Topic 5
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What is heat?
Form of energy that can be transferred from a higher to lower temperature object
What is temperature?
Average Ek of particles in a substance
What is enthalpy?
The heat absorbed or released in a chemical reaction at constant pressure
Can enthalpy be measured?
No only enthalpy change
What is standard enthalpy change?
The enthalpy change of a reaction carried out under standard conditions with everything in its standard state ( 100 kPA, 298 k)
Features of a exothermic reaction
• Energy is released to the surrondings • Temp of the surrondings increases • Combustion and nuetralisation
Draw an enthalpy diagram for exothermic reaction
Features of an endothermic diagram
• temperature of surrondings decrease • energy is taken in • thermal decomposition and photosynthesis
What is the equation for energy
q=mc delta t
What is the equation for enthalpy change?
Delta H = q/n
Assumptions of calorimetry
• Assume there is no heatloss to the surrondings and the calorimeter • Assume that the SHC is the same as water • Assume that the combustion is complete
What is standard enthalpy change of formation
When one mole of a compound is formed from its constituent elements in their standard states with everything under standard conditions
What is the formula for enthalpy change?
Enthalpy change = sum of standard enthalpy change of formation of products - sum of standard enthalpy change of reactants
What is standard enthalpy change of combustion?
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions
Draw a basic enthalpy cycle diagram
What are the rules for Hess’s law
• DO everything in relation to the final equation - Compare the 1st and 3rd when cancelling
What is the definition for average bond enthalpy?
The energy required to break one mole of a bond in a gaseous molecule averaged over similar compounds
Why is bond enthalpy less accurate than using standard formation to find the enthalpy change?
Because it is an average
How do you use bond enthalpy to work out delta H?
Reactants - products OR Bonds broken - bonds formed
Acronym for making and breaking bonds
MEXOBENDO
Write the equations for the destruction of ozone?
O2 – O. + O. O.+ O2 – O3 O3– O2+ O. O3+ O. –2O2
Compare the structures of oxygen and ozone
• O3 is a resonance structure and has a bond order of 1.5 - The double bond in O2 is stronger than the one in O3 which means it requires more energy to break