Bond Enthalpy Flashcards
What is an exothermic reaction?
A reaction where the chemicals in the reaction loose energy to the surroundings, increasing the temperature of the surroundings
What is an endothermic reaction?
A reaction where the chemicals in the reaction gain energy from the surroundings, decreasing the temperature of the surroundings
What is Average Bond Enthalpy?
The amount of energy in a mole of a bonds averaged out across all the molecules that contain that bond
Equation to work out the change in energy in a reaction of chemicals whose mass/volume is known?
q=mcΔT
q = Energy (J)
m = mass (g)
c = Specific heat capacity (Jg⁻¹K⁻¹)
ΔT = Change in Temperature (K)
How to use energy released or taken in from a chemical reaction of known mass/volume to work out bond enthalpy?
ΔH = q/n
n = number of moles (mol)
q = Energy (J)
ΔH = change in energy in a mole of the bonds (Jmol⁻¹)
n = mass/Mr
n = volume(cm³)/1000 * concentration (moldm⁻³)
What is Hess’s Law?
Different pathways must have the same energy change
e.g.
Formation of a substance then reacting it
is the same as forming the products directly
What are the 2 Hess Cycles?
Formulation: Elements to Reactants to Products = Elements to Products
Combusion: Reactants to burnt stuff = Reactants to Products to burnt stuff
What dictates how we calculate the enthalpy of a reaction?
What data is given in the question
Average Bond Enthalpy: regular calculation
Formulation data: Formulation Hess Cycle
Combustion data: Combustion Hess Cycle
What is standard enthalpy change of formulation?
The energy released or taken in when forming 1 mole of a substance from its elements in their base states
What is standard enthalpy change of combustion?
The energy released when 1 mole of a substance undergoes complete combustion under standard conditions with everything in their standard states
What is the enthalpy change of a reaction?
Energy change when a reaction occurs in molar ration shown in the balanced equation.
