Topic 4: Energetics Flashcards
enthalpy change
Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.
standard enthalphy
Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature
Alkenes
double carbon bond
Steps to calculating enthalphy change
1) write balanced equation
2) work out how many of each bond is broken and total energy required
3) work out how many of each bond is made and total energy released
4) break - make
exothermic
a reaction that releases energy from the system in the form of heat
endothermic
a reaction that the system absorbs energy from its surroundings in the form of heat
system
surroundings
-System–> energy stored in the chemicals that are being studied (reactants and products)
-Surroundings –> everything around the system e.g the air in the lab etc
energy
symbol = Q
measured in joules
enthalphy
symbol = H
measured in KJ/mol
what is enthalphy change
Enthalphy change of a process is the heat energy that is transferred between the system and the surroundings at constant pressure
types of enthalphy change
exothermic
endothermic
exothermic and endothermic
In an exothermic reaction products have less energy than the reactants
In an endothermic reaction products have more energy than the reactants
what happens to enthalphy in exothermic system
decreases
what happens to enthalphy in an endothermic system
increases
bond enthalphy equation
Molecule + bond enthalpy –> molecular fragments
what state must molecules be in
gaseous
define bond energy
Bond energy –> the energy needed to break one mole of a particular covalent bond
define bond enthalphy
Bond enthalpy –> the enthalpy change when one mole of a bond in the gaseous state is broken
why is enthalphy change always positive
Enthalpy change is always positive as the energy required to break the strong covalent bond attractions between a shared pair of electrons and nuclei
mean bond enthalphy
-When bonds are in different compounds there are slightly different bond enthalpies
-bond enthalpy decreases as there are less elements
-mean bond enthalpy is the enthalphy change when one mole of a bond averaged at over many different gaseous molecules is broken
CH4 (g) –> C(g) + 4H(g)
Mean bond enthalpy –> +Q /4 –> ¼ CH4 (g) –> ¼ C (g) + H (g)
bond energy calculations
Negative = exothermic
Positive = endothermic
types of exothermic reactions
combustion
oxidation
neutralisation
types of endothermic reactions
thermal decomposition
standard conditions
100KPa and 298K
types of enthalphy change
standard enthalphy change of reaction
standard enthalphy change of formation
standard enthalpy change of combustion
standard enthalphy change of neutralisation
equation for enthalpy change
q = m c ∆ T
q = heat loss/gain (j)
m = mass of water (g)
c = specific heat capacity of water (4.18 J/g/K)
∆T = temperature change of water
calculate how much heat would be given out when 8g of methane is burnt completely
mass / mr = moles (8/16 = 0.5)
moles x KJ = ans
how does pressure affect the amount of heat energy given out by reactions that involve gases
greater atmospheric pressure = more energy needed to push gas away
this means that less energy remains to be given out as heat by the reaction
∆fH (formation)
enthalphy change when one mole of substance is formed from consistent elements under standard conditions
∆cH
enthalphy change when one mole of a fuel is completely burnt in excess oxygen under standard conditions with elements in their standard states
what physical property should be kept constant when measuring an enthalpy change
pressure
standard enthalpy change
ΔH°
Standard enthalpy changes are measured at a standard constant pressure of 100 kPa and a temperature of 298K
definition of standard enthalphy changes
Standard enthalpy change of reaction is the enthalpy change measured at 100kPa and 298K when the number of moles of substances as written in the equation react
what would happen if the number of moles half
If the number of moles of an equation half the enthalpy change will also half
Standard enthalpy change of formation: ΔH°f
-the enthalpy change measured at 100kpa and 298K when one mole of a substance is formed from its elements in their standard states
-standard enthalpy of formation of an element in its standard state must be zero
-elements must be in their standard state e.g F2(g)
equation for enthalphy change of formation
-only ever 1 mole of products
-Methane –> C(s) + 2H2 (g) –> CH4
endothermic and exothermic
Bond breaking is an endothermic process, because it requires energy.
Bond forming is an exothermic process, because it releases energy.
if pressure is constant then…
Q = enthalphy change
if temperature increases then…
it is an exothermic reaction (forming bonds)
true or false - incomplete combustion is less exothermic
true
elements in their standard states
= zero
suggest two disadvantages of using a glass beaker on a tripod and gauze
-glass beaker is a poorer conductor
-reduced heat transfer to tripod and gauze
suggest two reasons why the value of enthalphy of combustion is less exothermic than data book value
incomplete combustion
heat loss to surroundings
suggest one addition to this apparatus that would improve the accuracy of the enthalphy value obtained
use wind shield to reduce heat loss
outline a simple practical experiment that the engineer woulc use in order to determine the enthalphy of combustion for one of the unknown alcohols
-weigh alcohol before and after combustion
-place water in calorimeter
-measure mass of water
-burn alcohol to heat the water
-measure temperature change in water
Calorimetry sources of error
-heat loss e.g not all heat used to heat the water
-evaporation of the fuel
-incomplete combustion of the fuel e.g soot under beaker
-neglecting specific heat capacity of calorimeter
-non-standard conditions e.g water vapour is produced
enthalphy change of combustion
-The enthalpy change measured at 100kPa and 298K, when one mole of a fuel (hydrocarbons/alcohols) is completely burned in excess oxygen with all substances in their standard states
requirements for standard enthalphy of combustion
-the standard enthalphy of combustion is always negative (temperature increases)
-should only be 1 mol in products and water produced should be a liquid
Measuring enthalphy change of combustion (Calorimetry)
1) measure initial mass of the fuel
2) determine the volume and initial temp of water in the copper can
3) heat water with fuel and record temperature every minute
4) measure final temperature
5) reweigh the fuel and determine the moles that have been combusted
mean bond enthalpies
different covalent bonds require different amounts of energy to be broken
values can be found experimentally using calorimetry methods
why is a copper cup better to use than a glass beaker
Copper cup is better to use than a glass beaker because the heat will transfer directly to the solution being heated.
Glass is a poorer conductor and less heat would reach tripod+gauze
errors in enthalphy of combustion
-heat loss
-incomplete combustion
-experiment was not completed under standard conditions
-evaporation of fuel
3 factors that affect the amount of thermal energy of a substance
-temperature
-mass
-material it is made from
specific heat capacity
-Water has the highest specific heat capacity
Specific heat capacity is the amount of energy needed to increase the temperature of 1kg of a substance by 1 degree
4.18
1g of water 1 degree hotter = 4.18J (energy required)
1 of water 2 degree hotter = 4.18 x 2 = 8.36J
equation for energy
Q = m c Δ T
Q = energy transferred in joules
M = mass of substance being heated (water) in grams
C = specific heat capacity of water in J/K/g
ΔT = change of temperature in kelvin
Volume/density = mass
calculating enthalphy change
1) Q = m c Δ T
2) Q / mol = enthalpy change
calorimetry sources of error
-heat loss e.g not all heat used to heat the water
-evaporation of the fuel
-incomplete combustion of the fuel e.g soot under beaker
-neglecting specific heat capacity of calorimeter
-non-standard conditions e.g water vapour is produced
what is calorimetry
experimental method for finding enthalphy change by measuring temperature over time
what is measured change in temperature proportional to
energy change
what is the state symbol for carbon
solid
Hess’s law
-the enthalphy change accompanying a chemical change (reaction) is independent of the route by which the chemical change occurs
-the enthalphy change will always be the same but the route may be different
why is it difficult to measure enthalphy change directly
-when carbon is burned biproducts are formed such as soot
-it would require a significant amount of thermal energy to cause reactants to react
-hard to distinguish between the amount of energy required and the amount of energy released
hess’s law
-overall enthalphy change for a reaction is independent of the route it takes as energy can only be transferred and change its form
-There is a direct route e.g A–>B
-there are indirect routes x 2 e.g A—C—B or A—D—B
So ΔH1 = ΔH2 + ΔH3
direction of the arrow and sign of the value for formation
-the direction of the arrow determines the sign of the value
e.g if the arrow points up but you need to go down the value will be the opposite sign
Arrow goes from elements to products for enthalphy change of formation
steps for calculating the hess cycle (formation)
1) write equation for reactants —> products (ignore single elements e.g O2 as their value is zero)
2) Work out elements required to form the molecules and ensure it is balanced with the state symbols in standard states
3) Place arrows in correct directions with ΔH labels –> ensure if there is more than 1 product formed multiply by 2
4) two arrows can be drawn if more than 2 products
5) Find ΔH using Hess’s law e.g (+239.1) + (-393.5) + 2(-285.8) = -726 KJ/mol
which direction do the arrow’s point for combustion
down
steps for hess cycle of combustion reactions
1) State products of combustion
2) Add oxygen to both the reactants and products in the main reaction
3) Write out equation for each reaction to help you
4) check arrow direction (should be downwards)
enthalpies of combustion
Answer should be endothermic (positive) –> enthalpies of combustion
what should you do if there are two fuels in the reactants
multiply by 2
Explain how the experimental method and use of apparatus can be improved to provide more accurate data
Describe how this data from the improved method can be used to determine an accurate value for the temperature change
-use burette instead of measuring cylinder
-use a polystrene cup instead of a beaker to insulate it
-use powdered solid and reweigh watch glass
-measure initial and final temperature and measure temp at regular intervals
-plot graph with temp and time and determine overall change in time
how would u calculate Q
mol x enthalphy change
suggest how to reduce percentage uncertainty in the temperature change
increase the concentrations of the solution
state why the heat change calculated from the bomb calorimeter experiment is not an enthalphy change
pressure is not constant
Give one reason why the bond enthalphy calculated is different from the mean bond enthalphy in the data book
data book value derived from different compounds
suggest the most likely reason for the large difference between the students experimental value and the data book value
heat loss = less exothermic
suggest how the students method and analysis of results could be improved to determine a more accurate enthalphy change
-insulate beaker/use polystyrene cup to reduce heat loss
-record initial temp and record temp at regular intervals
-exbalish maximum temperature on graph
suggest why the value for the standard enthaply of formation of liquid Sb is not zero
liquid is not Sb’s standard state
reaction profiles
exothermic = products less than reactants
endothermic = products more than reactants
calorimetry experiment
washes the equipment (cup and pipettes etc) with the solutions to be used
dry the cup after washing
put polystyrene cup in a beaker for insulation and support Measure out desired volumes of solutions with volumetric pipettes and transfer to
insulated cup
clamp thermometer into place making sure the thermometer bulb is immersed in solution
measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes
At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance. If using a solid reagent then use ‘before and after’ weighing method
stirs mixture (ensures that all of the solution is at the same temperature) Record temperature every minute after addition for several minutes
mean bond enthalphy
The mean bond energy is the enthalpy needed to
break the covalent bond into gaseous atoms, averaged over
different molecules.
combustion
negative final value
improve the apparatus
-polystyrene cup
-calculate mass by difference
-powdered solid
-burette instead of measuring cylinder
enthalphy of formation
products - reactants
errors in calorimetry experiments
- energy transfer from surroundings (usually loss)
- approximation in specific heat capacity of solution. The method assumes all
solutions have the heat capacity of water. - neglecting the specific heat capacity of the calorimeter- we ignore any
energy absorbed by the apparatus. * reaction or dissolving may be incomplete or slow. - density of solution is taken to be the same as water.
errors in combustion
- Energy losses from calorimeter * Incomplete combustion of fuel * Incomplete transfer of energy
- Evaporation of fuel after weighing
- Heat capacity of calorimeter not included
- Measurements not carried out under standard conditions as
H2O is gas, not liquid, in this experiment
combustion vs formation
combustion = reactants - products (only combustion if it says changes from combustion)
formation = products - reactants
as carbon atoms increases
enthalphy of combustion increases
if calculating temperature =
you can keep it in celcius
calorimetry
heavier water = smaller change in temperature
reduce heat loss =
increase volume of solution
types of calorimetry
Flame calorimetry is used to calculate the enthalpy change of a reaction in which a substance is burned.
There are two types of flame calorimetry: simple flame calorimetry and bomb calorimetry.
A 22.0cm3 sample of hydrochloric acid is neutralised by exactly 18.0cm3 of potassium hydroxide.
627 J of heat energy is released.
Calculate the temperature change in Kelvin.
Assume that both solutions have the same initial temperature, densities of
1.00gmcm3 and specific heat capacities of
4.18
1) Mass of HCL
2) Mass of KOH
3) Add mass together
Q / m c
reason for less exothermic
incomplete combustion
environmental problem =
acid rain = SO2
