topic 8 Flashcards
what is enthalpy and what is it measured in
heat change in a raction
units = KJmol^-1
what are the standard conditions of enthalpy change
100 KPa
298 K
draw and endothermic reaction profile
reactants lower than products
what is endothermic and example
absorb energy from surroundings
eg. thermal decomposition
what is the enthalpy change for an endothermic reaction
positve
draw an exothermic reaction profile
reactants higher than products
what is an exothermic reaction and example
release energy to surroundings
eg. combustion
what is the enthalpy change for an exothermic reaction
negative
what is the standard ethalpy chnage of reaction
the enthalpy change of a reaction according to the molar quantities in the equation under standard conditions
what is the standard enthalpy change of neutralisation
the enthalpy change when an acid and alkali react to form 1 mole of water under standard conditions
what is the standard enthalpy change of formation
the ethalpy change when 1 mole of compound is formed from its elements in their standard conditions
what is the standard enthalpy change of combustion
the enthalpy change when 1 mole of a substance is completely burned in oxygen to make CO2 and H2O under standard conditions
draw a reaction profile for bonds broken
reactants lower than products
see flash card
what happens when bonds are broken in terms of energy and enthalpy
energy absorbed
endothermic
positive enthalpy
if more energy is needed to break bonds that energy given out when bonds formed = endothermic
draw and bonds made energy reaction profile
reactants higher than products
arrow down
see flashcard
what happens when bonds are made in terms of energy and enthalpy
energy released
exothermic
negative enthalpy
if more energy is required when bonds are formed than what is needed to break = exothermic
how to work out enthalpy chnage in terms of bonds broken and made
include units
= total energy to break bonds - total energy released forming bonds
units = KJ mol^-1
what is calorimetry used for
work out enthalpy change of combustion
describe the process of calorimetry
- a fuel is burnt to raise the temp of water to be a specific amount, weigh fuel before and after to work out mass burnt
- energy transfered to water although some is lost to the surroundings
- a lid is placed on top to prevent heat loss and wind sheilds around flame to prevent draught moving flame
how to work out enthalpy from calorimetry with units of all parts of equation
q = mcAT
q- energy lost/ gained (J)
m- mass od water (g)
c- specific hear capacity of water ( 4.18Jg^-1K^-1)
T - temp change (K)
enthalpy = q/ moles
moles - fuel mass/Mr
what is hess’ law
the total enthalpy change of a reaction is independent to the route taken
what would a hess cyle of formation look like
see flash card
arrows up with formation data
how do you work out enthalpy change using hess cycles
go with the arrow - keep sign same
go against arrow - change sign
draw a hess cycle of combustion
see flash card
arrows down
