Chapter 9 Enthalpy Flashcards
what is enthalpy
measure of heat energy in a chemical
what is the symbol for enthalpy
H
what is enthalpy change
the difference in enthalpies between the products and reactants
what is the enthalpy change formula
ΔH = H(products) - H(reactants)
what is conservation of energy
fundemental law of science that states that energy cannot be created or destroyed
what is EXOTHERMIC
transfer of energy from the system to the surroundings
what is ENDOTHERMIC
transfer of energy from the surroundings to the system
what does an enthalpy profile diagram show
the relative enthalpies of the reactants and products and the enthalpy change
how does the enthalpy diagram show if a reaction is endothermic or exothermic
exothermic reactions have a negative enthalpy change
endothermic reactions have a positive enthalpy change
what is activation energy
the minimum amount of energy that must be supplied to the system before the reaction can take place
define standard conditions
conditions under which most experiments are and most closely resemble typical working conditions of temperature and pressure
how are standard physical conditions represented
⦵ in the superscript
what is the standard pressure value
100kPa
what is the standard temperature value
298K
25C
what is the standard concentration value
1moldm^-3
what is the standard state
the normal physical state of a substance under standard temperature and pressure
what is the standard enthalpy change of reaction
the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions
what is the symbol for standard enthalpy change of reaction
ΔrH⦵
what is the standard enthalpy change of formation
the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions
what is the symbol for standard enthalpy change of formation
ΔfH⦵
what is standard enthalpy change of combustion
the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions
what is the symbol for standard enthalpy change of combustion
ΔcH⦵
what is standard enthalpy change of neutralisation
energy change that accompanies the reaction of an acid by a base to form one mole of H2O(l) under standard conditions
what is the symbol for standard enthalpy change of neutralisation
ΔneutH⦵
what three quantities are required to work out energy change
mass
specific heat capacity
temperature change
how is mass calculated
it is the mass of the water in the aqueous solutions so you take the volume and times it by the density; 1g/cm
what is the specific heat capacity value
4.18
what is the heat energy equation
q = m x c x ΔT energy = mass x specific heat capacity x temperature change
what are the three steps to calculate ΔcH
> calculate the energy change
calculate the amount in moles of substance burned
do q/n to find ΔcH
what are the four reasons why ΔcH may be different from the theoretical or expected
> heat loss to surroundings
incomplete combustion
evaporation of substance
non-standard conditions
what are the three steps to calculate ΔrH
> calculate the energy change , q
calculate the amount of moles of the reactant that is not in excess
do q/n to find ΔrH
what are the three steps to calculate ΔneutH
> calculate the energy change , q
calculate the amount in moles of the base + acid that was reacted
do q/n to find ΔneutH
what is average bond enthalpy
the energy required to break one mole of a specified type of bond in a gaseous molecule
why does average bond enthalpy have limitations
the actual bond enthalpy can vary depending upon the chemical environment of the bond
which energy is endothermic and which is exothermic
energy required to break bonds is endothermic
energy released when bonds break is exothermic
what is the enthalpy change of reaction equation
ΔrH = Σ(bond enthalpies in products) - Σ(bond enthalpies in reactants)
what is Hess’ Law
the total enthalpy change for an indirect route is the same as that for the direct route
what is the value for all ΔneutH⦵ reactions
-57kJmol^-1
