Energetics UNIT 2 Flashcards
How do you work out the molar enthalpy change
1) work out the energy change
2) number of moles
3) divide the energy change by the number of moles
Hess’ law
States that the total enthalpy change of a chemical reaction is the same no matter which pathway is taken
How to calculate enthalpy change
Total energy absorbed - total energy released
Calculated using average bond enthalpies
Enthalpy change
Heat energy transferred in a reaction per mole of substance at constant temperatures and pressures
Standard enthalpy change
Enthalpy change when molar quantities of reactants in their standard states react in their standard conditions
Standard molar bond enthalpy
The energy required to make or break one of a specific bond in a specific molecule
What is the enthalpy change for an exothermic reaction
Negative
What is the enthalpy change for an endothermic reaction
Positive
Example of an exothermic reaction
Oxidation
Examples of endothermic reaction
Thermal decomposition, photosynthesis
Breaking bonds is always an ………….. process, so mean bond enthalpies are always ……..
Endothermic. Positive
How to find out enthalpy change in a lab
Need to know number of moles and change in temp
Some take place in container (polystyrene beaker) with thermometer to measure temp change
Combustion reactions are trickier as reactant is burned in air. A copper calorimeter containing a known mass of water is often used. You burn a known mass of the reactant and record the temp change of water
What happens when you increase the concentration of the products in an eqm reaction
The eqm will shift to left to reduce this increase, as it tries to remove the extra product
Changing the pressure only affect equilibria involving what?
Gases
