Module 3 - Enthalpy Flashcards
What is enthalpy
The total chemical energy inside a substance is called the enthalpy
What is the units for enthalpy
kJ mole ^ -1
ΔH
What is an exothermic reaction
Heat energy is given off by the reaction to the surroundings
What is an exothermic reaction enthalpy change
ΔH is negative as heat energy is transferred to the surroundings.
What is an endothermic reaction enthalpy change
ΔH is positive as heat energy is taken from the surroundings.
What should be included in an enthalpy profile diagrams
The energy level of the reactants
The transition state(s)
The energy level of the products
The activation energy (Ea)
Axis labelled
What is activation energy?
The minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction
What standard condition should be measured for enthalpy to occur
298K
100kPa
standard state
Define standard enthalpy change of formation (ΔH⦵f) (2)
The enthalpy change when one mole of a substance is formed from its stated elements, under standard condition.
State the meaning of the term standard enthalpy of combustion. (2)
(ΔH⦵c)
when 1 mole of a substance combusts fully in oxygen under standard conditions
What is standard enthalpy change of neutralisation (ΔH⦵neut)
when an acid and alkali neutralise under standard conditions, to form 1 mole of water
Formula for calculating the heat energy
Q = mcΔT
q = heat energy (J)
m = mass of water or solution (g)
c = specific heat capacity of water, 4.18
ΔT = temperature change (K)
Formula for calculating the enthalpy change
enthalpy change= heat energy/ number of moles burnt
ΔH = enthalpy change (kJ mol-1)
q = heat energy (kJ)
n = number of moles of the limiting reactant (mol)
Enthalpy changes are typically expressed in kJ.
Meaning average bond enthalpy
Mean energy required ot break 1 mol of bonds in gaseous molecules
What type of reaction MAKES bonds?
EXOthermic
What type of reaction BREAKS bond?
ENDOthermic
Define the term enthalpy change. (1)
Heat (energy) change at constant pressure
Some reactions of H2O2 are exothermic.
Use ideas about the enthalpy changes that take place during bond breaking and bond
making to explain why some reactions are exothermic.
- Bond breaking absorbs energy AND bond making releases energy
ALLOW bond breaking is endothermic AND bond making is exothermic - More energy released than absorbed
ALLOW exothermic change transfers more energy than
endothermic change OR bond making transfers more energy than bond breaking
