save my exams energetics Flashcards
why enthalpy changes take place
because bonds are being broken and formed
Energy is needed to overcome attractive forces between atoms
bond breaking exo and endo
endothermic
bon forming is exo or endo
Energy is released from the reaction to the surroundings (in the form of heat) when new bonds are formed
therefore exothermic
what determines whether the reaction is overall exothermic or endothermic
The sum of the energy in and energy out
endothermic
If more energy is required to break bonds than energy is released when new bonds are formed
exothermic
more energy is released when new bonds are formed than energy is required to break bonds
energy level diagram i
hows the energies of the reactants, the transition state(s) and the products of the reaction as the reaction proceeds
transition state
stage during the reaction at which chemical bonds are partially broken and formed
why transtion state is unstable
a molecule in the transition state cannot be isolated and is higher in energy than the reactants and products
activation energy
nergy needed to reach the transition state
he minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
exo diagram
eactants are higher in energy than the products
eactants are therefore closer in energy to the transition state
exothermic reactions have a lower activation energy compared to endothermic reactions
endo diagram
reactants are lower in energy than the products
therefore further away in energy to the transition state
This means that endothermc reactions have a higher activation energy compared to exothermic reactions
enthalpy
total chemical energy inside a substance
enthalpy change symbol
ΔH (
pos or neg
temp of ev in exo
Heat energy is given off by the reaction to the surroundings
The temperature of the environment increases - this can be measured with a thermometer
The energy of the system decreases
enthalpy decrease
negative
why exo is thermodynamically possible
because the enthalpy of the reactants is higher than that of the products
endo environment
The temperature of the environment decreases - this can be measured with a thermometer
The energy of the system increases
enthalpy increase
positive
standard enthalpy conditions
pressure of 100 kPa
A temperature of 298 K (25 oC)
standard physical state (solid, liquid or gas)
the standard enthalpy change symbol
ΔHꝊ
reaction
enthlpy change when reactants in stoichiometric equtaion recat to give products under standard conditions
formation
the enthalpy change when one mole of a compound is formed fom its elements under sdtandard conditions
combustions
enthalpy change when one mole of a substance is burnnt inexcess oxygen under standard condtions
neutralisation
enthalpy change when one mole of water is formed by reacing acid and alkali undee standard conditions
reac tion exo or endo
both
formation exo and endo
both
combustion exo or endo
exo
neutralisation exo or endo
exo
calorimetry
measurement enthalpy changes in chemical reactions
how to make calorimeter
polystyrene drinking cup, a vacuum flask or metal can
specific heat capacity
The energy needed to increase the temperature of 1 g of a substance by 1 oC
spec heat cap of water
4.18 J g-1 oC-1
energy transferred
q=mcxdelta t
bond dissociation energy
The amount of energy required to break one mole of a specific covalent bond in the gas phase
bond energy or bond enthalpy
Average bond energy
Bond energies are affected by other atoms in the molecule (the environment)
Therefore, an average of a number of the same type of bond but in different environments is calculated
mean bond enthalpies
average values over a range of compounds
