12. Thermochemistry

exothermic reactions? i studied them before they were cool!

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