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CGP Chemistry > Section 5 - Physical Chemistry P1 > Flashcards

Section 5 - Physical Chemistry P1 Flashcards

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1
Q

Define an exothermic reaction:

A

An exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature of the surroundings.

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2
Q

Define an endothermic reaction:

A

An endothermic reaction is one which takes in energy to the surroundings, usually in the form of heat and usually shown by a fall in temperature of the surroundings.

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3
Q

What is enthalpy change?

A

The overall change in energy in a reaction is called the enthalpy change.
It has the symbol ΔH.

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4
Q

What are the units for enthalpy change?

A

kJ/mol

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5
Q

What is the enthalpy change value in an exothermic reaction?

A

-ΔH

because energy is given out

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6
Q

What is the enthalpy change value in an endothermic reaction?

A

+ΔH

because energy is taken in

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7
Q

What can you use to find out enthalpy change?

A

Calorimetry

-it allows you to measure the amount of energy transferred in a chemical reaction

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8
Q

What are the two different types of experiments you can do using calorimetry for measuring enthalpy change?

A
  • Dissolving, displacement and neutralisation reactions

- combustion

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9
Q

Describe a practical for how you can use calorimetry for dissolving and displacement reactions:

A

You take a temperature of the reactants, mix them and take a temperature at the end of the reaction.
-mix the reactants in a polystyrene cup

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10
Q

What is the biggest problem with using a polystyrene cup for a calorimetry experiment?

A

The amount of energy lost to the surroundings

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11
Q

How can the accuracy of a calorimetry - dissolving, displacement and neutralisation reaction be improved?

A
  • put the polystyrene cup in a beaker with cotton wool (more insulation)
  • put a lid on the polystyrene cup (to reduce energy lost by evaporation)
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12
Q

Describe a practical for how you can use calorimetry for a neutralisation reactions:

A
  • put 25cm³ of HCl and NaOH in separate beakers
  • place both beakers in a 25⁰C water bath
  • add the HCl and NaOH into a polystyrene cup
  • take the temperature every 30secs, record the highest temperature
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13
Q

Describe a practical for how you can use calorimetry for a combustion reaction:

A

You can burn the fuel and use it to heat up water.

  • put 50g of water in a copper can and record its temperature
  • weigh the spirit burner and lid
  • light the spirit burner under the copper can, stir the water constantly until the temperature is 50⁰C
  • put out the flame and measure the final temperature of the water
  • weigh the spirit burner and lid again
  • calculate enthalpy change
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14
Q

What are the issues with using a calorimetry for a combustion reaction?

A

Heat loss to the environment
-so you have to reduce draughts as much as possible,
using draught excluders, put the flame close to the copper container of water, have a lid on the copper container

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15
Q

What is the equation for how to calculate heat energy transferred?

A 
Q = m x c x ΔT
Q - heat energy transferred (J)
m - mass of the liquid being heated (g)
c - specific heat capacity (J/g/⁰C)
ΔT - change in temperature of the liquid (⁰C)
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16
Q

What is the specific heat capacity of water?

A

4.2J/g/⁰C

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17
Q

How do you calculate molar enthalpy change?

A
  • calculate heat energy change in kJ
  • calculate how many moles of the substance there is (moles = mass/Mr)
  • molar enthalpy change = kJ/mol
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18
Q

Define the rate of a reaction:

A

it is how fast the reactants are changed into products

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19
Q

What is an example of a slow reaction?

A

rusting

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20
Q

What is an example of a moderate speed reaction?

A

magnesium reacting with an acid to produce a gentle stream of bubbles

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21
Q

What is an example of a fast reaction?

A

burning is fast

explosions are faster

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22
Q

What are the two ways you can find the speed of a reaction?

A
  • the amount of product formed over time

- the amount of reactant used up over time

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23
Q

At the the start of a reaction what would the amount of reactants be? And at the the end of a reaction what would the amount of reactants be?

A

Start-high

End-low

24
Q

What happens to the amount of reactants as a reaction takes place? And when does the amount of reactants change fastest?

A

It decreases

At the start

25
Q

How could you sketch a graph to show what happens to the amount of reactants during a reaction?

A

Draw a graph of concentration(mol/dm³)[y] against time(s)[x]
The graph would be a curve starting high on the concentration curving down towards time and levelling out (like the shape of an elongated l)

26
Q

How could you sketch a graph to show what happens to the amount of products during a reaction?

A

Draw a graph of concentration(mol/dm³)[y] against time(s)[x]

The graph would be a curve starting at the origin curving up and levelling out(like the shape of an f without the cross)

27
Q

On a rate of reaction graph what can the gradient mean?

A
  • the steeper the gradient the faster the reaction
  • over time the gradient becomes less steep as the reactants are used up
  • when the gradient = 0 the reaction has finished
28
Q

What does the rate of a reaction depend on?

A
  • collision frequency

- energy transferred during a collision

29
Q

How does collision frequency affect the rate of reaction?

A

the more collisions, the faster the reaction is

-double the frequency, doubles the rate of reaction

30
Q

How does the energy transferred during a collision affect the rate of reaction?

A

particles have to collide with enough energy for the collision to be successful

31
Q

Define activation energy:

A

The minimum amount of energy that particles need to react is the activation energy

32
Q

What factors affect the rate of reaction?

A
  • temperature
  • surface area
  • concentration of a solution or pressure of a gas
  • the presence of a catalyst
33
Q

Hoes increasing the temperature affect the rate of reaction?

A

When temperature increases,,,

  • particles gain kinetic energy and move faster, so they have more collisions
  • particle collisions have more energy (reactions only happen if particles collide with enough energy)
  • more successful collisions, so the rate of reaction increases
34
Q

How does increasing the concentration or pressure affect the rate of reaction?

A

If a solution is more concentrated…
-there are more particles of reactant in the same volume, this makes collisions more likely, so the rate of reaction increases
If the pressure of a gas is increased…
-particles as closer together so collisions of particles will be more frequent, so the rate of reaction increases

35
Q

How does increasing the surface area (having smaller solid particles) the rate of reaction?

A

If a reactant is a solid & the surface area is increased…

  • will increase its surface area to volume ratio
  • particles around it will have more area to work on, frequency of collisions will increase
  • so the rate of reaction is faster for solids with larger surface are to volume ratios
36
Q

Define a catalyst:

A

A catalyst is a substance which increases the rate of reaction without being chemically changed or used up in the reaction.

37
Q

Do you need a large or a small amount of catalyst in a reaction?

A

Only need a small amount because it isn’t used up

38
Q

How do catalysts work?

A
  • by decreasing the activation energy
  • they provide an alternative reaction pathway that has a lower activation energy
  • so more particles have at least the minimum amount of energy needed to start a reaction when the particles collide
39
Q

What is the equation for how to calculate rate of reaction?

A

rate of reaction =
amount of reactant used or amount of product formed
÷ time

40
Q

What are three experiments you can do to measure the rate of reaction?

A
  • precipitation
  • change in mess(usually gas given off)
  • the volume of gas given off
41
Q

Describe how you can use a precipitation reaction to measure rate of reaction:

A
  • put a flask on a piece of paper with a cross on
  • mix the two clear reactant solutions in the flask and start a timer
  • observe how long you can see the cross for and stop once you can’t, the faster it disappears, the faster the reaction
42
Q

When does a precipitation reaction work for measuring the rate of reaction?

A

For a reaction where you are mixing two clear solutions which produce a precipitate

43
Q

What is the problem with the precipitation reaction for measuring the rate of a reaction?

A

The result is subjective

-different people might not agree on when the mark disappears

44
Q

Describe how you can measure change in mass from a reaction to measure rate of reaction:

A
  • need a reaction that produces a gas,place it on a mass balance, with a piece of cotton wool in the flask top
  • the quicker the reading on the balance drops, the faster the reaction
  • reaction has finished when the mass stops changing
  • can use your results to plot a graph of change in mass against time
45
Q

Why is cotton wool placed in the top of the flask for the mass loss experiment for measuring the rate of reaction?

A
  • it lets gases through

- stops any solid, liquid or aqueous reactant flying out during the reaction

46
Q

What is a safety precaution that may need to be taken depending on the product produced in a mass loss experiment?

A

If the gas released is harmful then the experiment should be carried out in a fume cupboard

47
Q

Describe how you can use the volume of gas given off in a reaction to measure rate of reaction:

A
  • use a gas syringe to measure the volume of gas given off
  • reaction has finished when no more gas is produced
  • use your results to plot a graph of gas volume against time elapsed
48
Q

What is a safety precaution that may need to be taken depending on the reaction in a volume of gas given off reaction?

A
  • need to use the right sized gas syringe

- if the reaction is too vigorous it could blow the plunger out of the end of the syringe

49
Q

Describe a practical for how you can measure how surface area affects rate:

A
  • put dilute HCl and marble chips into a flask attached to a bung with a gas syringe
  • measure the volume of gas produced @ 20s intervals
  • record your results in a table, plot a graph of volume against time
  • repeat the experiment with the same volume and concentration of acid, and the same mass of marble chips, just more crunched up and then as a powder
50
Q

Describe a practical for how you can measure how concentration affects rate:

A
  • put dilute HCl and marble chips into a flask attached to a bung with a gas syringe
  • measure the volume of gas produced @ 20s intervals
  • record your results in a table, plot a graph of volume against time
  • repeat the experiment with the same mass and size of the marble chips, and the same volume of acid just varying the concentration
51
Q

Describe an experiment for how reaction rate is affected by temperature:

A

-have sodium thiosulphate and HCl in water baths at different temperatures (20, 30, 40)
-mix the two solutions of the same temperature together in a flask over a cross and time how long it takes for the yellow precipitate to cover it
-record the results in a table
[this can also be used to test the effects of concentration, vary the conc. instead of the temp.]

52
Q

Describe a practical for how you can measure how using a catalyst affects rate:

A

The decomposition of hydrogen peroxide

  • put hydrogen peroxide and MnO₂ powder into a flask and attach a gas syringe
  • measure the volume of gas produced at regular time intervals, record the results in a table
  • repeat the experiment with a different catalyst but keep the same volume and conc. of H₂O₂ and the same mass of catalyst
  • plot a graph of volume against time
53
Q

What is the equation for the decomposition of hydrogen peroxide?

A

2H₂O₂ (aq) -> 2H₂O (l) + O₂ (g)

54
Q

What catalyst speed up the decomposition of hydrogen peroxide?

A

manganese(IV) oxide (MnO₂)
copper(II) oxide (CuO)
zinc oxide (ZnO)

55
Q

What is a reversible reaction?

A

A reversible reaction is one where the products of the reaction can react with each other and convert back to the original reactants.
A + B ⇌ C + D

56
Q

What is an example of a reversible reaction?

A
  • The thermal decomposition of ammonium chloride

- The dehydration of copper(II) sulphate

57
Q

Describe the thermal decomposition of ammonium chloride:

A
  • ammonium chloride is a white solid, when heated it breaks down into the gases ammonia and hydrogen chloride (forward reaction)
  • if the ammonia and hydrogen chloride cools they react to re-form the white solid (backward reaction)

CGP Chemistry (14 decks)

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