|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|
Thermochemistry Cheat Sheet
Thermochemistry deals with heat (energy) changes in chemical reactions. In chemical reactions heat is released or absorbed. If reaction absorbs heat then we call them endothermic reactions and if reaction release heat we call them exothermic reactions.
In endothermic reactions, potential energy of reactants are lower than potential energy of products. To balance this energy difference, heat is given to reaction. Potential energy is shown with H.
Condensation of gases, combustion reactions are examples of exothermic reactions. In these reactions, potential energies of reactants are higher than potential energies of products. Excess amount of energy is written in right side of reaction to balance energy difference.
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".
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
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
2H2O(g) → 2H2(g) + O2(g) ; ∆H=116 kcal
Properties of Thermochemical Reactions
Hess' law states that, you can sum one more than one reactions to form new reaction. While doing this, you apply same changes also on enthalpy changes of used reactions.
1) Standard Molar Enthalpy of Formation:
Enthalpy change of formation of 1 mole compound from its elements is called standard molar enthalpy of formation and expressed in kcal/mol or kjoule/mol. Be careful when writing formation reactions an pay attention on following suggestions;
2) Standard Enthalpy of Decomposition:
Enthalpy change of decomposition of 1 mole compound into its elements is called standard molar enthalpy of decomposition.
H2O(l) → H2(g) + 1/2 O2(g) ; ∆H=68 kcal
3) Standard Enthalpy of Combustion:
It is the heat released from the reaction of one mole element with O2(g).
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ; ∆H=-212 kcal
4) Standard Enthalpy of Neutralization Reaction:
It is the enthalpy change of neutralization of 1 mol acid and one mol base. These reactions are exothermic reactions.
Acid + Base → Salt + Water + Heat
H+ + OH- → H2O + 13,5 kcal
Forming chemical bond atoms become more stable and their energies decrease and this energy is released outside. While breaking this bond same amount of energy is required. Energy released during formation of one mol bond and required for breaking one mole bond is called bond energy.
Reactants → Products ; ∆H=?
∆H=∑(Bond Energies)Reactants-∑(Bond Energies)Products
Where ∑ shows sum of given quantities.
In a reaction If;
Most of enthalpy change can be measured experimentally. This process is called " measuring heat transfer " calorimetry. Calorimeters are devices used in measuring heat flow. In calorimeters;
Heat Absorbed = Heat Released
Heat flow in calorimeter is calculated with following formula;
Q=mcal.ccal.∆T + mwater. cwater.∆T
mcal= mass of calorimeter, in g.
ccal=specific heat capacity of calorimeter
mwater= mass of water in g.
cwater= specific heat capacity of water
Enthalpy of the reactions depends on;