Topic 8 Energetics Flashcards
define enthalpy change
heat change from a reaction under constant pressure
define standard enthalpy change
enthalpy change of a reaction under 100kPa and 298K
define standard enthalpy change of reaction
enthalpy change of materials reacting under standard conditions, with everything in standard state
define standard enthalpy change of formation (3 marks)
enthalpy change when 1 mole of compound (1) is formed from its elements (1) under standard conditions (1atm, 298K) , everything in standard state (1)
define standard enthalpy change of combustion
enthalpy change when 1 mole of compound is burned completely under oxygen in standard conditions and state
define standard enthalpy change of neutralisation (2 marks)
enthalpy change of 1 mole of water is formed when an acid and alkali are mixed together
What is the definition of bond enthalpy?
Energy required to break the one mole of the exact same bond in gaseous state
What is the definition of mean bond enthalpy? 2 marks
Enthalpy change to break one mole of the same type of bond in different molecules (1) in gaseous state (1)
What does Hess’s law state?
Enthalpy change of a reaction is independent to the route the reaction takes
Q =
mcΔT
what is the unit for enthalpy?
kJ mol ^-1
What is the unit of enthalpy change and how to calculate?
+/-
mc ΔT / moles
kJ mol -1
Find Mc ΔT for the solution that had change in temp
Find Mole for solid
ΔH = -Q/mol = Jmol-1
Use bond breaking and forming to explain why combustion of ethanol gives out energy
Energy taken in to break bonds (endo)
Energy released from forming bonds (exo)
More energy is used forming than breaking bonds
ΔHr (using mean bond enthalpies)=
Σ Bonds broken - Σ bonds formed
How is sodium hydrogencarbonate used in cooking?
Baking soda
Carbon dioxide released causes breads to rise
Why are bond enthalpy values always positive?
Bond breaking requires energy, and bond breaking is endothermic
What assumptions are made when doing Hess calculations?
- no heat is lost
- reactions went to completion
What makes propanol very good fuels? 2 marks
- ignites easily
- releases thermal energy when it burns
How to calculate bond enthalpies?
ΔH = Σ bond energies broken - Σ bond energies made
What effect does using a copper beaker instead of glass would affect the enthalpy change in an exothermic reaction?
Glass is a better heat insulator
Reaction will be more negative since it’s more exothermic as more heat is conducted to water
So more accurate change in temp
In Q = mc ΔT, what is m?
Total volume of solution used, ALL solutions add up the volume pls
Explain why the data book value for the standard enthalpy change of neutralisation of ethanoic acid with NaOH is -55.2kJmol-1 but the value for HCl is -57.1kJmol-1. (2 marks)
- HCl is a strong acid, (so fully dissociates itself)
- energy used to fully ionise ethanoic acid (while no energy needed to dissociate HCl)
- so reaction with ethanoic acid is less exothermic
