Energetics 3.1.4 Flashcards
(21 cards)
Define enthalpy change
Heat energy chnage measured under conditions of constant pressure
Define standard enthalpy change
Enthalpy chnage measured under standard conditions
100KPa and 298K
Define bond enthalpy
Enthalpy required to break a covalent bond into gaseous atoms
Is breaking bonds endothermic or exothermic
Is forming bonds endothermic or exothermic
Breaking bonds is endothermic
Forming bonds is exothermic
Define mean bond enthalpy
Enthalpy required to break a covalent bond into gaseous atoms averaged over a range of different Compounds
Why is enthalpy change from bond enthalpies and a data book different
-Values averaged from a range of different Compounds
-bond enthalpies ignore intermolecular forces
Enthalpy Samsung notes
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Why is a polystyrene cup used when measuring enthalpy chnages
Good insulator
Low heat capacity
What are some sources of error in the practical for measuring enthalpy chnages
-Heat loss to surroundings
-Reaction or dissolving may be incomplete or slow
-Density of solution taken too be the same as water
-Assuming that all solutions have the same specific heat capacity as water
-Energy absorbed by polystyrene cup is ignored
What is the unit for enthalpy
kJmol-1
What are the calculations involved in the experiment for measuring enthalpy chnage and what are the units
q=mc◇T
q-heat gained or lost in J
m - mass in grams
c - Specigic heat capacity of water (4.18)
◇T - Change in temperature
◇H° - q/n
n - number of moles
In a simply calorimeter, what are some sources of error
Flames affected by draughts
Heat lost by convection from the top of calorimeter
Heat lost by conduction through colorimeter
Incomplete combustion of fuel
Evaporation of fuel
What are 3 types of calorimeter
Simple calorimeter
Flame calorimeter
Bomb calorimeter
Define standard enthalpy change of formation
The enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products are in their standard states
Define standard enthalpy chnage of combustion
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions. All reactants and products in their standard states
Define Hess’s law
The total enthalpy chnage for a reaction is the same whatever route is taken from reactants to products
What is the standard enthalpy change of formation for an element
0
What is the acronym for standard enthalpy change of formation
FORPAR
Formation is products takeAway reactants
What is the acronym for standard enthalpy change of combustion
COMRAP
Combustion is reactants takeAway products
HESS Samsung notes
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When using FORPAR and COMRAP, when they give you both formation and combustion in the question, which one do you use
Whichever one the data is
