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Chemistry > energetics > Flashcards

energetics Flashcards

1
Q

what is enthalpy

A

the heat energy that is stored in a chemical system

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2
Q

how is enthalpy shown

A

H

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3
Q

how is enthalpy measured

A

from temperature changes when a chemical reaction takes place

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4
Q

delta H

A

heat energy change at constant pressure

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5
Q

what are the units of heat energy change (delta H)

A

kJ per mole

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6
Q

what happens in terms of heat in an exothermic reaction

A

heat energy is given out to the surroundings

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7
Q

what happens to the temperature of the surroundings when an exothermic reaction taxes place

A

temperature of surroundings increases

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8
Q

why is delta H negative in exothermic reactions

A

the chemicals lose heat energy

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9
Q

important exothermic reactions

A

combustion of fuels and respiration

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10
Q

what happens in terms of heat in endothermic reactions

A

heat energy is taken in from the surroundings

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11
Q

what happens to the temperature of the surroundings in endothermic reactions

A

the temperature of the surroundings decreases

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12
Q

why is delta H positive in endothermic reactions

A

the chemicals gain heat energy

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13
Q

what do endothermic reactions require

A

the input of heat energy

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14
Q

important endothermic reactions

A

thermal decomposition of calcium carbonate

photosynthesis

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15
Q

what can enthalpy profile diagrams be used to illustrate

A

the enthalpy change for a reaction

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16
Q

what do enthalpy profile diagrams show

A

the products, reactants and enthalpy change

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17
Q

activation energy

A

the minimum energy required to start a reaction by the breaking of bonds

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18
Q

do the products or reactants have a higher enthalpy in exothermic reactions

A

reactants

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19
Q

do the products or reactants have a higher enthalpy in endothermic reactions

A

products

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20
Q

what is required to start the exothermic reaction

A

energy is required to break the bonds, even though the products have a lower energy than the reactants

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21
Q

what will provide the energy needed to continue to overcome the activation energy

A

the net energy, once the barrier has been overcome

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22
Q

why is energy needed in endothermic reactions

A

the break the bonds and start the reaction

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23
Q

what do most endothermic reactions need to provide the necessary energy

A

to be heated continuously

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24
Q

what is bond enthalpy

A

the energy required to break one mole of a given covalent bond in the molecules in the gaseous state

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25
why are bond enthalpy values always positive
bond breaking is endothermic
26
What do bond enthalpies give an indication of
The relative strength of a covalent bond
27
How do bond enthalpies give an indication of the relative strength of a covalent bond
The stronger the bond, the more endothermic the bond enthalpy
28
Why do chemists use mean bond enthalpy values
The same covalent bond may appear in different compounds and the value of the bond will be slightly different in each compound
29
Mean bond enthalpy
The energy required to break a covalent bond, averaged for that type of bond in a range of different compounds
30
What happens in terms of the bonds in a chemical reaction
Bonds in the reactants are broken and new bonds are formed to make the products
31
What is required to break bonds
Energy
32
What is released when new bonds form
Energy
33
What is the enthalpy change of a reaction
Sum of bonds broken - some of bonds formed
34
Limitations of bond energy calculations
Using mean bond enthalpies to calculate the enthalpy change of a reaction often leads to a value that is less accurate than a value obtained from Hess's Law
35
Why is using mean bond enthalpies to calculate enthalpy change less accurate than a value obtained from Hess's law
Bond enthalpies used are only an average value- not specific to compound in question We are assuming all species are in the gaseous state- lots of compounds won't be eg. Ethanol
36
What are the standard conditions for standard enthalpy of formation
298k and 100kPa
37
Standard state
The physical state of a substance under standard conditions
38
Standard enthalpy of formation
The enthalpy change that occurs when one mole of a compound is formed from its constituent elements with all reactants and products in their standard states
39
What is the standard enthalpy of formation of an element in its standard state
0
40
What do all products of standard enthalpy of formation need to be
1 mole
41
What is the relationship between negativity of standard enthalpy of formation of a compound and the stability of it
More negative = more stable
42
Standard enthalpy of combustions
The enthalpy change that occurs when one mole of a compound reacts completely in oxygen with all reactants and products in their standard states
43
Calorimetry
The method used to determine enthalpy changes by experiment
44
What does calorimetry involve
Measuring the temperature change of a given amount of water as the reaction occurs- and converting this to a quantity of heat energy
45
Specific heat capacity of water
4.18 J required to hear 1g of water
46
What does heat change =
Mass of water x specific heat capacity x temperature change Q=mc DELTA T
47
What is q
Heat energy released or absorbed in joules
48
What is m
Mass of water in grams
49
What is the mass of water in grams equal to
Volume in cm3
50
What is c
The specific heat capacity of water
51
What is delta t
Temperature change in K | Same as change in celcius
52
What must you do to q when you've initially calculated it using mcDELTA T
Divide it by 1000 to convert J to kJ
53
Heat given out or taken in by one mole=
Q(in kJ)/ no of moles
54
What are the units of delta H for experimental determination of enthalpy changes
kJmol-1
55
When is delta H negative in calorimetry
If temp increases during reaction (exothermic reaction)
56
When is delta H positive during calorimetry
If temperature decreases during a reaction (endothermic)
57
Why may the experimental value for enthalpy of combustion be different to the data book value
- heat loss to surrounding - incomplete combustion - some methanol may evaporate - some water may evaporate
58
What can cooling curves be used for
As a method of accounting for heat loss with reactions in solutions
59
How to use data to find maximum temperature rise
Plot a graph of temperature against time using results and determine max temperature change accompanying reaction Extrapolate back to where lines meet (ish) to establish max temperature rise
60
First law of thermodynamics
Energy cannot be created or destroyed but it can be changed from one form to another
61
Where does hess's law apply the first law of thermodynamics
To chemical reactions
62
Hess's law
The enthalpy change for a chemical reaction is independent of the route taken
63
Why is it not always possible to measure the enthalpy change of a reaction directly
- reaction may have more than one possible product | - practically impossible to measure
64
What is Hess's law used for
To calculate enthalpy changes which cannot be measured directly using an enthalpy cycle
65
What can we do using Hess's law if we know two of the enthalpy changes
Calculate the third
66
What does delta H1 equal
Delta H 2 + delta H 3
67
How to remember enthalpy cycles of combustion
C=R-P
68
Where should you always do the vectors for enthalpy of combustion
Products to element
69
Where should you do vectors in calculations for reactions from enthalpy of formation values
Elements to products
70
How to remember enthalpy cycles of formation
F=P-R

Chemistry (42 decks)

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