3.2.1 - Enthalpy Changes
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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?
Endothermic reaction Energy is taken in to break bonds
What energy change is bond making associated with?
Exothermic Energy is released to make bonds
What is an endothermic reaction?
A reaction with an overall positive enthalpy change Enthalpy of products > enthalpy of reactants
What is an exothermic reaction?
A reaction with an overall negative enthalpy change Enthalpy of products < enthalpy of reactants
Draw an enthalpy change diagram for an endothermic reaction, and one for an exothermic reaction
diagram
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
Give an example of an equation which represents standard enthalpy of formation
H2 (g) + 1/2 O2 (g) ———> H2O (l)
Define enthalpy change of combustion
The energy change that takes place when 1 mole of a substance is completely combusted
Give an example of an equation which represents standard enthalpy of combustion
C (s) + O2 (g) ———–> CO2 (g)
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 a reaction mean?
The energy change associated with a given reaction
How can you calculate enthalpy change from experimental data
Use the equation Q = mc Delta T Where m is the mass of the substances being heated (usually water) C is the specific heat capacity of that substance (water SHC = 4.18J-1K-1) Delta T is the change in temperature
Draw a simple calorimeter
calorimeter
What are the advantages of using a bomb calorimeter? (2)
Minimises heat loss Pure oxygen used –> ensures complete combustion
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
Delta H = Bond enthalpies of reaction - bond enthalpies of products