enthalpy

Created by

liberty ross

Cards (43)

enthalpy is heat energy
enthalpy change is the amount of heat energy that is released or taken in during a chemical reaction
calorimetry is a method used to determine the amount of heat energy given off by a chemical reaction, the idea is to make all the energy released heat up a measured amount of water
it takes 4.18 J of energy to raise the temperature of 1 gram of water by 1 degree
to convert degrees celsius to Kelvin add 273
standard conditions are an agreed set of conditions which allow fair comparisons between different sets of of experimental data.
standard conditions are taken to be when temperature is 298K / 25 degrees celsius and the pressure is 100 kPa
enthalpy change of reaction is the enthalpy change when reactants as shown in the chemical equation convert to products under standard conditions. shown by the symbol ΔHr
enthalpy change of formation is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions. it is shown by the symbol ΔHf
enthalpy change of combustion is the enthalpy change when 1 mole of an element or compound is burned completely in oxygen under standard conditions. it is shown by the symbol ΔHc
in practice it is impossible to burn a fuel under standard conditions so in enthalpy change of combustion it is burned the normal way and then adjustments are made to allow for the non-standard conditions
bond enthalpy/bond dissociation enthalpy is the amount of energy needs to break 1 mole of a particular covalent bond in a gaseous molecule under standard conditions
average bond enthalpy is the average amount of energy required to break 1 mole of covalent bonds in gaseous molecules
average in average bond enthalpy refers to the fact that a C-C bond in ethane isn’t quite the same as a C-C in glucose, the value quoted is the average amount of energy required to break a particular bond in a range of molecules
energy transferred = mass x specific heat capacity x change in temperature
$E=$ $mcΔT$
Hess’ Law is the total enthalpy change accompanying a chemical change is independent of the route by which a chemical reaction takes place provided the initial and final conditions are the same for each route
breaking bonds requires energy/ takes energy in
making bonds releases energy
a positive enthalpy change value means the reaction is endothermic and the energy is absorbed by the reactants
a negative enthalpy change means the reaction is exothermic as the reactants release energy to the surroundings
in exothermic reactions the products are described as being more kinetically stable than the reactants
the higher the activation energy to the slower the reaction as there a very few number of particles that meet of exceed this energy level
when drawing endothermic enthalpy change diagrams the reactants are at a lower energy level than the products
on enthalpy change diagrams enthalpy is on the y-axis and progress of reaction is on the x-axis
when drawing exothermic enthalpy change diagrams the reactions are a higher energy level than the products
change in enthalpy on diagrams is labelled from the reactants to the products with a single headed arrow
activation energy on a diagram is drawn as the distance from the reactants to the top of the curve
to calculate enthalpy change from the energy released in calorimetry divide energy released by the number of moles of the desired substance
$ΔH=$ $E/n$
when drawing a Hess cycle for the enthalpy change age of combustion the point of the triangle is always carbon dioxide + water
when drawing a Hess cycle for the enthalpy change of formation the point of the triangle is always the elements of the reactants and products in their elemental for. like H2
when drawing a Hess cycle for bond enthalpies the point of the triangle is always the atoms of the reactants that have been broken down for example 4H +4O
Hess’ Law is used to measure enthalpy changes of unknown or difficult to measure reactions indirectly
standard enthalpy change of neutralisation is the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce one mole of water
how to construct a Hess cycle for a reaction:
write out the reaction left to right
find the substances in common between the reactants and products and put these at the bottom of the cycle
work out whether the arrows go down or up to bottom/products/reactants
recognise and include balancing numbers (if balancing number is 2 tunes the bond enthalpy by 2)
construct expression for the enthalpy change of the unknown reaction
in a Hess cycle if you are travelling the wrong way up an arrow change the sign, for example if the bond enthalpy is negative it will become a positive
reasons why a calorimetry value may differ:
energy lost to the surroundings
some incomplete combustion
non-standard conditions
overall enthalpy change of formation = enthalpy change of products - enthalpy change of reactants
for bond enthalpies:
overall enthalpy = bond enthalpies of reactants - bond enthalpies of products
in endothermic reactions more energy is needed to break the reactant binds than is released when the product bonds are formed
in exothermic reactions more energy is released when product bonds are formed than the energy needed to break the reactant bonds