Topic 8 Energetics 1 Flashcards
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 negativeenthalpy change (-ΔH) → enthalpy of products < enthalpy of reactants
Describe the diagram for an endothermic reaction
Reactants lower than products
Describe the diagram for an exothermic reaction
Reactants higher than products
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
H 2 (g) + ½ O 2 (g) → H 2O (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) + O 2 (g) → CO 2 (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 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
Draw a simple calorimeter
[TOP]
thermometer in beaker with known amount of water
gauze
tripod
spirit burner
heatproof mat
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
