Module 3: Section 2 - Physical Chemistry
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Enthalpy changes Definition of enthalpy change AH (delta H)
Energy transferred between a system and its surroundings at a constant pressure and final temperature is the same as the start temperature
Enthalpy changes What are the units for enthalpy change
kJ mol ^-1
Enthalpy changes What is a system
Chemical or chemicals being studied and if system is closed there is no exchange between the system and the surroundings
Enthalpy changes What does standard enthalpy changes refer to
Reactions done under standard conditions and with everything in their standard stats
Enthalpy changes What are standard conditions
298 K (25*C) Pressure of 1atm (100kPa) Where solutions involved have concentration of 1 mol dm-3
Enthalpy changes What is exothermic change
Give out heat to surroundings
Enthalpy changes Examples of exothermic changes
Combustion Respiration Hot packs
Enthalpy changes Is delta H positive or negative for exothermic changes
Negative as the products are at a lower energy level than reactants
Enthalpy changes On an enthalpy profile diagram for exothermic changes where are the products
Products are below the reactants
Enthalpy changes What are endothermic changes
Take in energy from the surroundings
Enthalpy changes Examples of endothermic changes
Melting and vaporisation Photosynthesis
Enthalpy changes Is delta H popsoitve or negative for endothermic changes
Positive as products have more energy than the reactants
Enthalpy changes In and energy profile diagram are the products above or below
Products above reactants
Enthalpy changes What must happen for a reaction to occur
Bonds must be broken and new ones formed so energy must be supplied to start both endothermic and exothermic reactions
Enthalpy changes What is activation energy Ea
Minimum amount of energy needed to begin breaking reactants bonds and start a chemical reaction
Enthalpy changes Units for activation energy Ea
kJ mol-1
Enthalpy changes How is the symbol of delta H modified fr standard enthalpy change
A H °298(small) Delta symbol triangle then capital H then degree sign with lien through horizontally then small 298 at bottom
Enthalpy changes What is the enthalpy change of a reaction
Enthalpy change associated with a stated reaction
Enthalpy changes Sign for enthalpy change of reaction
AH r Delta symbol then capital H them small r
Enthalpy changes What is enthalpy change of combustion
Enthalpy change when one mole of a substance undergoes complete combustion
Enthalpy changes Sign for enthalpy change of combustion
A H c Delta sign then capital H the small c
Enthalpy changes What are enthalpy change of combustion always
Always exothermic so negative delta H
Enthalpy changes Example of enthalpy change of combustion for ethanol
C2H5OH (l) 3 O2 (g) —> 2 CO2 (g) + 3 H2O (l)
Enthalpy changes Important for equation of enthalpy change of combustion
Include state symbols, always balanced, COMPLETE combustion