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CHEM10007
Week 6 (Heat Flow (Calorimetry, Hess Law), Galvanic Cells)
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Cards (97)
Heat flow, q
Measure of
heat flow
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Calorimetry
Science of measuring
heat
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DH is
proportional
to mole
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Dissolving sodium nitrate in water
1.
NaNO3(s)
→
NaNO3(aq)
2. ΔH = +
21.0
kJ mol–1
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Sodium nitrate dissolving in water
Temperature
of water
rises
or falls
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Determining enthalpy change
1.00
g of solid
NaNO3
dissolved in water
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Relationship between heat and temperature
Flow of
energy
Measure of average
kinetic
energy
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Finding q and therefore ΔrH
Relate
q
, the flow of energy in/out of a system and the
change
in temperature of the system, or the surroundings
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What should be understood and done by the end of the lecture
Understand calorimetry relates
heat
and
temperature
Define and use Specific
Heat Capacity
to find q
Find q and ΔH heat of
reaction
, or enthalpy change of a reaction using
Coffee Cup
calorimeter
Find q and ΔH heat of reaction, or enthalpy change of a reaction using
Bomb
calorimeter, using
Ccal
the calorimeter constant
Write the
thermochemical
equation
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Heat capacity
Quantity of heat needed to change the
temperature
of a specified amount (gram or n) of material by
1oC
(ΔT = 1oC = 1K)
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Hess'
Law
Predict ΔH for a given
reaction
, using
thermochemical
data of related reactions
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Hess' Law - Predicting enthalpy change of reactions that can't be measured
1. Find another
path
to get from the reactants to products in the
target reaction
2. Add the values of
ΔH
=
ΔH
(iii) - ΔH (ii)
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Specific heat capacity
Heat capacity
per gram, amount of energy needed to
raise
the temperature of 1 g of the substance by 1oC (or 1K)
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Specific heat capacity values
water:
4.18
aluminium:
0.89
carbon:
0.71
iron:
0.45
mercury:
0.14
sand:
0.48
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Enthalpy change,
ΔHreaction
is a state function, meaning the path we take will not change the final value of
ΔHreaction
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Molar heat capacity
Energy required to heat 1 mole of a substance by
1oC
(=
1K
)
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If the reaction is
reversed
The sign of the ΔH is
reversed
, while the magnitude remains the
same
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Heating
200.0
mL of water from
20.0oC
to 100.0oC
1. q = C. m. ΔT
2. q = 4.18 × (200.0 ×1.000) × 80.0
3. q = 66900 J = 66.9 kJ
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Galvanic
cell
Energy
source
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If the coefficients of a balanced reaction have been multiplied by an integer x
ΔH must be
multiplied
by
x
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Unattributed images taken from Cengage Learning resources
Mahaffy
et al, HACR and
Zumdahl
et al
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Heating
200.0 g block
of iron from
20.0oC
Tf = ?, m=200.0 g, Ti =
20.0oC
, q = 7.5 kJ, C(iron) =
0.45 JoC-1 g-1
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Chemistry
see
LMS
for details
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General Method for solving Hess' Law problems
1. Look for reactions containing the desired reactants, and
rearrange
reaction (and amounts) to place that species on the
lefthand
side of reaction
2. Look for reactions containing the
desired
products, and rearrange reaction (and amounts) to place that species on the
righthand
side of reaction
3. Add the reactions together,
cancelling
any species in the same state that appear on the left and right of the equation
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C2H4 (g) + H2O (l) →
C2H5OH
(l)
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Determining molar heat capacity of water
1. Specific heat capacity =
4.18
J g-1 oC-1
2. Mass of
1.00
mole of water =
18.0
g
3. Energy required to heat 1 mole (
18.0
g) of H2O by
1.00oC
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Balance a redox reaction equation in acidic solution
1. Balance all elements except
hydrogen
and
oxygen
2. Balance oxygen atoms using
H2O
molecules
3. Balance hydrogen atoms using
H+
ions
4. Balance
charge
, using
electrons
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Calorimetry
Science of measuring heat, based on observation of
temperature
changes when a body
absorbs
or releases heat
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Half cell
Part of a
galvanic
cell where a
redox
reaction takes place
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Redox chemistry
Electron transfer reactions
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Coffee Cup Calorimeter
Measures
temperature
changes and relates this to the heat associated with a
chemical
reaction
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Salt bridge
Allows flow of
ions
between half cells to maintain
electrical neutrality
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C6H12O6(s) +
6
O2(g) →
6
CO2(g) + 6 H2O(g), ΔrHo= -2803 kJ mol-1
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Determining change in enthalpy for neutralisation reaction
1. 50.0 mL of 1.00 M HCl solution at
25.0oC
combined with 50.0 mL of 1.00 M NaOH solution at
25.0oC
2. Temperature increases to
31.9oC
3. Specific heat of solution =
4.18
J g-1 0C-1, density of solution = density of
water
, no heat loss
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Redox reaction in a galvanic cell
1.
Oxidation
at the
anode
2.
Reduction
at the
cathode
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Redox reactions
Rusting:
4Fe
(s) +
3O2
(g) → 2Fe2O3 (s)
Capturing energy(sunlight) as carbohydrates in plants: 6 CO2(g) +
6 H2O
(g) →
C6H12O6
(s) + 6 O2(g)
'Burning' carbohydrates for energy in the body:
C6H12O6
(s) + 6 O2(g) →
6 CO2
(g) + 6 H2O(g)
'Burning' gas for heat (combustion): CH4(g) + 2O2(g) → CO2(g)+
2H2O
(l)
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Writing thermochemical equation for
neutralisation
reaction
1. Net
ionic
equation
2. Calculate number of
moles
of reactants
3. Calculate
heat
released to the
surroundings
= the solution: qsurroundings = Csolution .msolution .ΔTsolution
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Oxidation
Reaction with
oxygen
, losing
electrons
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Determine the change in
enthalpy
for the neutralisation reaction, DH in
kJ mol-1
, and write the thermochemical equation for the reaction.
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Sketch a galvanic cell
Draw the
cell
diagram from the equation for the cell
reaction
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