Energetics - TN Flashcards
what is the enthalpy change of a reaction
the heat energy it releases or absorbs per mole
what is the symbol for enthalpy change
units of enthalpy change
kJ / mol
what is a positive enthalpy change
- endothermic
- when heat energy is absorbed
- when breaking bonds
what is a negative enthalpy change
- exothermic
- heat is released
- when bonds are created
exothermic energy level diagram
heat is RELEASED to surroundings so LOST from the reactants
total enthalpy change =
energy from bonds broken - energy from bonds created
reaction profile diagram for an exothermic reaction
- heat energy is absorbed, then lost as bonds are broken then created
what does a reaction profile diagram show
the path the reaction actually takes from the reactants to the products
what is the activation energy on a reaction profile diagram
it is the energy needed to break the bonds and is shown by an arrow from the reactants up to the top of the hump
what needs to happen to bonds to create an exothermic reaction
bond making - heat energy released
what needs to happen to bonds to create an endothermic reaction
bond breaking - heat energy absorbed
what is bond energy
the amount of energy that is absorbed or released when bonds are broken or made
how to calculate enthalpy change
bonds broken - bonds made
draw a reaction profile diagram for an exothermic reaction
what is calorimetry
an experimental method used to measure enthalpy change
explain the observations of an exothermic reaction
formula for the amount of heat energy in Joules released in an experiment
energy = (mass of water or solution being heated by the fuel) x (specific heat capacity of water - 4.18J/kg*C) x (change in temp of water)
what is a positive enthalpy change
when heat is taken into a substance
what is the equation for enthalpy change
(-energy / 1000) / moles
what is the unit for enthalpy change
kJ/mol
why would the calculated enthalpy change be different to the experimental enthalpy change
- heat lost to surroundings
- incomplete combustion
also for solution calorimetry - the water being heated is not longer pure water as it has things dissolved in it. this means its specific heat capacity is no longer exactly 4.18
how to carry out combustion calorimetry
- weigh out 50g water and place above the burner
- measure initial mass of burner and temp of water
- ignite the burner, stir water for 30secs then extinguish the burner
- measure the final mass of the burner and the maximum temperature of the water
what are the two types of calorimetry
combustion calorimetry and solution calorimetry
how to carry out a solution calorimetry
measuring the enthalpy change for the displacement reaction between Mg and CuSO4
- measure out 25cm3 CuSO4 into a polystyrene cup
- measure initial temp
- add 1g Mg and stir continuously
- measure the maximum temp of the solution
