Energetics 1 Flashcards
(21 cards)
What does system mean in a
chemical reaction?
The atoms and bonds involved in the
chemical reaction
Explain the law of conservation
The amount of energy in an isolated system
remains the same. Energy cannot be destroyed
or created, It can only be transferred from one
form to another
What energy change is
breaking bonds associated
with?
Energy is taken in to break bonds →
endothermic reaction
What energy change is
making bonds associated
with?
Energy is released to make bonds →
exothermic reaction
What is an endothermic
reaction?
A reaction with an overall positive
enthalpy change (+ΔH) → enthalpy of
products > enthalpy of reactants
What is an exothermic
reaction
A reaction with an overall negative
enthalpy change (-ΔH) → enthalpy of
products < enthalpy of reactants
What does activation energy
mean?
The minimum energy required for a
reaction to take place
Which way does the arrow for
activation energy point on an
enthalpy profile diagram?
Always points upwards
What are the standard conditions
100 kPa
298 K
What does “in standard state” mean?
The state an element / compound exists
at in standard conditions (100 kPa, 298 K
Define enthalpy change of formation
The energy change that takes place
when 1 mole of a compound is formed
from its constituent elements in their
standard state under standard conditions
Define enthalpy change of
combustion
The energy change that takes place
when 1 mole of a substance is
completely combusted
Define enthalpy change of
neutralisation
The energy change that takes place
when 1 mole of water is formed from a
neutralisation reaction
What does enthalpy change of reaction mean?
The energy change associated with a
given reaction
How can you calculate
enthalpy change from
experimental data?
Use the equation Q = mcΔT, where m is the
mass of the substance being heated (usually
water), c is the specific heat capacity of that
substance (water’s SHC = 4.18gJ-1K-1) and ΔT
is the change in temperature
Why might experimental
methods for enthalpy
determination not be
accurate?
Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion
What does average bond
enthalpy mean?
The mean energy required to break 1
mole of bonds in gaseous molecules
Why will using bond enthalpies
not be as accurate as using
standard enthalpy of
combustion/formation?
Bond enthalpies are a mean for the same bond
across different molecules whereas standard
enthalpy of combustion and formation apply just
to that molecule, therefore they are more
accurate.
How to calculate enthalpy
change of reaction using
average bond enthalpies?
ΔH = Σ (bond enthalpies of reaction) -
Σ (bond enthalpies of products)
What is Hess’s Law
States that the enthalpy change for a reaction is
the same regardless of the route taken
