CHEMICAL REACTIONS Produce (lose) energy = exothermic reaction Absorb (require) energy = endothermic reaction.
Post on 19-Jan-2016
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ENERGY EXCHANGES IN
CHEMICAL REACTIONS
CHEMICAL REACTIONS Produce (lose) energy = exothermic reaction
Absorb (require) energy = endothermic reaction
ENERGY The energy (in a reaction) is linked to either MAKING or BREAKING bonds
BOND BREAKING energy absorbing endothermic
BOND MAKING energy releasing exothermic
The big BUT… In most reactions the bonds of the reactants are broken and the new bonds of the products are formed (so we have both exo and endo reactions
ex. Combustion of methaneCH4(g) + 2O2(g) CO2(g) + 2H2O(g)
All the C-H bonds and O-O bonds are brokenThis requires energy – call this Ein (ACTIVATION
ENERGY)C=O bonds and O-H bonds need to be formedEnergy is released – call this Eout
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
The ΔH can therefore be thought of asΔH = Ein – Eout
When methane burns ΔH is negative because it is an exothermic reaction. This means that forming bonds releases more energy than what was absorbed to break the bonds in the reactants Eout > Ein
In general If the total energy input is greater than the output ΔH is positive and the reaction is endothermic
If the total energy output is greater than the input ΔH is negative and the reaction is exothermic
ex. 2 Formation of Hydrogen Iodide
H2(g) + I2(g) 2HI(g)
The reaction is endothermic, therefore the products possess more total formation enthalpy than the reactants
In general: exothermic reactionsΔH < 0 kJThe energy input is less than the energy
output
In general: endothermic reactionsΔH > 0 kJThe energy input is greater than the energy
output
CatalystsA catalyst is a chemical that, when added to a reaction, speeds up the reaction and also remains when the reaction is over (it is not INVOLVED in the reaction)
Catalysts and Hess’s Law Hess’s Law states that the enthalpy of a reaction depends ONLY on the enthalpies of the initial and the final states, no matter what series of steps exist in between.
A catalyst has NO effect on the enthalpy change of a reaction
Catalyzed reactions lose heat more rapidly if reaction is exothermic
gain heat more rapidly if reaction is endothermic
Catalysts speed up a reaction by providing an alternate reaction pathway that has lower activation energy
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