Physical chemistry Flashcards

(65 cards)

1
Q

What is enthalpy

A

the thermal energy (heat) stored in a chemical system

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What do endothermic reactions do

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Give examples of exothermic reactions

A
  • combustion

- respiration

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Give examples of endothermic reactions

A
  • thermal decomposition
  • photosynthesis
  • electrolysis
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is chemical energy

A

A special form of potential energy that lies within chemical bonds

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is a system

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is a surroundings

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Heat loss in a chemical system

A

heat gain to the surroundings

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Heat gain in a chemical system

A

heat lost to the surroundings

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Describe an exothermic graph

A
  • product enthalpy smaller than reaction enthalpy
  • heat loss
  • -ve change in H
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Draw an exothermic graph

A

DRAWWWW IT NOW

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

DRAW AN ENDOTERMIC GRAPH

A

DRAWWWW IT NOW

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What is calorimetry

A

calorimetry is the quantitive study of energy in a chemical reaction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

you cannot measure the enthalpy directly but…

A

You can measure the enthalpy change

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Change in enthalpy equation
``` 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 ```
26
describe the process for the direct measurement of enthalpy of a reaction
- 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
27
Describe the direct measurement of enthalpy of combustion
- 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
describe a bomb calorimeter
- minimises heat loss | - uses pure oxygen to ensure complete combustion is achieved
29
What is thermodynamics
this focuses on energy in a chemical system which is the reactants and products
30
Why do we use standard conditions
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
What are the standard states
the physical state a substance is in under the standard conditions 100Kpa, 298K
32
What is the average bond enthalpy
The mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission and break
33
How to break and made a bond
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
IN AN ENDOTHERMIC REACTION MORE
ENERGY IS NEEDED TO BREAK THE REACTANT BONDS THEN IS RELEASED WHEN TEH RPODUCT BONDS ARE FORMED - TAKES IN ENERGY
35
IN AN EXOTHERMIC REACTION MORE
ENERGY IS RELEASED WHEN THE PRODUCT BONDS ARE FORMED THAN IS NEEDED TO BREAK REACTANT BONDS - RELEASES ENERGY
36
How do you use average bond enthalpy data to predict enthalpy change
enthalpy change = sum of bond enthalpies of reactants - sum of bond enthalpies of products
37
why is it hard to calculate the enthalpy change for incomplete combustion
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
define the enthalpy change of reaction
this is the energy change associated with a given reaction
39
define the enthalpy change of formation
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
define the enthalpy change of combustion
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
define the enthalpy change of neutralisation
this is the energy change associated with the formation of 1 mole of water from a neutralisation reaction under standard conditions
42
what does Hess's law state
it states that the enthalpy change in a chemical reaction is independent of the route that it takes
43
What are enthalpy cycles used to measure
they are used to measure the enthalpy changes that are impossible to measure directly - they indirectly calculate the enthalpy change
44
what does the enthalpy cycle do
an enthalpy cycle is a pictorial representation showing alternative routes between reactants and products
45
ENTHALPY CALCULATIONS
PG 144-145
46
How do you work out the rate of reaction
change in concentration/time - measure the gradient, draw a straight line at the point that you want to measure
47
describe how the rate of reaction progresses during a reaction
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
What factors effect the rate of a chemical reaction
- 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
How does concentration affect reaction rate
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
How does pressure affect reaction ate
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
Draw enthalpy change of formation hess diagram
DRAW IT
52
Draw and enthalpy change of reaction from enthalpy changes of combustion diagram (Hess)
DRAW IT
53
whats the standard enthalpy change of formation of n2
- 0KJmol-1, as N2 element is in standard state under standard conditions
54
why is the experimental value different from the actual result
- heat loss - may not happen under standard conditions - may not have complete combustion - fuel evaporated
55
when are average bond enthalpies used
- when calculating an enthalpy change but only when each species in a reaction is a gas
56
why calculations of enthalpy change of reaction using bond enthalpy values are not standard values
- 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
what does Hess law allow us to calculate
- indirect calculation of the enthalpy change of reaction
58
how can a enthalpy change of reaction be determined experimentally
- measure formation or combustion | - construct an Hess cycle
59
what two factors are required for a collision between two particles to be successful in causing a reaction
- 2 or more reactants collide with enough energy to overcome the activation energy - there are reactive areas
60
how can you calculate rate of reaction from results between change in concentrtion over time
gradient of a line
61
how does a solid catalyst work in a reaction where the reactants are solutions
- the solutions adsorb onto the surface of the catalyst
62
in boltzman distribution why does the curve never touch the X axis
- molecules do never have the maximum amount of energy
63
what happens when a reaction reaches equlibrium
- no net movement of concentrations and they remain cosntant but are constantly interchanging - foward rate of reaction = reverse rate of reaction
64
what determines the extent of a reaction
- equilibrium
65
what must you add to yellow potassium chromate to cause a colour change
- addition of acid - yellow to organge - chromate becomes dichromate