3.1.4 Energetics Flashcards
Thermochemistry
Exothermic - energy is given out
Bond breaking - endothermic
Negative energy output
Endothermic - energy has been taken in
Bond making - exothermic
Pos energy output
Quantities in endo and exo reactions
Amount of heat given out depends on quantity of reactants
Depends on temperature, pressure or conc of solutions
Usually in Kj mol -1
Need to give the equation
To look at ratios
Examples of exothermic and endothermic reactions
Exothermic
Neutralization
Endothermic
Heating copper sulphate heat energy must be put in
Cold pack reaction
NH4NO3(s) + aq -> NH4NO3(aq)
Enthalpy change
Heat change at a constant pressure
The heat given out to the surroundings while the reaction mixture cools is the enthalpy change
Not over till products back to temp they started at
Standard conditions
100kPa
298K
Pressures effect in enthalpy conditions
If gas given out energy is required to push away the atmosphere
Greater the atmospheric pressure the more energy required
Meaning less energy available to be given out as heat by reaction
Physical states of reactants and products effect on enthalpy change
Heat must be put in to change liquid to gas
And given out when gas is changed to a liquid
Means you have to include state symbols
Standard enthalpy of formation
The standard molar enthalpy of formation is the enthalpy change w hen one mole of substance is formed from its constituent elements under standard conditions. all reactants and products being in their standard states.
Standard enthalpy of combustion
The standard molar enthalpy of combustion is the enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions, all reactants and products being in their standard states.
Relationship between heat and temperature
Temperature is independent if the amount of particles present
Heat is a measure of the total energy of all the particles present in a substance - does matter how many particles present
Measuring the enthalpy change of a reaction
You need to know:
Mass of a substance that is being heated up or cooled down
Temperature change
Specific heat capacity of the substance
Specific heat capacity
The amount of heat needed to raise the temperature of 1g of substance by 1K
J per g per k
4,18 JgK to raise the temp of 1 g of water by 1 K
Enthalpy change equation
Q = mc delta T
Calorimeter
You burn the fuel to heat a known mass of water
Then measure the temperature change of the WATER
Assume all heat goes to heating the water
Improve result by reducing heat loss
Flame calorimeter
Used for measuring enthalpy change of combustion designed to reduce heat loss even further
Spiral chimney is made of copper , flame is enclosed, fule burns in pure oxygen rather than air
Measuring enthalpy change of reactions in solutions
The heat is generated in the solution themself - has to be kept in the colourimeter
Expanded polystyrene beakers are often used in colorimeter - good insulators low heat capacity
Measuring the enthalpy change of a Neutralisation reactions
Exothermic give out heat
To measure enthalpy change you used the quantities given out in moles given by the balanced equations
To find the mass add the alkaline and the acids mass together
Measuring the enthalpy change of a displacement reaction
When a more reactive metal displaces a less reactive metal
If compound dissolves in water - reaction can be measured in a polystyrene beaker
Metal is the fuel
Allowing heat loss
Pot a cooling curve
Best estimate of the temperature immediately after mixing - draw a line of best fit through graph
Extrapolate back to the time of mixing
Hesses law
Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
Shown in a thermochemical diagram
Thermochemical cycle for the formation
If it’s going to an element on its own - value = 0
arrows go up
Thermochemical cycle general structure
Go clockwise from the reactants to the products
If the arrow is going the opposite direction the take it away
Thermochemical cycle for combustion
arrows go down
Enthalpy of elements
The enthalpies of all elements in their standard states (i.e., the states in which they exist at 298K and IOOkPa) are taken as zero. (298 K and lOOkPa are approximately normal room conditions.)
If element such as pure carbon can exist in a number of states - the most stable allotropes state is used
Plot the enthalpy change of combustion against the carbon number
Straight line means enthalpy change of combustion same amount for each extra carbon in the chain
Bond enthalpy
A definite amount of energy to a particular bond
Bond breaking and making
Bond breaking - endothermic
Bond making - exothermic
Bond dissociation enthalpy
Enthalpy change required to break a covalent bond with all species in a gaseous state
The mean bond enthalpy
Average amount of energy to make a bond
Slightly different for each molecule
Useful quick and easy to use
Comparing the results with that from a thermochemical cycle
Thermochemical cycles give accurate values for bond enthalpies for that molecule
Mean bond enthalpies means you can calculate an approximate means bond enthalpy for a compound never been made
In q = mc delta t
To workout the enthalpy of combustion of a fule
It is (mass of WATER) (specific heat capacity if water 4.18)(temperature change)
Divide by 1000 to get kj
Divide by mols of fule to get kj mol -1
Must put a sighn - if water is being heated to a higher temp then heat released by fule so exothermic so neg sighn
Reason enthalpy change of combustion is less exothermic than using hess to calculate
Incomplete combustion.
Heat loss
Enthalpy change
Heat change at a constant pressure
Enthalpy change formualr using bind enthalpies
= bonds broken - bonds made
= reactants - products
When bond enthalpy of 0=0 always the same
O2 is the only substance that contains 0=0 bond
Table tor ecord the readings needed ot workout the experimental value of combustion
One side
Temp
Initial
Final
Delta T
Other side
Mass g
Burner befor
Burner after
Mass …. Burnt
Q = mc delta t or neutralisation
The m is the mass of the acid + base
And to calculate mol dm-3 you have to use the mols of either the acid or base
if using (TFinal - Tinital) for delta T rule
then gives you always -q
(stick a neg sigh in front)
when doing uncertainty questions on initial and final reading for temperature
you have to do ( 2 x uncertainty/ (final - initial reading ) x 100
why heat change CALCULATED FROM A BOMB CALORIMITER EXPERIMENT IS NOT ENTHALPY CHANGE
PRESSURE IS NOT CONSISTNT IN A BOMB CALORIMETER
when calculating enthalpy change f combustion of a neutralization what do you have to assume
that they are both starting at the same initial temperature
reaction for the enthalpy change of formation equation
the mols of product must be one - so use fractions or what not to fix equation to make sure not multiple mols of product
