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
