enthalpy Flashcards
(19 cards)
Enthalpy change meaning?
The heat energy change (that is transferred between system & surroundings) at constant pressure aka ΔH
- substance in standard state under standard conditions
Standard conditions?
100 kPa
298 K (25 degrees Celsius)
- under these conditions - enthalpy change is ‘standard enthalpy change)
- no. moles change enthalpy
Exothermic v Endothermic?
exo - where heat energy transferred from system (reaction mix) to surroundings so -temp of mix increases
endo - where heat energy transferred from surroundings to system - temps of mix decreases
Energy diagrams?
exo - final enthalpy of system less than initial (endo is reverse)
- change in enthalpy aka ΔH; Hproducts - Hreactants
exo - Hreactants > Hproducts so - ΔH -ve
- endo - Hreactants < Hproducts so - ΔH +ve
- arrow alw points towards products
- y axis: ‘Enthalpy, H’
- formulae for reactants & products
- values for ΔH
Standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substance is completely burned in O2 to make CO2 & H2O under standard conditions (100kPa & 298K)
Experiment to find ΔH combustion of a liquid - calorimetry?
*known mass of liquid burned - heat energy produced used to heat known vol of water (raise temp by specific amount)
1. spirit burner containing liquid weighed
2. known vol of water added to copper can
3. temp of water measured
4. burner is lit- mixture constantly stirred w thermometer
5. when temp of water reached approx. 20 Celsius above initial temp 6. flame extinguished & burner immediately reweighed- final temp measured
Calculating enthalpy change of combustion?
- Q=mcΔT - calculate heat energy transferred to water (convert to kJ)
- m = mass of water/sol
- c = specific heat capacity of water (4.18) - calculate amount, n (moles), of ethanol burned (of fuel)
- ΔH = -Q/n
Sources of error & assumptions made in experiment?
- some of heat energy produced in burning transferred to air not water
- some of ethanol may not burn completely to form CO2 + H2O (incomplete combustion would cause soot to form on bottom of copper can)
- some of heat energy produced in burning transferred to copper can not water
- conditions are not standard e.g. water vapour not liquid water produced
- as experiment takes long time - not all of heat energy transferred from water to surroundings is compensated for
Preventing probs of experiment?
- lid placed on top of copper can
- wind/draught shields placed by/near flame - prevent a draught moving flame
Standard enthalpy change of neutralisation?
The enthalpy change when acid & alkali react to form 1 mole of water under standard conditions (100kPa & 298K)
- for strong acids & alkalis - assumption that r fully ionised in aq sol reaction - H+ + OH- –> H2O - why enthalpy changes r so similar
Experiment to find ΔH neutralisation?
- using pipette fitted w safety filter, place 25cm3 of 1moldm-3 acid into expanded polystyrene cup
- measure temp of acid
- place 25cm3 of alkali into beaker
- measure temp of alkali
- add alkali to acid - stir w thermometer & measure max temp reached
- q=mcΔT (cm3 = g)
- n = cv
- ΔH = q/n
Sources of error & assumptions?
- only 2 sig figs as ΔT only 2 sig figs
- uncertainties of measurement w use of pipette & thermometer
- heat energy transferred to thermometer & polystyrene cup
Standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions (100kPa & 298K)
Hess’s Law?
The enthalpy change of a reaction is independent of the path taken in converting reactants into products - provided initial & final conditions r the same in each case
Bond enthalpy definition?
(ΔBH) enthalpy change when 1 mole of a bond in gaseous state is broken
e.g. Cl2(g) –> 2Cl(g)
e.g. CH4 –> CH3 + H
THEN… CH3 —> CH2 + H etc.
Calculating enthalpy change using mean bond enthalpies?
bonds broken - bonds made
limitations of bond enthalpies to calculate enthalpy change?
bond enthalpies measured in gaseous state & compounds may not be gases
Standard enthalpy change of reaction?
The enthalpy change of a reaction according to the molar quantities in the equation under standard conditions (100kPa & 298K)
Bond broken v bonds made?
to break - energy needs to be absorbed
- if more energy needed to break bonds than energy given out when bonds formed - endothermic
to form - energy released
- if more energy released when bonds r formed than what was needed to break initial bonds - exothermic
