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exothermic reactions? i studied them before they were cool!
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thermochemistry
the study of heat changes that accompany chemical reactions
exothermic reaction
a chemical reaction that gives out heat, temperature goes up
endothermic reaction
a chemical reaction that takes in heat, temperature goes down
exothermic Δh value
negative
endothermic Δh value
positive
heat of reaction (Δh)
the heat change when a reaction takes place according to a balanced equation
heat of reaction standard unit
kJ mol^-1
bond energy
the average energy required to break one mole of a particular covalent bond and to separate the neutral atoms completely from each other
heat of combustion of a substance
the heat change when ONE mole of a substance is completely burned in excess oxygen
to write combustion equation
write formula of substance, add O2, -> show CO2 and H2O as products, balance equation
calorimeter
any container used for the determination of heat changes
how does a calorimeter work
known mass of substance whose heat of combustion is being measured is placed inside the crucible in the bomb. filled with oxygen under pressure. bomb placed in a known quantity of water in calorimeter. substance ignited using an ignition coil of wire. rise in temperature is measured and used to calculate heat produced.
formula to calculate heat produced
m x c x Δt
kilogram calorific value of a fuel (kJ/kg)
the heat energy produced when 1kg of the fuel is completely burned in oxygen
heat of formation
the heat change that takes place when ONE mole of a compound in its standard state is formed from its elements in their standard state
to write formation equation
write formula of substance on rhs, write reactants in standard state on lhs, balance
hess's law
if a chemical reaction takes place in a number of stages, the sum of the heat changes in the separate stages is equal to the heat change if the reaction is carried out in one stage
heat of neutralisation
the heat change when one mole of H+ ions from an acid reacts with one mole of OH- ions from a base
what kind of heat of reaction would heat of neutralisation reactions be expected to have?
negative
to calculate heat of neutralisation
heat given out = m x c x Δt
density of solution
same as density of water
what value do you use if the acid and base are at two different temperatures at the beginning of experiment?
use the average value
if the acid is dibasic (sulfuric acid)…
divide by 2
if given heat capacity (capital C)…
use c x Δt