unit 3 aos 1 Flashcards
sources of energy
Endothermic reactions
1) absorb thermal energy (gets colder)
2) thermal energy»_space;> chemical energy
3) delta H = positive
Exothermic reactions
1) release thermal energy (gets hotter)
2) chemical energy»_space;> thermal energy
3) delta H = negative
Energy profile diagrams
- Vertical axis: enthalpy (kJ/mol or KJ)
- horizontal axis: progress of reaction
- exothermic: reactants higher than products
- endothermic: reactants below products
Complete combustions
fuel + 02 (g)»_space;> CO2(g) + H2O(l)
- product is CO2(g)
- more oxygen used overall
incomplete combustion
- products: carbon monoxide (CO) or carbon particulates (C)
- less oxygen used overall
Thermochemical Equations
MUST have states and delta H value
FORMULA to convert between mass and moles
n = m/M
FORMULA to find energy released by a fuel
q = delta H x n
with help from data book
different FORMULAS of delta H
1) if in kJ/mol: delta H = q/n
2) if in kJ/g: delta H = q/m
3) if in kJ/mL: delta H = q/v
FORMULA of density
d = m/v
- volume can be in both ml & L
FORMULA to find energy absorbed by water
q = MCAT
- given specific heat capacity of water is 4.18 J/g/K
how to find delta H experimentally??
1) find temp change of water
- use q = MCAT formula
2) find change of mass of fuel
- use n = m/M
3) then calculate delta H
- use delta H = q/n
FORMULA of percentage efficiency of a ‘fuel’
% eff = actual / theoretical x 100
AKA
% eff = output / input x 100
what are systematic errors?
links to ACCURACY
(how close a measurement is to the true value)
what are random errors?
link to PRECUSION
(how close a measurement is to each other)
WAYS to minimise heat loss (in the experiment)
1) add a lid
2) bring flame closer
3) insulate the sides of the beaker
4) place heat shields on sides
stoichiometric ratios
- ALWAYS in moles
unknown / known
what are SLC conditions?
temperature @ 25 degrees celsius
pressure @ 100kpa
redox: ph @ 0
universal gas law
PV = nRT
- not tested in chem 3/4
FOMULA for volume occupied by gas IN SLC CONDITIONS
n = V / Vm
what is the molar volume (Vm) of gases?
24.8L/mol
volume to volume stoich:
CONDITIONS + how
1) temperature is constant
2) pressure is constant
3) all substances must be gas
- just multiply ratio with volume
how to find limiting reagent?
1) find moles of both reactants
2) divide both by their respective coefficient
3) the lower one = limiting reagent
how to find how much excess reagent is left after all of limiting reagent is used up
1) find amount of excess used
2) subtract that value from how much there is left
initial - used
features of a calorimeter
- in 3/4 a SOLUTION calorimeter is tested
- the sides are insulated to minimise heat loss, so most of the energy goes straight to the water
! heat is transferred directly to water
purpose of stirrer
to evenly distribute heat
+
ensure there is a constant temperature
purpose of glass bulb
initially separates the reactants and prevents reaction to occur
what is a calorie
IT is the energy required to heat up 1g of water by 1 degree celcius
- there are 4.18 joules in ONE calorie
DEF: calibration factor
aka: CF
IT is a measure of how much energy is required to change the temperature of a substance in the calorimeter by 1 degree celcius
FORMULA of calculating the CF
CF = energy inputted (E) / the temperature change observed (AT)
DEF: electrical calibration
- when the calorimeter can be calibrated using the electric heater
FORMULA to find the electrical calibration factor CF
CF = E / AT
E = VIT
v: voltage (volts(v))
I: current (amps)
T: time (seconds)
DEF: chemical calibration
- the calorimeter can be calibrated chemically by combusting an organic molecule with a known change in enthalpy value
FORMULA for chemical calibration factor CF
CF = E / deltaT
E = delta H x n (if in kj/mol)
E = delta H x m (if in kj/grams)
FORMULA of concentration
c = n / V
temperature - time graphs
what is a renewable fuel? (DEF)
a fuel that can be replenished by natural processes within a relativly short period of time.
How is coal formed
from the fossilisation of dead? plant and animal matter
crude oil
- non renewable
- consists of a nixture of hydrocarbons, namely alkanes, ranging from 1 to 30 carbons.
- carbon is sperated by fractional distilation
WHAT is natural gas
- about 95% CH4 & 5% CO2
- a gas found underground
- non-renewable
- can be found in coal sam gas deposits found in coal mines
WHAT is biogas
- renewable
- produced from biomass/biowaste
how is biogas produced
- formed from the anerobic breakdown of organic matter by bacteria
Bioethanol
- renewable
- formed from the fermentation of glucose with the help of yeast
Fermentation equation
C6H12O6 (aq) -> 2C2H5OH(aq) + 2CO2
Distillation
- a process of sperating a liquid mixture by boiling the mixture.
- mixture is heated tp the target temperature
- that temperature is between two substances boiling points
distillate
the substance that is evaporated and condensed back back down in distillation
purpose of distillation
- ## increases efficiency upon combustion of ethanol as less water is present
what biodiesel
- renewable
- made up of fatty acid methyl ester
biodiesal - formation
- from plant and animal fats/oils
- transesterification
partially oxidised
A fuel that already contains oxygen in the molecular formula (eg. methanol = CH3OH)
THUS HEAT OF COMBUSTION WOULD BE LOWER bc partially oxidised = ‘partially burnt’
Carbon neutrality
when the carbon dioxide released when using the fuel is previously offset during the production of fuel.
reasons for carbon neutrality
the production of glucose via photosynthesis
6CO2(g) + 6H2O(l) -> C6H12O6(aq) + 6O2(l)
-> catalyst: UV light + chlorophyll
Reasons AGAINST carbon neutrality
other factors which contribute to carbon emmissions during the production of fuel:
- farming equipment
- transportation vehicles
carbohydrates
store/provide energy (quick energy) and fiber (remove waste)
Fats/oils
store/provide energy (slow energy), shock absorbtion/insulation, and storage of fat-soluble vitamins
proteins
helps with growth/ repair of cells, helps with immunity and transport of molecules,a nd enzymes
______________
___________________
Redox reaction
- the transfer of electrons
- must occur in pairs
redox half-equations
- the two half equations cannot occur bu themselves. MUST OCCUR TOGETHER
- one gives away electrons and the other takes in electrons
OIL RIG
oxidation is loss
reduction is gain
oxidation number
- the hypothetical charge of an atom if the bonding is purely ionic
- according to the OCTET rule, atoms generally want eight electrons in it’s outer shell
oxidising agent
AKA (oxident)
- causes the oxidation of another substance
- UNDERGOES reduction
reducing agent
AKA (reducatant)
- causes the reduction to another substance
- UNDERGOES oxidation
congugate redox pairs (x^+/x)
oxident/reductant
balancing redoox equations in ACIDIC conditions
KOHES
balancing redoox equations in BASIC conditions
1)KOHES
2) add OH- on both sides
3) simplify/cancel out
net ionic equation
a balanced full equation with spectator ions omitted
spectator ions
(ions which do not change states)
- compounds which is present but do not participate in the reaction
where is the strongest oxident on ELECTROCHEMICAL series?
top LEFT
where is the strongest reductant on ELECTROCHEMICAL series?
bottom RIGHT
spontaneous slopes
negative slopes
IMAGINE a ball being able to roll down BY ITSELF without help
FROM THE LEFT
what happens if there are multiple oxidents and reductants?
only the strongest oxidents will react with the strongest reductant
ENERGY CONVERSATIONS:
for direct contact spontaneous redox equation
chemical TO thermal
ENERGY CONVERSATIONS:
for indirect contact spontaneous redox equation
chemical TO electrical
why may a reaction not occur for some metals
- rate of reaction too slow
THE electrochemical series does not predict the state of reaction
the strenghts of oxident/reductant DOES NOT affect the rate of reaction
Galvanic cell
an electrochemical cell that converts the chemical energy of spontaneous redox reactions into electrical energy
- reactants ARE NOT in direct contact
INDIRECT redox reaction
3 main part/components of a galvanic cell
- 2 x half cells = i galvanic cells
- external circuit (load + wire)
- salt bridge (internal circuts)
electrode
electrical conductors, which carry electrons to/from the non-metallic part in ana eledtrolyte
- allows movement of ELECTRONS
electrolyte
contains ions which can conduct electricity due to the movement of ions
- allows movement of IONS
FUNCTION: to complete the circuit by allowing the flow of charges particles through the cell
is a gas is a product, is a gas tube required?
NO
Inert electrode
platinum (Pt) & graphite (c)
How do you know what reaction happens at each electrode
AO -
anode = oxidation (-)
RC +
reduction = cathode (+)
electron flow in galvanic
anode TO cathode
internal circut
- used to balance the build -up of charge
PROPERTIES:
- inert (does not react)
- soluble (does not doem a percipitate)
MOST COMMON: KNO3(aq)
FORMULA: electromotive force (EMF)
E(oxident) - E(reductant) = EMF
top - bottom
CONDITIONS: slc + 1.0M
CHANGES is size/mass of electrodes
at cathode mass INCREASES
at anode, mass DECREASES
Changes in PH
(H+)
in OXIDATION: if concentration of H+ ions increase = more acidic = PH decrease
in REDUCTION: H+ ions increases = less basic = PH increases
Changes in PH
(OH -)
in OXIDATION: concentration of ions increase = more basic = PH increase
in REDUCTION
concentraton of ions decrease = less basic = PH decrease
Gases
if a gass is a product , Bubbles are formed in reaction
if gass reactant, use a gas tube
colour
in OXIDATION:
- product = NI 2+
SO ion concentration increases = colour increases
in REDUCTION:
- ions are being used up = colour decreases
“more/less intesly… [colour]”
LOOK AT DATA BOOK
Fuel cell has:
- has a shared electrolyte
- no salt bridge
- continous suppply of reactants
define fuel cell:
a galvanic cell that produces electrical energy from chemical energy, and is characterised by a continuous supply of reactants
- uses FUEL as a reactant
Fuel cell: overall reaction
combustion reaction
Fuel cell: energy converstations
chemical TO electrical
Fuel cell: electrolyte
- acts similar to salt bridge, balances the build up of charge
Fuel cell: electrolyte movement
cations -> cathode
anions -> anodes
Fuel cell : oxygen + fuel
O: ALWAYS undergoes REDUCTION
fuel: always undergoes oxidation
balancing half-equations for carbon containing fuels
- unless specified, assume carbon containing fuel oxidises into CO2(g)
- use KOHES to balance
Properties of fuel cell ELECTRODES acronym
PICCY
Porous electrode
1) ensures that the gaseous, fuel/oxygen can come into contact with the eledtrode
2) increases surface area = increased rate of reaction
inert electrode
electrode is unreactive, thus ensures that no other reaction occurs
catalyst electrode
increases rate of reaction
electrode conducts electricity
ensures that electrons can flow through the external circut
energy efficiency and pollution in a fuel cell (vs power stations)
- fuel cell has direct energy conversion (chemical TO electrical)
- meaning less energy is wasted and thus is more efficient
- therefore, in comparison to a power station, less fuel is required to release tje same amount of energy . Also, therefore, less polutants and greenhouse gases would be produced
green chemistry principle FOR fuel cells
1) design for energy efficiency
2) use of renewable feedstocks
ADVANTAGES of fuel cells
1) more energy efficient, less fuel required
2) quiet
3) low start/stop time
DISADVANTAGES of fuel cell
2) hydrogen storage/safety
what is both inert and a catalyst in fuel cells
platinum
is there a difference between the overall ewation in acidic & basic conditions?
NO, they are the exact same.
- the charge is also the same 1.23v EMF
PRIMARY cell
a galvanic cell which produces electrical energy and CANNOT be recharged
PRIMARY cells: electrolyte purpose
- to allow for the movement of ions to balance charges around the electrodes
(acts similar to the salt bridge)
- provides the internal circuit in the cell
what is the most common metal used in batteries
LITHIUM:
- it is the stronger reductant in the electrochemical series
- THUS it results in a higher EMF production
primary vs fuel cells: type of reaction
both are sopntaneous
primary vs fuel cells: reactants
primary: are stored in each half-cell
fuel: there is a continuous supplu of reactants
primary vs fuel cells: electrodes
P: conduct electricity
F: PICCY
primary vs fuel cells: reactivity of electrodes
P: may participate in the chemical reaction
F: will not participate in the chemical reaction
primary vs fuel cells: price
P: cheap
F: expensive
primary vs fuel cells: electrolyte
P: each half cell has its own electrolyte BUT they may also share an electrolyte
F: shared electrolyte
primary vs fuel cells: energy efficency
BOTH hsve high energy efficiency because they both have direct energy conversions from chemical to electrical
electric charge
the amount of charged particles (e.g electrons) that are present
- denoted by: Q
- si units: columbs (C)
electric current
the rate of flow of charged particles per second
- denoted by: I
- si units: amps (A)
electrical charge FORMULA
Q = IT
Faraday’s first law of electrochemistry
Q is DIRECTY proportional to mass
- eg. as MORE electrical charge passes throught he cell, the amount of substance that react/are produced also INCREASES
elementary charge
the charge carried by the elementary particle (electrons)
value: -/160 x 10 ^-19
faraday’s constant
the quantity of charge carried by one more of electrons
- 96500 C / mol
FORMULA linking electric charge and moles of electrons
n(e^-) = Q / F
Farafay’s second law of electrochemistry
TO consider the stoichiometric ratio between n(e^-) : n(metals)
STEPS to derive a charge of a ion
1) find n(metal) by n(metal) = m/Mr
2) find n(e^-) by Q/F
3) find stoichometric ration by divinding the smallest number present
intramolecular bonding
within molecules
TYPES of intramolecular bonds
ionic bonding
covalent bonding
metallinc bonding
intermolecular bonding
between molecules
TYPES of intermolecular bonds
- dispersion forces
- dipole - dipole
- hydrogen bonds
what are dispersion forces?
Dispersion forces are weak attractions caused by temporary electron shifts in molecules.
what are dipole-dipole forces?
Dipole-dipole forces are attractions between polar molecules due to their partial positive and negative charges.
They are stronger than dispersion forces but weaker than hydrogen bonds.
what are hyrdogen bonds?
Hydrogen bonds are strong dipole-dipole attractions between molecules containing H bonded to N, O, or F.
They are stronger than regular dipole-dipole forces but weaker than covalent bonds.
differences within fuel vs galvanic experiments
1) continuous supply vs fixed amount of reactants
2) electrodes act as catalyst and inert vs electrodes may be consumed part of reaction
3) more energy efficient vs efficiency may vary and typically decrese as cell discharges
possible random errors
1) enviromental conditions: changes in temp, humidity or air pressure can affect the experiement -> leads to flunctuation in reaction rates and measuremnts
2) variations in readings of measurements: MAY be due to human error or equipment sensitivity. EG. slight changes in voltages reading
possible systematic errors
1) equipment may not be properly calibrated (voltmeters/thermometers/PHmeters) or innacurate timers = lead to consistent deviations in measuremnts
2) worn or improperly prepared elctrodes = consistent errors in measurements = affecting the cell’s performance
sample resopnse for fuel cell vs conbustion
direct energy conversion from chemical →
electrical energy.
LESS energy loss, MORE energy efficient.
LESS fuel is required in a fuel cell
to produce the SAME amount of
energy.
Less overall CO2 emissions overall.
