Enthalpy Flashcards
Reaction when WATER is added to SOLID CALCIUM CHLORIDE
CaCl2(s) + aq —— CaCl2(aq)
Observations ~ test tube feels HOT
EXOTHERMIC:
- heat is given out from the system to the surroundings during the reaction.
Reaction when WATER is added to SOLID AMMONIUM NITRATE
NH4NO3(s) + aq —- NH4NO3(aq)
Observations ~ test tube feels COLD
ENDOTHERMIC:
- heat is taken from the surroundings into the system during the reaction
what is meant by :
- CHEMICAL SYSTEM
- SURROUNDINGS
- UNIVERSE
CHEMICAL SYSTEM ~ includes the reactants reacting with each other to form products.
SURROUNDINGS ~ everything outside the chemical system.
UNIVERSE ~ system + surroundings
Law of conservation of energy
- During chemical reactions , no heat is LOST
- It is only TRANSFERRED from one place to another.
Enthalpy , H
Describes the HEAT ENERGY stored in a chemical system
i.e in the the BONDS of a substance
Enthalpy diagram for an EXOTHERMIC reaction
- Heat is GIVEN OUT from the system to the surroundings
- The enthalpy of products is LOWER than that of the reactants
Change in enthalpy:
H products - H reactants
- Has a NEGATIVE value
- An exothermic enthalpy change is indicated by a DOWNWARD arrow drawn from the reactants to the products enthalpy line
Enthalpy diagram for an ENDOTHERMIC reaction
- Heat is TAKEN from the surroundings into the system
- The enthalpy of products is HIGHER than that of the reactants
Change in enthalpy :
H products - H reactants
- Has a POSITIVE value
- An endothermic enthalpy change is indicated by an UPWARD arrow drawn from the reactants to the products enthalpy line.
Activation energy
- Reactions are NOT SPONTANEOUS
- They need a MINIMUM amount of energy
- This energy is required to start BREAK THE BONDS in the reactants
- Once the reaction has started , the heat released by the reaction continues to supply the activation energy ~ reaction becomes SELF-SUSTAINING
- The activation energy prevents substances from SPONTANEOUSLY COMBUSTING
Enthalpy change
- The heat energy at a constant pressure
- By performing the reactions under the same conditions, called STANDARD CONDITIONS , the enthalpy changes for different reactions can be tabulated and compared.
Standard conditions of :
- pressure
- concentration of a solution
- temperature
pressure ~ 100KPa
Concentration of a solution ~ 1mol dm-3
Temperature ~ 298 K (25 C)
T(K) = T(C) + 273
Standard states
- For a standard enthalpy change , all reactants and products must be in their standard states
- In other words, their physical state under standard conditions.
Examples:
H2(g)
C(s)
H2O(l)
Standard enthalpy change of REACTION
The enthalpy change that accompanies a reaction in MOLAR QUANTITIES shown in a chemical equation under STANDARD CONDITIONS ,
with all reactants and products int heir STANDARD STATES
Standard enthalpy change of FORMATION
- This is the enthalpy change when ONE MOLE of a substance is formed from its CONSTITUENT ELEMENTS ,
- under STANDARD CONDITIONS
- with all reactants and products being in their STANDARD STATES
The standard enthalpy change of FORMATION of an ELEMENT
- The formation of ONE MOLE of an elements from its element.
- There is NO ENTHALPY CHANGE
- So all elements by definition have an enthalpy change of formation of 0kJ mol-1
Standard enthalpy change of COMBUSTION
- This is the enthalpy change when ONE MOLE of substance reacts COMPLETELY WITH OXYGEN
- under STANDARD CONDITIONS
- with all reactants and products being in their STANDARD STATES
- The PRODUCTS are the oxides of all the elements in the substance
