3.2.1 Enthalpy changes

enthaly change

is the heat energy transferred in a reaction at constant ressure ∆H

standard conditions for enthaly changes

100kPa

298K

ΔH°

standard states

hysical states of reactants under standard conditions

endthermic reaction

when energy is taken in from the surroundings so roducts have more energy than reactants heat taken in

endothermic enthaly change

ositive

exothermic reaction

when energy is released to surroundings roducts have less energy than reactants heat is given out

enthaly change of exothermic reaction

negative

overall enthaly change equation

energy needed to break bonds(reactants - energy released making bonds (roducts

activation energy

minimimum amiunt of energy for a chemical reaction to tak lave

activation energy on enthaly rofile diagrm

difference btween reactants and to of hum

exothermic enthaly rofile diagram

endothermic enthaly rofile diagram

standard enthaly change of reaction

• ∆r H

  • enthaly change when a reaction occurs in the molar quantities shown in the chemical equation under standard conditions with all reactants and roducts in their standard states

standard enthaly change of fromation

• ∆f H

  • enthaly change when 1 mole of a comound is formed from its elements in their standard states under standard conditions

standard enthaly change of combustion

• ∆c H

  • enthaly change when 1 mole of a substance is comletely burned in oxygen under standard conditions with all reactants and roducts in their standard syayes

standard enthaly change of neutralisation

• ∆neutH

• enthaly change when solutions of an acid and alkali react together to form 1 mole of water under standard conditions

calorimetry

exerimetnak method for finding enthaly change by measuring temerature chnage over time

how to measure enthlay changes in lab

• neutralisation,dislacement stick thermometer into solution in olysteren beaker

• combustion - copper calorimeter containing known mass of water burn and measure temerature change

equation for enthaly change

q=mc∆T

• q=energy change(j

• m=mass g

• c= secific heat caacity jg^1K^1

• ∆t temerature chnage K

secific heat caacity

energy required to raise 1g of subsatance by 1K without change of state

how to calculate nergy change er mole

qx10^-3/moles or kj/g x mr

errors with enthaly change exepriments

• calorimetry not comleteyly accurate

• heat loss can occurbut can insulate

• use secific heat cacity of water not solution

average bond enthaly

energy required to break one mole of the stated bond in a gaseoys state under standard conditions

hess law

total enthalychanges of reaction is alwys the ame no matter which route is taken

enthaloy change of formation hess law

arrows oint up

-Hf of reactanta + hf of roducts

enthaly change of cobustion hess law

oint down

H2O and CO2

reactants - products