OCR A 3.2.1 Enthalpy changes

enthalpy

-measure of heat energy in a chemical system (atoms, molecules or ions making up chemicals)

-energy stored within bonds

enthalpy change

-heat change in a reaction at constant pressure

-units: kJmol^-1

-(enthalpy of products - enthalpy of reactants)

average bond enthalpy

when 1 mole of bonds of gaseous covalent bonds is broken

standard condition

-100kPa

-298K/25^oC

exothermic reactions

-reaction that release energy to surrounding

-reactant higher in energy than product

-negative enthalpy change

endo reaction profile

endothermic reactions

-reaction that absorb energy from surrounding

-products higher in energy than reactant

-positive enthalpy change

endo reaction profile

standard enthalpy change of reaction

-enthalpy change of reaction according to molar quantities in the equation under standard conditions

standard enthalpy change of neautralisation

-the enthalpy change when an acid and alkali react to form 1 mole of water, under standard conditions

standard enthalpy change of formation

-the enthalpy change when 1 mole of a compound is formed from its element in their standard states, under standard conditions

standard enthalpy change of combustion

-the enthalpy change when 1 mole of substance is completely burned in Oxygen to make CO₂ and H₂O under standard conditions

what happens when bonds are broken

-to break bond energy needs to be absorbed

-bonds broken in reactants

-enthalpy change is positive; endo

endo and bonds

-when more energy is needed to break bonds than energy given out when bonds are formed

what happens when bonds are made between

-energy is released

-bonds made when products is being produced

-enthalpy change is negative

exo and bonds

-when more energy is released when bonds are formed than what was needed to break initial bonds

mean bond enthalpy (backstory)

-bonds of same type don’t have all have same amount of energy

-eg: CH₄ dif energy needed to break 1st bond than 2nd bond

-so measure the enthalpy of break all bonds and find average

calorimetry combustion

-used to work out enthalpy change of combustion

-fuel is burned to raise temp of water by specific amount

-weigh fuel mass before and after

-energy from fuel often lost to surroundings instead of heating water

-lid placed on top to prevent heat loss

-wind shields to prevent draught moving flame

-calc using q=mcAt

calorimetry solution

-add acid and measure temp

-add solid/alkali measure temp change

use q=mcAt

hess’s law

total enthalpy change of a reaction is dependant of the route taken

hess cycle formation

hess cycle combustion