Module 3 - [ch9] (enthalpy) Module 5 - [ch22a] (lattice energy)
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what is a system in a chemical reaction
atoms and bonds involved in a chemical reaction
explain law of conservation
energy in an isolated system remains the same energy cannot be created nor destroyed. only transferred from one form to another
endothermic reaction
energy taken in to break bonds ΔH is positive
What is meant by the term exothermic? [1]
energy released to make bonds ΔH is negative
activation energy
minimum energy required for a reaction to take place
standard conditions
1atm - 101kPa 298K - 25 degrees
what is standard state
the state an element/compound exists in at standard conditions
Define the term enthalpy change of formation. [2]
energy change when1 mole of substance is formed from its constituent elements in their standard state under standard conditions
define the term enthalpy change of combustion [2]
change in energy when 1 mole of substance is completely combusted
enthalpy change of neutralisation
energy change when 1 mole of water is produced from a neutralisation reaction
enthalpy change of reaction
energy change associated with a given reaction
enthalpy change {}
Q = mcΔT energy change = mass * specific heat capacity * change in temperature shc = 4.18g^-1K^-1
what are the advantages to using a bomb calorimeter
minimises heat loss to surroundings pure oxygen used to ensure complete combustion
Suggest three reasons why standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretical values [3]
• heat is released to surroundings • non-standard conditions • incomplete combustion (reaction may not go to completion)
average bond enthalpy [2]
average energy required to break 1 mole of bonds in gaseous molecules
Use ideas about the enthalpy changes that take place during bond breaking and bond making to explain why some reactions are exothermic. [2]
bond breaking absorbs energy whereas bond making releases energy in some reactions more energy is released than absorbed, these reactions are exothermic
State le Chatelier’s principle [1]
The position of equilibrium will shift so as to minimise the effect of any change in conditions
It is very difficult to determine the standard enthalpy change of formation of hexane directly. Suggest a reason why [1]
many different hydrocarbons would form OR activation energy very high
Why do bond enthalpies have positive values? [1]
bond breaking is endothermic so energy has to be put in to break a bond
Write the equation, including state symbols, that represents the standard enthalpy change of formation for carbon monoxide, CO [2]
C(s) + 1/2O2(g) -> CO(g)
what is Hess’ law
enthalpy change is independent of route taken
what is lattice enthalpy
measure of the strength of the ionic bonding in a giant ionic lattice the enthalpy change when 1mole of an ionic compound is formed from its gaseous ions under standard conditions
born haber cycle
gaseous ions at top /\ | | | gaseous atoms | /\ | lattice energy | | elements in | standard states | | | | | \/ \/ ionic lattice
define standard enthalpy of atomisation
the enthalpy change when 1 mole of gaseous atoms are formed from the elements in its standard state under standard conditions