Developing Fuels: Thermochemistry Flashcards
Definition enthalpy
Measure of heat content of a substance
Definition enthalpy change ( delta H )
Change in heat content at constant pressure
Symbol for enthalpy change
Delta H
Standard conditions (delta H°) (pressure and temp )
10^5 Pa
298 K
Describe reaction profile / energy level diagram for EXOthermic reactions
- products = lower energy than reactants
- ve delta H
Describe reaction profile / energy level diagram for ENDOthermic reactions
- products = higher energy than reactants
- +ve delta H
Definition of standard enthalpy change of reaction ( delta(r) H°)
Enthalpy change for a reaction with the quantities shown in the chemical equation
= value should always be quoted along with the equation
Symbol for standard enthalpy change of a reaction
Delta(r) H°
Definition of standard enthalpy change of formation ( delta(f) H°)
Enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions
Symbol for standard enthalpy change of formation
Delta(f) H°
Write equation to show standard enthalpy change of formation ( delta(f) H°) of H2O(l)
H2(g) + 1/2 O2 (g) —> H2O(l)
Definition of standard enthalpy change of combustion ( delta(c) H°)
Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions
Symbol for standard enthalpy change of combustion
Delta(c) H°
Write equation to show standard enthalpy change for combustion of CH4
CH4(g) + 2 O2(g) —> CO2(g) + 2 H2O(l)
Definition of standard enthalpy change of neutralisation ( delta(neut) H°)
Enthalpy change when 1 mole of WATER is formed in a reaction between an acid and alkali under standard conditions
Symbol for standard enthalpy change of neutralisation
Delta ( neut) H°
Write equation to show standard enthalpy change of neutralisation ( delta(neut) H°) of sulfric acid and sodium hydroxide
1/2 H2 SO4 (aq) + NaOH (aq) —>
1/2 Na2 SO4 ( aq) + H2O(l)
Equation for heat energy transferred ( given out / absorbed )
q= mcAT
What does each letter stand for in q= mcAT
- q= heat energy transferred ( J )
- m= mass of substance ( liquid / solution ) heated
- c = specific heat capacity ( 4.18)
- AT (delta T ) = change in temp ( in K but no conversions required )
Equation for ( molar ) enthalpy change (per mole )
delta H = q ( KJ) / no. of moles reacting
What does enthalpy change always have to have
+ or -
In a calorimetry experiment if temp INCREASES what kind of reaction is it
EXOthermic ( -ve delta H )
In a calorimetry experiment if temp DECREASES what kind of reaction is it
ENDOthermic ( +ve delta H )
Describe steps to carry out dissolving / displacement / neutralisation calorimetry
- Using a measuring cylinder pour a known volume of a known concentration of acid into an insulated vessel
- Measure temp of acid before adding the alkali
- Using a measuring cylinder add a known volume of a known concentration of alkali to the acid
- Stir
- Record highest temp reached
- Calculations: q=mcAT, mols of acid/alkali ( one which is not in excess ) , delta H
Describe steps to carry out combustion calorimetry
- Using a measuring cylinder pour a known volume of water into copper calorimeter
- Weigh spirit burner before lighting
- Measure temp of water before lighting
- Light the wick to heat the water & stir
- Extinguish burner & record temp of water after heating
- Reweigh the spirit burner
- Calculations:q=mcAT,mols fuel burnt, delta H
Define average bond enthalpy ( 2 marker )
- av. enthalpy change when 1 mole of bonds
- of gaseous covalent bonds is broken
BENDO MEX
- Breaking bonds is endothermic
- Making bonds is exothermic
Energy change= ???
Break - Make
( Reactants - Products )
( energy needed to break bonds - energy released making bonds )
Sign for EXOthermic reaction
-ve
Sign for ENDOthermic reaction
+ve
Explain what happens in an EXOthermic reaction
More energy is released making bonds than is needed to break bonds
( heat energy given out = increase temp)
Explain what happens in an ENDOthermic reaction
More energy is needed to break bonds than is released making bonds
( heat energy taken in = decrease temp)
Why are many bond enthalpies describes as averages
They are averaged out over different compounds containing the same bond
