|Matters and Properties of Matters|
|Atomic Structure with Examples|
|The Mole Concept with Examples|
|Gases with Examples|
|Chemical Reactions with Examples|
|Nuclear Chemistry (Radioactivity)|
|Acids and Bases|
|--Enthalpy and Thermochemical Reactions|
|--Hess's Law (Summation of Thermochemical Reactions)|
|--Bond Energies and Enthalpy|
|--Measuring Enthalpy and Calorimeter|
|--Thermochemistry Cheat Sheet|
|Rates of Reactions (Chemical Kinetics)|
|Exams and Problem Solutions|
Enthalpy and Thermochemical Reactions
Physical and chemical changes are done under constant pressure. Gained or lost heat in reactions under constant pressure is called enthalpy change. Enthalpy is the total kinetic and potential energy of particles of matter. It is denoted by letter "H". Enthalpy of matters can not be measured, however, enthalpy change can be measured. We can find change in enthalpy as given below;
Reactants → Products
If HR is the enthalpy of reactants and HP is the enthalpy of products, change in enthalpy becomes,
HP<HR so; ∆H<0
HP>HR so; ∆H>0
In endothermic reactions, enthalpy of system increases. For example, enthalpy of water is larger than enthalpy of ice at same temperature.Graph given below shows enthalpy of endothermic reactions;
In exothermic reactions;
Reactions showing both changes of matters and energy are called thermochemical reactions. Examples of thermochemical reactions;
C(s) +O2(g) → CO2(g) ; ∆H=-94 kcal
This reaction tells us that, 1 mol C(s) reacts with 1 mol O2(s) and produce 1 mol CO2, and 94 kcal heat released.Reaction becomes;
C(s) +O2(g) → CO2(g) + 94 kcal
2H2O(g) → 2H2(g) + O2(g) ; ∆H=116 kcal
This reaction explains us, 2 mol H2O absorbs heat and decompose into 2 mol H2 and O2.
2H2O(g) + 116 kcal → 2H2(g) + O2(g)
Properties of Thermochemical Reactions
Example: Find heat released from reaction in which 2 mol CH4 and 2 mol Cl2 react to form CCl4 and HCl.
CH4(g) 4Cl2(g) → CCl4(g) + 4HCl(g) + 104 kcal
Reaction given above is balanced for 1 mol CH4, we should find limiting matter first.
1 mol CH4 react with 4 mol CCl4
? mol CH4 react with 2 mol CCl4
?=0,5 mol CH4
2-0,5=1,5 mol CH4 is not used in this reaction since CCl2 is limiting matter.
Heat releases from reaction is calculating by considering limiting matter;
4 mol Cl2 release 104 kcal heat
2 mol Cl2 release ? kcal heat
?=52 kcal heat is released from the reaction of 2 mol Cl2.
CO(g) + 1/2O2(g) → CO2(g) ; ∆H=-68 kcal
When we multiply reaction with 2;
2CO(g) + O2(g) → 2CO2(g) ; ∆H=2(-68)=-136 kcal
2H2O(g) → 2H2(g) + O2(g) ∆H=116 kcal
2H2(g) + O2(g) → 2H2O(g) ∆H=-116 kcal
As you can see from the example, when we change direction of reaction, sign of enthalpy change also changes.
H2(g) + 1/2O2(g) → H2O(g) ∆H=-58 kcal
H2(g) + 1/2O2(g) → H2O(l) ∆H=-68 kcal
As you can see from the examples, enthalpy of water at liquid state is lower than enthalpy of water at gas state.