5.2 ENERGETICS Flashcards
Define an enthalpy change of a reaction
The heat energy it releases or absorbs per mole
Is heat energy absorbed or released in an ENDOthermic reaction (and so what enthalphy sign is it)
Absorbed (+)
Is heat energy absorbed or released in an EXOthermic reaction (and so what enthalphy sign is it)
Released (-)
Units of enthalpy
kJ/mol
How do you draw an energy level diagram for an ENDOthermic reaction
- Enthalpy scale on y-axis, and time/progression on x-axis
- Reactants drawn on line on bottom of diagram
- Vertical line up with an arrow (labelled with enthalpy)
- Products of horizontal line above line
How do you draw an energy level diagram for an EXOthermic reaction
- Enthalpy scale on y-axis, and time/progression on x-axis
- Reactants drawn on line at top of diagram
- Vertical line down with an arrow (labelled with enthalpy)
- Products of horizontal line below
When bonds are broken, what type of reaction is it
Endothermic - energy is absorbed
When bonds are made, what type of reaction is it
Exothermic - energy is released
Unit for bond energy
kJ/mol
Equation for calculating with bond energies
Enthalpy change = Bonds broken - bonds made
How to calculate a bond energy question
- Add up each individual bond energy for each bond
- Add them together for products and reactants
- Subtract products from reactants
In an exothermic reaction, what is the ‘ratio’ of energy absorbed / released
Released > Absorbed
In an endothermic reaction, what is the ‘ratio’ of energy absorbed/released
Absorbed > Released
In an endothermic reaction, what is the ‘ratio’ of energy absorbed/released
Absorbed > Released
What is the activation energy
The energy needed to break the bonds
How do you make an energy level diagram, a reaction profile diagram
- There is a hump from the products to the reactants
- In the centre of the hump, and arrow up
- Labelled activation energy and the actual enthalpy (if avaliable)
Q =
Q=mc 0t
Enthalpy Change equation
-Q / moles
Describe how to carry out a combustion calorimetry experiment
- Weigh 50g of water and plae in a can above a burner
- Measure inital mass of burner and intital temperature of water
- Ignite the burner, stir water for 30s then extinguish burner
- Measure final mass of burner, and final temperature of water
What are the main sources of error in a combustion calorimetry experiment
- Heat loss - most of the thermal energy heats the air
- Incomplete combustion - releases less heat energy
How do endothermic reactions get given a ‘+’ value for enthalpy change when being calculated
Q=mc0t will be negative and so -Q/mol (negative multipied by negative) will be positive
Describe how to carry out a solution calorimetry experiment
- Pipette 25cm^3 of solution into polystyrene cup
- Measure intital temperature of solution
- Add 1g of metal and stir continuosly
- Mesure the maximum temperature of solution
What are the sources of error in a solution calorimetry experiment
- Heat loss - thermal energy used to heat up air
- Incomplete reaction - not stired/left for long enough
- Solution is no longer pure water - won’t have same SHC
Describe how to carry out a solution calorimetry experiment
- Pipette 25cm^3 of solution into polystyrene cup
- Measure intital temperature of solution
- Add 1g of metal and stir continuosly
- Mesure the maximum temperature of solution
How to calculate enthalpy change from a combustion calorimetry experiment
- Q=mc0t (of water)
- CONVERT TO KJ
- Moles of fuel
- Enthalpy = -Q/moles
In an exothermic reaction, what is the ‘ratio’ of energy absorbed / released
Released > Absorbed
When bonds are broken, what type of reaction is it
Endothermic
