Energetics
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What is there usually when there a chemical reaction happens?
Change in energy Enthalpy change
Define Enthalpy change
∆H (delta H) of the heat energy transferred in a reaction at constant pressure. The units of ∆H are kJ mol-1
Ho of Enthalpy change means
Ho of Enthalpy change Substances were in standard states and measurement was made under standard conditions
Standard conditions for Enthalpy change are?
Standard conditions are 100kPa pressure 298K Stated temperature ΔH298
Describe exothermic
Give out energy ΔH is negative
Describe endothermic
Takes in energy ΔH is positive
Exothermic reactions are usually
Oxidation e.g. Combustion of a fuel like methane CH4(g) + O2 (g) → CO2(g) + H2O(l) ΔcH=-890 kJ mol-1 Oxidation of carbohydrates eg. C6H12O6 in respiration
Endothermic examples
Thermal decomposition of calcium carbonate CaCO3(s)-> CaO(s) + CO2(g) ΔH=+178kJmol-1 Main reactions of photosynthesis
In exothermic reactions what Happens to temperature
Increase
In endothermic reactions what Happens to temperature
Decrease
What happens in bonds when a reaction happens?
Reactant bonds are broken and product bonds are formed
What do you need to break bonds?
Energy so bond breaking is endothermic ΔH is positive Stronger bonds take more energy to break
What happens when bonds are formed?
Energy is released so bond making is exothermic ΔH if negative Stronger bonds release more energy when they form
What’s Enthalpy change for a reaction
The overall sum of these two changes If you need more energy bonds then released when bonds made ΔH if positive If less ΔH is negative
How do you get mean enthalpies do?
Up in data books
What’s bond Enthalpy?
The energy required to break bonds
When you break the same type of bond does it always require the same amount of energy?
No the energy needed to beak a bond depends on the environment its in
What’s mean bond Enthalpy?
The average energy needed to break a certain type of bond over a range of compounds
Example of mean bond Enthalpy
Water has two O-H bonds First bond Enthalpy= +492kJ mol-1 Second bond Enthalpy= +428kJmol-1 Mean bond Enthalpy 492+428/2=+460kJmol-1
Bond enthalpies are?
Always positive because breaking bonds is always endothermic
What happens in a chemical reaction during bond formation?
Energy absorbed to break bonds and given out during bond formation
Enthalpy change=
Enthalpy change= Total energy absorbed-total energy released
What is so because bond energies are averages?
Enthalpy changes aren’t exact They are slightly less accurate than using Hess’s law
Define standard Enthalpy of formation?
ΔfHo Is the Enthalpy change when 1 mole of compound is formed from its elements in their standard states