Section 9 chemical energy Flashcards
Enthalpy definition
The measure of chemical energy stored in a substance .
What is enthalpy measured in
KJ mol-1
Exothermic reactions definition
A reaction where the products have a lower enthalpy and potential energy than reactants, meaning heat energy is released by the reaction.
Endothermic reaction definition
A reaction where the products have a higher enthalpy and potential energy than reactants, meaning heat energy is absorbed.
Industrial cost of exothermic reactions
Heat energy must be removed, since if temperature rises far too quickly there may be an explosion.
Industrial cost of endothermic reactions
Heat energy may be needed to be constantly supplied to keep the reaction occurring.
Heat energy equation
Eh = cm delta T
Specific heat capacity of water
4.18 KJ kg-1C-1
What is measured in an energy change experiment
Temperature change and mass of water.
How do you calculate the enthalpy of combustion
Eh = cm delta T
N = m/gfm
Eh/n
Enthalpy of combustion definition
The enthalpy change when one mole of a substance completely burns in oxygen.
Ways to minimise error in enthalpy of combustion experiment
Copper can to ensure heat energy is conducted.
Heat shield to prevent heat loss into environment.
Equations of enthalpy of combustion key characteristics
The substance burned in the reaction must be one mole.
Examples of equations of enthalpies of combustion
H2 +1/2O2 —> H2O
C + O2 —-> CO2
Hess’s law explanation
The enthalpy change for a reaction depends on the reactants and products, not on the route taken.
Hess’s law equations
Manipulate all target equations, to get reactants on their own, then add the enthalpy change of the reactions together.
Why are Hess’s law equations used
To calculate the enthalpy change for reactions that can’t be carried out.
Molar bond enthalpy
The energy required to break one mole of bonds in one mole of a diatomic molecule.
Mean molar bond enthalpy
The average energy required to break one mole of bonds in one mole of a molecule, for a bond that occurs in a number of compounds.
Enthalpy change of a reaction break down
Breaking bonds is endothermic and requires for energy to be absorbed.
Forming new bonds is exothermic and releases energy.
What state must the substances be in to do a bond enthalpy.
Gas
Bond enthalpy equation
Add the enthalpy of the bonds on the left side, take away the bonds on the right side.
