energetics I Flashcards
(29 cards)
what is enthalpy change
energy change for a chemical reaction
△H
whats the standard enthalpy change of combustion
The enthalpy change when one mole of a fuel is combusted completely in oxygen under standard conditions (1atm and 298K).
what are standrd conditions
A specified temperature, usually 298K
100 kPa pressure
Solutions at 1 moldm-3
△H° (sometimes the circle has a dash through it)
what is standard enthalpy change of formation
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
what is standard enthalpy change of neutralisation
The enthalpy change when an acid and an alkali react together under standard conditions to give one mole of water.
what is standard enthalpy change for a reaction
The enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction.
hwo to calculate enthalpy changes
Q=mc△t
Q= energy in J
m= mass of water heated(g) (1g/cm3)
c= specific heat capacity 4.18 JKg-1
△t=change in temperature
how to calculate △H
Q/moles - Jmol-1
(Q/1000)/moles - kJmol-1
has a sign can be + (endothermic reaction so energy of system goes up, energy of surroundings go down) or - (exothermic reaction so energy of system goes down, energy of surroundigns go up)
moles =
mass/mr
hess’ law
The enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.
how to calculate the standard enthalpy change of combusiton
- write the equation for the reaction
we want to find △H for. - work intermediates underneath.
- Balance
- Complete arrows - arrows point downwards
- Add O2
- Calculations
products of combustion are at the bottom and equation for formation on top
△H1=△H2-△H3
how to calculate the standard enthalpy change of formation
- write the equation for the reaction
we want to find △H for. - work intermediates underneath.
- Balance
- Complete arrows - point upwards
- Calculations
proucts of formation are at the bottom and equation for combustion on top
△H1=△H3-△H2
what is bond enthalpy
is the enthalpy change when one mole of bonds in the gaseous state is broken.
what is bond dissacosiation
- enthalpies refer to the breaking of one bond in a diatomic molecule, e.g.: H2(g) –> 2H(g) - bond dissociation enthalpy = +436kJ mol-1
- They are always endothermic (positive)
why are mean bond enthalpies used
- Each successive bond that is broken in a molecule containing more than two atoms will have a slightly different bond enthalpy as the remaining bonds have a slightly different electronic environment
- Bond enthalpies for a particular type of bond in different covalent environments are slightly different
method for enthalpy of combusiton practical and hwo to process the data
- Measure 100 cm3 of water into a can.
- Accurately weigh a ‘burner’ containing one of the alcohols.
- Arrange the apparatus in such a way that the burner can be used to heat the water in the can. You should try to protect your apparatus from draughts.
- Take the temperature of the water before you start heating.
- Use your burner to heat the water. Stop heating when you have heated the water enough to give a reasonable temperature rise.
- Take the final temperature of the water and reweigh the burner
processing data: - Q=mc△t
sources of error in enthalpy of combustion practical
- Heat transferred to air and equipment and not to water.
- Incomplete combustion.
- Conditions not standard i.e. H2O(g) formed not liquid
- Takes time to heat so energy transferred from water to surroundings i.e. water being heated by flame but also losing heat energy to the surrounding from where it is not being heated.
method for energy changes during displacement pracitcal
- Using a 25 cm3 pipette, measure 50 cm3 of the copper(II) sulfate solution into the polystyrene cup held securely in a beaker.
- Place the thermometer in the copper(II) sulfate solution. Gently stir the solution with the thermometer, start the timer and record the temperature to at least the nearest 0.5°C in the table below.
- Continue to stir the solution gently, recording its temperature every half minute for 3.0 minutes.
- At exactly 3.5 minutes, add the zinc powder to the copper(II) sulfate solution, stirring as you do so. Do not attempt to record the temperature at 3.5 minutes.
- Record the temperature of the solution, to at least the nearest 0.5°C, from minute 4.0 to minute 9.0 at half minute intervals. Continue to stir the solution throughout.
prcoessing data for energy changes during displacement pracitcal
We can’t measure the actual maximum temperature because the reaction rate means that in the time taken for the reaction to complete and the maximum to be reached, it has already started cooling down.
Use a graphical method to determine the maximum temperature that reduces the effect of these heat losses.
y axis - temperatyre, don’t start at 0
x axis - time mins
do an extrapalation, exteneding the line of best fit after zinc was added backwards to find △t
then use 1=mc△t
imporvements for energy changes during displacement pracitcal
- Use an electric stirrer. This could help to increase rate and also make sure the reaction goes to completion.
- Use a lid on the cup to reduce heat losses.
Use a pipette to measure volumes to make them more accurate and so make m in mcDT more accurate
method for enthalpy of neutralisation
- Clean and rinse a 25cm3 glass pipette and then pipette out 25cm3 of the acid into a polystyrene cup (within a glass beaker).
- Record the initial temperature of the alkali solution as accurately as you can. Put the thermometer through the hole of a lid.
- Clean and rinse a 25cm3 glass pipette and then pipette out 25cm3 of the alkali into the polystyrene cup.
- Place the lid and thermometer back on the cup and keep swirling whilst monitoring the temperature.
- Record the maximum temperature reached.
But we get the same issues with reaction rate and losing heat energy while we wait for reaction to happen.
So make DT more accurate we can: - Clean, rinse and fill a burette with the acid solution you are using.
- Clean and rinse a 25cm3 glass pipette and then pipette out 25cm3 of the alkali into a polystyrene cup (within a glass beaker).
- Record the initial temperature of the alkali solution as accurately as you can.
- Add 5.00cm3 of acid to the alkali from the burette, stir the solution thoroughly using the thermometer, and record the temperature in the table below.
- Continue to add 5.00cm3 aliquots of acid and recording the temperature change until you have added 50.00cm3 of the acid.
processing data for enthalpy of neutralisation
using a graph with x axis being volume and y axis being temperature
drw a line of best for the curve, see where th elines cross and record the volume and temperature (idk what to do with that will check with cp book for now this si what u need to know u can figure it out urself in the exam trust xx)
improvements for enthalpy of neutralisation
using a pipette and burette to measure volumes.
energy profile for endothermic reaction
energy
|………………../—-\
|………………|….{..|products
|………………|….{[
|………………|….{[
|reactants|
|_______________________________
………………….time
{=△H
[=activation energy
