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Chemistry Module 3 > Physical chemistry > Flashcards

Physical chemistry Flashcards

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1
Q

What is enthalpy

A

the thermal energy (heat) stored in a chemical system

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2
Q

What does the law of conservation of energy state

A

states that energy cannot be created or destroyed only moved from one place to another

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3
Q

What do exothermic reactions do

A

they transfer the excess chemical energy into thermal energy which is transferred to the surroudnings, heat is gained

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4
Q

What do endothermic reactions do

A

transfer thermal energy from the surroundings to chemical energy, heat is lost

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5
Q

Give examples of exothermic reactions

A
  • combustion

- respiration

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6
Q

Give examples of endothermic reactions

A
  • thermal decomposition
  • photosynthesis
  • electrolysis
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7
Q

What is chemical energy

A

A special form of potential energy that lies within chemical bonds

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8
Q

What can you measure and cannot measure with enthalpy

A
  • it is impossible to measure the enthalpy of the reactants or products
  • but we can measure the energy absorbed or released to the surroundings
  • can vary but chemists usually measure energy changes in reactions by monitoring thermal energy
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9
Q

What is a system

A

the actual chemical reaction, so like the atoms and bonds involved in the chemical reactions

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10
Q

What is a surroundings

A

everything else such as the aqueous solution that the substances are dissolved in

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11
Q

Heat loss in a chemical system

A

heat gain to the surroundings

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12
Q

Heat gain in a chemical system

A

heat lost to the surroundings

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13
Q

An enthalpy change is…

A
  • the heat exchanges with the surroundings during a chemical reaction at constant pressure
  • the differences between the enthalpy of the products and enthalpy of the reactants
    ENTHALPY OF PRODUCTS-ENTHALPY OF REACTANTS
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14
Q

Describe an exothermic graph

A
  • product enthalpy smaller than reaction enthalpy
  • heat loss
  • -ve change in H
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15
Q

Draw an exothermic graph

A

DRAWWWW IT NOW

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16
Q

Describe an endothermic graph

A
  • enthalphy of products is greater than enthalpy of reactants
  • heat gain to the chemical system
  • +ve change in H
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17
Q

DRAW AN ENDOTERMIC GRAPH

A

DRAWWWW IT NOW

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18
Q

Describe how the activation energy is used in exothermic reactions

A

even though products have a lower energy than reactants there still has to be an input of energy to break the first bond and start the reaction
- activation energy is supplied by a spark this overcomes the activation energy and once this has overcome the net output of energy it provides the activation energy os the reaction can continue

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19
Q

What do simple enthalpy profile diagrams show us

A

it shows us what will happen to the enthalpies of the reactants and products during the course of the reaction

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20
Q

What happens if there was no activation energy in exothermic reactions

A
  • the exothermic reactions and fuels would spontaneously combust and be spontaneous
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21
Q

What is the activation energy

A

the activation energy is the minimum energy required to start a reaction by breaking bonds in the reactants

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22
Q

What is calorimetry

A

calorimetry is the quantitive study of energy in a chemical reaction

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23
Q

What is the specific heat capacity

A

specific heat capacity is the energy required to raise the temperature of 1g of a substance by 1K

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24
Q

you cannot measure the enthalpy directly but…

A

You can measure the enthalpy change

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25
Q

Change in enthalpy equation

A 
q=mc(triangle)T
Q= heat exchanged with surroundings measured in J
M= mass of substance heated or cooled in g
C= specific heat capacity
T= change in temperature 
then it goes to 
n=m/mr
or n= cxv
then (change)H =Q/n
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26
Q

describe the process for the direct measurement of enthalpy of a reaction

A
  • use insulated cup, put lid on with a hole for the thermometer to reduce heat loss to the surroundings
  • add a measured mass of reactant, take temperature every 4 minutes till it is stable
  • at 5 minute add the second reactant, do not measure the temperature in this minute
  • monitor temp every minute for another 5 minutes
  • plot a graph and repeat the experiment
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27
Q

Describe the direct measurement of enthalpy of combustion

A
  • measure the starting mass of fuel in the spirit burner
  • add a known mass of water to a copper calorimeter
  • take starting temperature
  • combust the fuel for a few minutes and take final temperature of water
  • take mass of unused fuel and calculate the mass of fuel burnt
28
Q

describe a bomb calorimeter

A
  • minimises heat loss

- uses pure oxygen to ensure complete combustion is achieved

29
Q

What is thermodynamics

A

this focuses on energy in a chemical system which is the reactants and products

30
Q

Why do we use standard conditions

A

for chemists calculate meaningful calculations so all values must be measured under standard consitons
100Kpa - 100,000 Pa
273K- add to covert degrees to kelvin
given by circle with a horizontal line through it symbol

31
Q

What are the standard states

A

the physical state a substance is in under the standard conditions
100Kpa, 298K

32
Q

What is the average bond enthalpy

A

The mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission and break

33
Q

How to break and made a bond

A
  1. reactant bonds are broken this take in energy and is endothermic
  2. atoms rearrange to form products
  3. products are formed and release energy this is exothermic
34
Q

IN AN ENDOTHERMIC REACTION MORE

A

ENERGY IS NEEDED TO BREAK THE REACTANT BONDS THEN IS RELEASED WHEN TEH RPODUCT BONDS ARE FORMED - TAKES IN ENERGY

35
Q

IN AN EXOTHERMIC REACTION MORE

A

ENERGY IS RELEASED WHEN THE PRODUCT BONDS ARE FORMED THAN IS NEEDED TO BREAK REACTANT BONDS - RELEASES ENERGY

36
Q

How do you use average bond enthalpy data to predict enthalpy change

A

enthalpy change = sum of bond enthalpies of reactants - sum of bond enthalpies of products

37
Q

why is it hard to calculate the enthalpy change for incomplete combustion

A

difficult or impossible to measure directly because there is often complete and incomplete combustion happing simultaneously

  • but you can use average bond enthalpy data to calculate the enthalpy change
  • values are averages for the bond in a variety of different environments and in different molecules this is not as accurate as direct measurements and using precise equipment such as the bomb calorimeter
38
Q

define the enthalpy change of reaction

A

this is the energy change associated with a given reaction

39
Q

define the enthalpy change of formation

A

this is the energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions

40
Q

define the enthalpy change of combustion

A

this is the energy change that takes place when 1 mole of substance is completely oxidised under standard conditions with the elements under standard states

41
Q

define the enthalpy change of neutralisation

A

this is the energy change associated with the formation of 1 mole of water from a neutralisation reaction under standard conditions

42
Q

what does Hess’s law state

A

it states that the enthalpy change in a chemical reaction is independent of the route that it takes

43
Q

What are enthalpy cycles used to measure

A

they are used to measure the enthalpy changes that are impossible to measure directly - they indirectly calculate the enthalpy change

44
Q

what does the enthalpy cycle do

A

an enthalpy cycle is a pictorial representation showing alternative routes between reactants and products

45
Q

ENTHALPY CALCULATIONS

A

PG 144-145

46
Q

How do you work out the rate of reaction

A

change in concentration/time - measure the gradient, draw a straight line at the point that you want to measure

47
Q

describe how the rate of reaction progresses during a reaction

A

at the start the concentrations of the reactants are the highest so the rate is at its fastest this is the initial rate
the rate slows down as the reactant concentrations decrease
when any reactant has a concentration of zero the reaction stops and the rate is 0

48
Q

What factors effect the rate of a chemical reaction

A
  • temperature (increased temperature = increased rate)
  • pressure (increase pressure = increased rate)
  • concentration ( increased concentration = increased rate)
  • surface area ( increased surface area = increased rate)
  • catalyst ( increases the rate of reaction)
49
Q

How does concentration affect reaction rate

A

increased concentration, this means that there are more molecules in the same unit volume,, the molecules will be closer together so there is a greater chance of them colliding with sufficient energy to overcome the activation energy
collisions of molecules will be more frequent
therefore more will occur in a certain amount of time
therefore increased rate of reaction

50
Q

How does pressure affect reaction ate

A

when pressure of a gas increased the molecules are pushed closer together
same number of molecules occupy a smaller volume
more frequent collisions therefore more collisions expected to overcome the activation energy
therefore rate of reaction increases when pressure increases

51
Q

Draw enthalpy change of formation hess diagram

A

DRAW IT

52
Q

Draw and enthalpy change of reaction from enthalpy changes of combustion diagram (Hess)

A

DRAW IT

53
Q

whats the standard enthalpy change of formation of n2

A
  • 0KJmol-1, as N2 element is in standard state under standard conditions
54
Q

why is the experimental value different from the actual result

A
  • heat loss
  • may not happen under standard conditions
  • may not have complete combustion
  • fuel evaporated
55
Q

when are average bond enthalpies used

A
  • when calculating an enthalpy change but only when each species in a reaction is a gas
56
Q

why calculations of enthalpy change of reaction using bond enthalpy values are not standard values

A
  • they have to be in a gaseous state in order to be able to use average bond enthalpies
  • gaseous state is not a standard state for most reactants and is not in standard condition so the enthalpy change of reaction is not standard enthalpy change
57
Q

what does Hess law allow us to calculate

A
  • indirect calculation of the enthalpy change of reaction
58
Q

how can a enthalpy change of reaction be determined experimentally

A
  • measure formation or combustion

- construct an Hess cycle

59
Q

what two factors are required for a collision between two particles to be successful in causing a reaction

A
  • 2 or more reactants collide with enough energy to overcome the activation energy
  • there are reactive areas
60
Q

how can you calculate rate of reaction from results between change in concentrtion over time

A

gradient of a line

61
Q

how does a solid catalyst work in a reaction where the reactants are solutions

A
  • the solutions adsorb onto the surface of the catalyst
62
Q

in boltzman distribution why does the curve never touch the X axis

A
  • molecules do never have the maximum amount of energy
63
Q

what happens when a reaction reaches equlibrium

A
  • no net movement of concentrations and they remain cosntant but are constantly interchanging
  • foward rate of reaction = reverse rate of reaction
64
Q

what determines the extent of a reaction

A
  • equilibrium
65
Q

what must you add to yellow potassium chromate to cause a colour change

A
  • addition of acid
  • yellow to organge
  • chromate becomes dichromate

Chemistry Module 3 (6 decks)

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