Energetics Flashcards
as chem
Meaning of enthalpy change + symbol + unit
the heat energy change in a reaction at constant pressure.
ΔH
kJ mol-1
What does ⦵ mean? and what are the conditions
It means the substance was in the standard state under standard conditions.
100kPa pressure
298K temperature
What are endothermic reactions?
Reactions that absorb energy from the surroundings
Products are higher in energy than reactants
eg) thermal decomposition
What are exothermic reactions?
Reactions that release energy to the surroundings
Products are lower in energy than reactants.
eg) combustion
What process is bond making and breaking
Bond breaking - endothermic (ΔH positive)
Bond making- exothermic
(ΔH negative)
How do you calculate enthalpy change from mean bond enthalpy?
Total energy to break bonds- total energy released forming
bonds breaking- bonds making
Describe calorimetry process using fuels.
- a fuel is burned to raise the temperature of the water by a specific amount.
- the fuel is weighed before and after to work out the mass of fuel burned.
- the energy from the fuel is transferred into the water although some is lost to the surroundings.
-the energy transferred can be calculated using q=mc ΔT
What is calorimetry used for?
To work out the enthalpy change of combustion
What’s the difference between enthalpy and energy?
Enthalpy is KJmol -1
Energy is KJ
How can energy be calculated from equation q=mc ΔT?
1) work out energy from information
M= mass of water/mass of liquids used
C= specific heat capacity of water
T= temperature change
2) calculate moles of solution
3) calculate enthalpy= energy/moles
(units KJmol -1)
Describe calorimetry experiment of solutions.
1) fill up a polystyrene cup with a lid and a thermometer.
2)add acid to solution and measure temperature
3)then add alkali/solid stir and measure temperature change.
Hess law meaning
The total enthalpy change of a reaction is independent of the route taken.
How is a formation hess cycle drawn?
- ensure data given is formation
- draw arrows upwards
reactants ——- products
\elements in standard states/
- substitute numbers ( x values if change in moles)
Equation for enthalpy of formation?
enthalpy of formation of products - enthalpy of formation reactants
How is combustion hess cycle drawn?
- ensure data given is combustion
- draw arrows downwards
reactants/elements ——- products
\elements in standard states/
- substitute numbers ( x values if change in moles)
Equation for enthalpy of combustion?
enthalpy of combustion of reactants - enthalpy of combustion of products
