Enthalpy changes Flashcards
define enthalpy (H)
heat content of reactions
define enthalpy change, ΔH
a change in the heat energy of a system, measured under constant pressure
what are exothermic reactions
- reactions where energy is given to the surroundings (-ve enthalpy change)
- less energy taken in to break bonds than released when making bonds
- enthalpy of products is smaller that that of the reactants
what are endothermic reactions
- reactions where energy is taken in from the surroundings (+ve enthalpy change)
- more energy taken in to break bonds than released when making bonds
- enthalpy of products is greater than that of reactants
what is activation energy Ea
the minimum energy required for a reaction to take place
pressure under standard conditions
100kPa
temperature under standard conditions
298K = 25C
concentration under standard conditions
1 moldm-3
define standard state
physical states under standard conditions
define standard enthalpy change of reaction
- enthalpy change when 1 mole of a substance is reacted under standard conditions
example of equation for standard enthalpy change of reaction ( Mg and O2)
Mg(s) + 1/2O2(g) –> MgO(S)
define standard enthalpy change of formation
- enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions
example of equation for standard enthalpy change of formation (H2 and O2)
- H2(g) + 1/2O2 —> H2O (l)
Define standard enthalpy change of combustion, ΔH°c
- enthalpy change when 1 mole of a substance combusts completely with oxygen under standard conditions
example of equation for standard enthalpy change of combustion, ΔH°c
(C4H10 and O2)
C4H10(g) + 6.5O2(g) —> 4CO2(g) + 5H2O(l)
Define standard enthalpy change of neutralisation
- enthalpy change when an acid and a base react together to form 1 mole of liquid water under standard conditions
example of equation for standard enthalpy change of neutralisation (HCl and NaOH)
HCl (aq) + NaOH (aq) —> NaCl (aq) + H2O (l)
define average bond enthalpy
average energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission
example of equation for average bond enthalpy (CF4)
1/4CF4(g) —> 1/4 C (g) + F (g)
do all reactions consist of an endothermic and an exothermic part
YES
how do you work out overall enthalpy change
Bb - Bf = O (overall)
equation for calculating heat change
q = mcΔT
q = heat change (J)
m = mass of solution, ignore solid (g)
c = specific heat capacity (Jg-1K-1)
ΔT = temperature change (K)
- then q/n
what’s coffee cup calorimetry used for
to directly determine the enthalpy change of reaction that involves 2 chemicals in a solution when mixed together
why is a polystyrene cup used in coffee cup calorimetry
insulating material - reduces heat loss from reaction to the surroundings
since density of water is 100gdm-3, 1cm3 is equal to
1g
what’s copper calorimetry used for
- used to indirectly determine the enthalpy change of combustion
- involves burning a fuel. The heat transferred from this reaction determined via measuring temp increase of a known mass of water
enthalpy change of neutralisation is always
sum(r) - sum(p)
-ve
enthalpy change of combustion is always
sum(r) - sum(p)
-ve
enthalpy change of formation can be
sum(p) - sum(r)
+ve or -ve
enthalpy change of reaction can be
sum(p)-sum(r)
+ve or -ve
for enthalpy change of formation (and reaction) what goes at the bottom of the Hess cycle
elements in standard states (arrows point away from these)
for enthalpy change of combustion (or neutralisation) what goes at the bottom of the Hess cycle
- CO₂(g) + H₂O(l) (arrows point towards these)
- for neutralisation you can also have salt and water
bond breaking is…
endothermic
bond breaking is…
exothermic
bond enthalpy values are always
+ve
which species don’t need arrows pointing towards or away from them
elements
why do calculated enthalpy values different from those in databases
….
