3.1.4 energetics Flashcards
change in enthalpy is
the change in heat energy measured at constant pressure, temperature and conc of 1M for solutions
those are the standard conditions needed for measuring enthalpy change
ΔH∘r
standard enthalpy of reaction
ΔH∘neut
standard enthalpy change of neutralisation
enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions with all reactants and products in their standard states
exothermic
how do you write an equation for the standard enthalpy of neutralisation
moles formed of water must be 1. use fractions for the other species if necessary
use standard state symbols
standard states
state you find a substance in at 298K and 100kPa
remember enthalpy change value is for the
specific molar quantities in that reaction. so its different for every reaction
ΔH∘c
standard enthalpy change of combustion
enthalpy change when 1 mole of substance is burned completely in oxygen under standard conditions with all the P/R in their standard states
(always referring to complete combustion)
exothermic
ΔH∘f
standard enthalpy change of formation
enthalpy change when 1 mole of substance is formed from its constituent elements under standard conditions with all products and reactants in their standard states
exothermic
what is the ΔH∘f of an element
0
by definition
there is no ΔH because no energy is required to change an element into an element
calorimetry
the process of measuring the heat of chemical reactions or physical changes as well as heat capacity.
is used to work out the ΔH of a reaction experimentally, using a calorimeter
units of ΔH
kJmol^-1
process of a calorimeter
- Use measuring cylinder to add known mass of water to a copper cup. that is the calorimeter
- take and record initial temp of water
- use stirrer to evenly distribute heat throughout the water
- clamp cup in the air. have screens on the sides to reduce heat loss to surroundings so more heat is transferred to water
- record mass of burner w lid on to prevent evaporation
- light it and begin. stir water to evenly distribute heat. ensure water is never brought to boil
- record final temp of water and final mass of burner with lid on
- calculate how much fuel is used up by doing initial mass burner - final mass
equation to find energy released or taken in from calorimeter results
q = mc∆t
where q is the energy released/taken in
m is the mass water heated
c is the shc of water (always given)
∆t is the temp change
how do you find the ∆H from calorimeter results
get q (q=mc∆t)
÷ by moles
for neutralisation you divide by moles water produced
for displacement you divide by moles used up in the reaction eg limiting reactant moles
why is calorimetry useful for comparing fuels
for combustion reactions this works out ΔH∘c for that recation/substance
so who ever has higher kJmol^-1 has higher energy output
for neutralisation the change in temp is so sudden. why
they are often v exothermic so more heat is lost at the moment of reaction than at any other point
for neutralisation the change in temp is so sudden. how can we get a somewhat accurate value for it
plot graph of time on X and temp on y
record temp of reaction before it happens (so j the acid/alk in a beaker basc) for a few minutes
record it after neutralisation for a few minutes (so after the other was added)
extrapolate both data plots w a line of best fit to when the substance is added
find difference
record temp at regular intervals
temp against time
2 lines of b.f
establish theoretical max temp by extrapolation
for the extrapolation of neutralisation temp graphs, how do you do it properly
- extrapolate no further than the point of addition
- the gap in time data will indicate when substance is added
- miss the point of inflection, will help you decide what way you extrapolation should go
- may be a straight line sometimes
what are energy cycles and Hess cycles used for
to calculate ∆H that are difficult to achieve in reality.
ex the enthalpy of formation of methane; H2 is v flammable
Hess law states that
∆H for any chemical reaction is independent of the intermediate stages, provided the initial and final conditions are the same for each route
basc to say that enthalpy change from reactant to intermediate products to products, all that added up, is the same as the enthalpy change from reactant to product
In a hess cycle if you travel opposite of the arrow then
you invert the sign of that ∆H
checklist for energy profiles
dotted line to extrapolate inherent energy in R/P to where the arrows come up/down from
yeah lol
In an exothermic change…
energy is transferred from the system to the surroundings
the products have less energy than the reactants
enthalpy change is negative
in an endothermic change…
energy is transferred for the surroundings to the system
the require an input of heat energy
the products have more energy than the reactants
Mean bond enthalpy
Average energy needed to break one mole of a certain type of bond in a gaseous state under standard conditions. this def only applies when all subatcnes start and end in a gaseous state
It is a mean of data from a range of different compounds containing that bond type
Always +ve, endothermic
Why is values from mean bond enthalpy calcs different from those determined by hess law
Mean bond enthalpies are not exact; they are averaged values of the bond enthalpies from various compounds
enthalpy of atomisation
Enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state under standard conditions
endothermic
hess cycle for when given enthalpy of formation data
constituent elements below
arrows pointing up towards the R->P
each arrow upwards representing the enthalpy change when constituent elements go to R or to P
r->p may/prob wont be a formation reaction
hess cycle for when given enthalpy of formation data
oxides on the bottom
arrows pointing down from R and P to their oxides
arrows representing the enthalpy change when R or P is completely combusted to form their oxides
r->p may/prob wont be a combustion reaction
hess cycle for when given mean bond enthalpy data
what else might you have to consider?
gaseous atoms on the bottom
arrows pointing down towards the gas atoms from R and P
arrows representing enthalpy change when moles of R or P have their bonds broken to form gaseous atoms
because mean bond enthalpy is taken when something starts and ends as a gas, have to make gaseous. so if R/P is in a non gaseous state, would have to add into arrow energy: the enthalpy change when they are vaporised (enthalpy of vaporisation)
enthalpy of vaporisation
enthalpy change when one mole of a liquid is turned to a gas standard conditions standard states
endothermic
enthalpy of fusion
enthalpy change when one mole of a solid is turned to a liquid
endothermic
