energy barrier so they would not collide successfully and therefore no reaction would occur

24
Q

what is a catalyst

A

a substance that increases the rate of reaction but is not used up during the reaction

25
Q

pros of catalyst

A
  • allow us to carry out reactions quickly without needing to increase temperature, this saves money
  • we can reuse them again
26
Q

why do particles with large amounts of energy cause a reaction

A

as they have a large amount of energy, causing them to overcome the activation energy barrier, so they are able to collide successfully causing them

27
Q

how do catalysts increase the rate of reaction

A

they provide a different pathway for the reaction that has a lower activation energy

28
Q

characteristics of catalysts

A
  • reduces the amount of energy needed for a successful collision
  • makes more collisions successful
  • speeds up the reaction
  • provides a surface for the molecules to attach to, which increases their chances into bumping into each other
29
Q

how do we change the direction of reversible reactions

A

by changing the condition of the reaction
e.g when heating hydrated copper sulfate it favours the forward reaction to produce anhydrous copper sulfate and water (thus reaction is endothermic as we are putting heat in)
when adding water to anhydrous copper sulfate it favours the backwards reaction and energy is released (thus the reaction is exothermic)
- the same amount of energy is transferrred in each case

30
Q

what happens when a reversible reaction takes place in a closed system

A
  • an equilibrium is achieved
31
Q

what is equilibrium

A

when the rate of the forward reaction is equal to the rate of the backwards reaction

32
Q

what is le chatiliers principles

A

if a system is at equilibrium and a change is made to the conditions then the system responds to counteract the change

33
Q

what happens if we increase the concentration of the reactants in a closed system

A

if we increase the concentration of the reactants then the system is no longer at equilibrium, this means that the concentration of the products has to change until equilibrium is reached

34
Q

what happens if we decrease the concentration of the products in a closed system

A

if we decrease the concentration of the reactants then the system is no longer at equilibrium, this means that the concentration of the reactants have to increase to produce more products so equilibrium can be reached

35
Q

what happens if we increase the temperature of the system

A

the equilibrium favours the endothermic side since then energy is taken in resulting in the temperature to reduce, so the amount of products formed by the endothermic reaction would increase and the amount of products formed by the exothermic reaction would decrease

36
Q

what happens if we decrease the temperature in a closed system

A

the equilibrium favours the exothermic side as energy is being released causing the temperature to rise so the amount of products formed by the endothermic reaction would decrease and the amount of products formed by the exothermic reaction would increase

37
Q

how does pressure affect equilibrium in a closed system

A
  • if we increase the pressure on a reversible reaction at equilibrium the position of equilibrium shifts to the side with the smallest number molecules
  • if we reduce the pressure then the position of the equilibrium shifts to the side with the larger number of molecules
  • if the number of molecules are the same on both sides changing pressure has no effects of the position of equilibrium

chemistry (13 decks)

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