Chapter 9 - Enthalpy Flashcards
1
Q
conservation of energy?
A
- chemical bonds are forces of attraction that bind atoms together
- chemical energy lies between these chemical bonds
- it’s a form of potential energy
- No energy is lost in a chemical reaction. It’s simply CONVERTED from 1 form to another
2
Q
what is enthalpy?
A
- H
- the amount of thermal energy that is stored in a system
3
Q
what CAN we measure?
A
- we can’ measure the direct enthalpy of the reactants and products
- instead, we CAN measure the amount of energy that is absorbed or released to the surroundings
4
Q
we can measure the change in energy by looking at?
A
the change in thermal energy
5
Q
temp ⬆ =
A
heat gain of surroundings
6
Q
temp ⬇ =
A
heat loss of surroundings
7
Q
enthalpy change =
A
delta H
the heat energy change at a constant pressure
8
Q
delta (triangle) means
A
change
9
Q
standard states?
A
the states substances are in under standard conditions
10
Q
system?
A
the chemical reaction (atoms & bonds)
11
Q
surroundings?
A
everything else
12
Q
enthalpy change EQUATION?
A
Hproducts - H reactants
13
Q
Exothermic reactions ?
A
release heat so deltaH is -
- enthalpy of products is smaller than enthalpy of reactants
14
Q
endothermic reactions?
A
- reaction absorbs heat
- product enthalpy > reactant enthalpy
- DeltaH is +
15
Q
activation energy?
A
- input of energy required to break the initial bonds & start the reaction
- once the AE has been overcome, the energy output of the reaction provides enough energy to sustain the reaction - the reaction becomes SELF - SUSTAINING
16
Q
SHC?
A
the energy required to raise the temp of 1g of a substance by 1K
17
Q
calorimetry - delta H remember?
A
REMEMBER SIGN
18
Q
what is calorimetry?
A
quantitative study of chemical energy in a chemical reaction
19
Q
2 types of calorimetry?
A
1 - polystyrene cup
2- spirit burner
20
Q
1 - polystyrene cup?
A
lid = insulates
- thermometer
- styrofoam cup - which could be held in another cup/ beaker
21
Q
calorimetry: error - evaporation?
A
some of the alcohol/water might evaporate
22
Q
what is average bond enthalpy?
A
the mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission
23
Q
what does the bond enthalpy provide us info about?
A
how much energy is needed to BREAK a bond
24
Q
bond enthalpy mnemonic?
A
Breaking bonds is E N dothermic D O M aking bonds is E Xothermic
25
in an endothermic reaction (bonds)?
more energy is needed to break the bonds in the reactants than is released when the bonds in the products are formed
26
in an exothermic reaction (bonds)?
more energy is given out when the bonds in the product are formed than is taken in when the bonds in the reactant are broken
27
delta H =
(bond enthalpy of reactants) - ( bond enthalpy of products)
28
some bonds occur in one envir but some occur in
multiple
29
bond strength varies based on?
the environment the bond is found in
30
actual bond enthalpy?
is specific to each individual molecule
31
use of enthalpy cycles?
- the enthalpy change of a reaction cannot always be directly measured
- we can use enthalpy cycles to help us indirectly calculate the enthalpy change
32
what is an enthalpy cycle?
pictorial representation showing the alternative routes of reaction between reactants and products
33
enthalpy cycles: rules?
- are triangular
- in each corner, there should be the same no. of each atom
- like energy, matter is also conserved
- the sides of the triangles r arrows
34
DIRECTION OF ARROW!!!!!
IF OTHER WAY ROUND MINUS INSTEAD OF PLUS
35
MOLES!!!!
MULTIPLY THE BOND ENTHALPY X MOLES
36
suggest why the enthalpy change of formation cannot be determined directly?
- rate too slow, Ea too high, other products may be formed instead (named)
37
the heat energy stored in a chemical system is called?
enthalpy
38
⭐ enthalpy equation?
Delta H = H(products) - H(reactants)
39
exothermic?
- if there is less enthalpy in the products than in the reactants, then Delta H is negative. The energy lost from the reaction is given out to the surroundings and the reaction is described as exothermic
40
endothermic reaction?
- if there is more enthalpy in the products than in the reactants, then delta H is positive. The energy gained by the reaction is taken in from the surroundings and the reaction is described as endothermic
41
{endo vs exo} heat is....
endo vs exo
| taken in from the surrounding vs given out to the surroundings
42
{endo vs exo} the temp?
endo vs exo
| decreases vs increases
43
{endo vs exo} enthalpy of products is...
endo vs exo
| higher vs lower
44
{endo vs exo} delta h is
endo vs exo
| positive vs Negative
45
e.g. of an exothermic reaction?
- Mg + Hcl
| - combustion of Mg ribbon
46
example of endo reaction?
- NaHCO3 + Hcl
| - photosynthesis
47
⭐enthalpy profile diagrams?
SINGLE HEADED ARROWS
48
what is activation energy?
- Ea
| - min energy a particle must have to take part in a successful collision
49
why are endothermic reactions less likely to occur than exothermic reactions?
endothermic reactions have a much larger Ea
| see reaction profile diagram
50
what is the value for standard pressure?
100kPa
51
standard temperature?
298k
52
standard conc?
1moldm(^-3)
53
standard states?
the standard state of a substance is its most stable state under standard conditions e.g. H2O - liquid
54
⭐standard enthalpy change of reaction DEFINITION?
The enthalpy change when amounts of reactants AS SHOWN IN THE REACTION EQUATION react together under standard conditions to give products in their standard states
55
⭐standard enthalpy change of formation DEFINITION?
- The enthalpy change when ONE MOLE OF A COMPOUND IS FORMED from its elements under standard conditions. Both the compound and elements are in their standard states - 298K, 100kPa
56
⭐ standard enthalpy change of neutralisation DEFINITION?
The enthalpy change when 1 MOLE OF WATER IS FORMED from the reaction of an acid with a base under standard conditions. both the reactants and products are in their standard states
57
⭐in definitions,?
always quote standard temp and pressure
58
⭐quantity of energy equation?
q = mcΔT
| - Q is in J
59
ALWAYS DIVIDE Q BY?
1000 TO GET ANSWER IN kJ
60
how to calculate enthalpy change from experimental data?
- q(in kJ)/ Moles
61
⭐ calculating enthalpies from experimental data remember?
- ADD THE SIGN
| - if the reaction is exothermic (temp increases) then ΔH is - and if the reaction is endo (temp decreases) then ΔH is +
62
q:?
- Quantity of energy
- Joules
- ^/1000 to get kJ
63
m:?
mass of water heated
64
c:?
SHC of WATER
| 4.18Jg(^-1)K(^-1)
65
ΔT:?
- temp change
| - K (but change is the same in degrees to)
66
explain why the ΔcH for the experiment is less exothermic than the published data book values?
- less exo bc:
- 1 - not under standard conditions
2 - heat loss to surroundings and apparatus (look if exo or endo)
3 - incomplete combustion
67
as the number of carbon atoms in the alcohol ⬆,
ΔcH gets more exothermic
68
bond breaking is an ______ process?
endothermic
69
bond forming is an ______ process?
exothermic
70
⭐ average bond enthalpy definition?
the average enthalpy change that takes place when breaking 1 mole of given bonds in molecules in the gaseous phase forming gasesous atoms.
e,g. CL2(g) -> 2CL(g)
71
why might the exact value of a bond enthalpy be different from the average bond enthalpy?
the exact value of a bond enthalpy depends upon the paticular compound in which the bond is found
72
The values for bond enthalpies are always ? and why
+ as the value is given for bond breaking which is an endo process
73
whic state are reactants and products always in for bond enthalpeis?
gaseous
74
to calculate energy change using bond enthalpies, must:
1 - write out the balanced equation using DISPLAYED FORMULA
| 2- use the equation ΔrH = (bond enthalpies of reactants) - bond enthalpies of products
75
enthalpy cycles - conservation of energy?
energy is neither created nor destroyed in a chemical reaction
76
what does Hess's Law state?
"the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place provided the initial and final conditions are the same"
77
what do energy cycles allow us to do?
- calculate the enthalpy change for reactions that do not occur directly or cannot be found by experiment (e.g. too dangerous or too slow)
78
⭐using hess's cycles to calculate ΔHr⦵ RULES?
1 - Look at the enthalpy data given in the question
| 2 - Draw the appropriate enthalpy cycle
79
when ΔfH⦵ data is given?
- in the circle, add elements of the reactants and products
- Formation = BUILD IT UP
- both arrows go up
80
when the arrow goes in the opposite direction in a cycle?
change the sign
81
when ΔcH⦵ is given?
- oxides of the elements in our reactants and products are in circle
- this is usually CO2 + H2O
- BURN IT DOWN - both arrows go down (from equation)
82
when going from reactants to circle to proucts?
the arrow goes down
| then up
83
SIGN - when do u add and when don't u add?
- Hess' law - no sign added!!! sign already there
| - calorimetry - add sign!! depending on if endo or exo
84
⭐standard enthalpy change of combustion definition?
the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions
