Thermochemistry Flashcards
thermochemistry
the study of the energy involved in a reaction
endothermic reaction
energy of products is more than the energy of the reactants as energy is taken in from the surroundings
exothermic reaction
energy of products is less than the energy of the reactants as it looses energy to the surrounding areas
delta -
heat of reaction
heat change when the numbers of moles of reactants from the balanced equation react completely
heat of combustion
heat change when ONE MOLE of a substance is completely burned in excess oxygen
if writing a balanced equation for combustion what is needed
reactant + O2 —–> CO2 + H2O
what is a bomb calorimeter used for
measure the heat of combustion
determine the kilocalories in food
how does a bomb calorimeter work
> record mass of substance being used
-> put in crucible and place inside bomb
->bomb is filled with oxygen under pressure
->this ensures complete combustion
->bomb is placed in a known quantity of water
-> temperature of water is recorded
->sample is ignited using an electrical wire
-> record rise in temperature of water
->heat produced = mass x specific heat capacity x change in temperature
->heat produced of ONE MOLE is calculated
kilogram calorific value of a fuel
heat energy produced when 1 kg of the fuel is completely burned in excess oxygen
bond energy
average energy required to break one mole of a particular covalent bond and to separate the neutral atoms completely from each other
conservation of energy
energy is neither created nor destroyed but is changed from one form to another
what are the steps for calculating bond energy
> draw out all of the separate bonds in each of the reactants and products
calculate the energy for the reactants and products seperately
minus the total of the products from the total of the reactants
if the answer is a minus number the reaction is exothermic if its positive its endothermic
heat of neutralisation
heat change when one mole of H+ ions from an acid react with one Mol of OH- ions from a base
are neutralisation reactions endo or exothermic
exothermic
when is energy released small in a heat of neutralisation reaction
if a strong acid and weak base are used
why is energy released small in a heat of neutralisation reaction if a strong acid and weak base are used?
weak acids do not fully dissociate in water
thus less OH- or H+ ions are produced so there are fewer ions available to combine thus fewer bonds are formed and less energy is given out
what is the formula for heat released
H= mass(kg) x specific heat capacity x temp change
how to calculate the heat of neutralisation from an experiment
- calculate the no of moles of HCl neutralised
- fill values into formula (mass is the mass of the acid and the base in kg)
3.calculate the energy released if one mole of HCl was neutralised
DONT FORGET MINUS SIGN!!!!!
heat of formation
heat change that takes place when one mole of a compound in its standard state is formed from its elements in their standard state
standard state
the state that the substance is in at 25 degrees C and 101 kPa
Hess’ law
if a chemical reaction occurs in a number of steps, the sum of the heat changes for each step is the same as the heat change if the reaction is carried out in one step
formula for heat of formation
delta H = sum of products - sum of reactants
