Enthalpy Flashcards
Explain system and surroundings
The system consists of the molecules which are reacting, the surroundings are everything else e.g. air, solution, glassware
What is the symbol for enthalpy change?
🔺️H
What happens during an exothermic reaction?
Products have lower enthalpy than reactants and energy is transferred from the system to the surroundings (surroundings get hot)
What is 🔺️H measured in?
kJ mol-1
What happens during an endothermic reaction?
Products have higher enthalpy than reactants and energy is transferred from surroundings to system (surroundings get colder)
Definition of activation energy
Activation energy (Ea) is the minimum energy required to start a reaction
Describe the arrows in an exothermic and endothermic profile
- Ea arrow goes to the top of the curve from the reactants, single-headed
- 🔺️H arrow goes from reactants to products (down exothermic, up endothermic), single-headed
What are the standard conditions?
Pressure of 100kPa
Temp of 298K (25°C)
Concentration of 1 mol dm-3
What must enthalpy change equations have?
State symbols
Definition of enthalpy change of combustion
The enthalpy change when ONE MOLE of a substance reacts COMPLETELY WITH OXYGEN under standard conditions
Definition of enthalpy change of formation
The enthalpy change when ONE MOLE of a compound is formed from its CONSTITUENT ELEMENTS in their STANDARD STATES
Definition of enthalpy change of neutralisation
The energy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form ONE MOLE OF WATER under standard conditions
At what enthalpy change is 🔺️neutH constant?
-57 kJ mol-1
What is the 🔺️fH of elements?
0 kJ mol-1
What are the 2 ways of determining enthalpy changes?
Polystyrene calorimeter and spirit burner
What is the equation for calculating energy exchanged?
q = mc🔺️T
What is the equation for calculating 🔺️H from q?
🔺️H = q/n
What are the 2 errors which can occur in calorimetry?
- Heat loss to surroundings
2. Not done under standard conditions
What impact does heat loss to surroundings have on q and 🔺️H?
q: 🔺️T is smaller (heat lost to air instead of water) than it should be so q is smaller
🔺️H: magnitude of 🔺️H decreases so 🔺️H is less exothermic than the actual value
How is heat lost for 🔺️cH specifically?
Liquid fuel might evaporate, heat capacity of beaker not taken into account, incomplete combustion
What does Bendo Mexo mean?
Bendo = breaking bonds is endothermic Mexo = making bonds is exothermic
What defines an exothermic reaction?
The energy required to break bonds is less than the energy released when making new bonds
What defines an endothermic reaction?
The energy required to break bonds is more than the energy released when making new bonds
Why are bond enthalpies always positive?
Bond-breaking is endothermic
What is the equation for calculating 🔺️H from bond enthalpies?
🔺️H = sum of bond enthalpies of bonds broken - sum of bond enthalpies of bonds made
What are the 2 limitations of bond enthalpies?
- Only used in GASEOUS state so if there are substances which aren’t gaseous under standard conditions the calculated 🔺️H will be different to the standard value
- It’s an average of the bond enthalpy in different compounds so different compounds will have slightly different enthalpies
Why can’t some enthalpy changes be determined DIRECTLY?
- High activation energy
- Slow reaction rate
- More than one reaction taking place
Describe combustion cycles
- Arrows point down (represent combustion reactions)
- Combustion products at bottom of arrows
- With the indirect route arrow = add, against the indirect route arrow = subtract
Describe formation cycles
- If formation data is given, use a formation cycle
- Elements at the bottom
- Arrows point up as we form those substances from elements
