Chapter # 10 [Thermochemistry]

 

UNIT# 10

THERMOCHEMISTRY

The chemical reactions during which material changes are accompanied with the change in heat energy are called thermo-chemical reactions. The branch of chemistry which deals with the study of heat changes in chemical reactions is Thermo chemistry and the reactions are called thermochemcial reactions which are:

·        Exothermic reaction

·        Endothermic reaction

Q#	Questions	Year
Q# 01	What is thermochemcial reaction?	[2009] [2007]
Q# 02	Define Exothermic Reaction with example	[2017][2014][2011] [2010] [2007] [2006]
Q# 03	Define Endothermic Reaction with example.	[2017][2016][2014] [2012][2010][2007] [2006]
Q# 04	Define Enthalpy	[2016][2015][2013][2011] [2009][2007]
Q# 05	Define Thermo chemistry	[2013][2007]

 

 

Q. Define Thermochemistry and thermochemical equation.

THERMO CHEMISTRY

The branch of chemistry concerned with the quantities of heat evolved or absorbed during chemical reactions is called thermo chemistry.

 

THERMO CHEMICAL EQUATION

Thermo chemical Equation is a balanced stoichiometric chemical equation which includes the change in enthalpy, ΔH. In variable form, a thermochemcial equation would look like this
X + Y → Z

ΔH = (±) #

Where {X, Y, Z} are the standard agents of a chemical equation with coefficients and “(±) #” is a positive or negative numerical value, usually with units of KJ.

 

Q. What is heat of reaction?

HEAT OF REACTION

Amount of heat evolved or absorbed during a chemical reaction is called heat of reaction.

 

Q. Define exothermic reaction and endothermic reaction. Give examples of each.

EXOTHERMIC REACTION

Those reactions which accompanied with the emission of energy are called exothermic reaction.

During exothermic reaction, heat energy is released because total internal energy is greater than the total energy of the products. Change of heat represents by ∆H with negative sign.

It usually forms the bond.

 

FOR EXAMPLE

Reactant         →        Product + Heat

C + O₂ →        CO₂                             ΔH= -393.7 K.J/mol

H₂ + ½ O₂      →        H₂O                             ΔH= -286 K.J/mol

CH₄ + 2O₂      →        CO₂ + 2H₂O               ΔH= -890 K.J/mol

 

ENTHALPY FOR EXOTHERMIC REACTION

If enthalpy of product is smaller than enthalpy of reactants than ∆H will be negative and heat is evolved.

H₂ + ½ O₂      →        H₂O                             ΔH= -286 K.J/mol

 

APPLICATIONS OF EXOTHERMIC REACTION

To warm the food contents

The armed forces don’t use fire to warm their foods. Their food pouches contains flameless radiation heater. The heater contains chemicals (Mg, Fe, NaCl) which reacts with water to produce heat in order to , warm the food, pouch is kept in a bag and water is added, thus the temperature of food reached up to 60^o C within 15 minutes.

Mg +2H₂O     →        Mg(OH)₂ + H₂           ΔH= -3.53 K.J/mol

Reaction of Magnesium with water is slow because of formation of MgO film. Its reactivity can be increased by the presence of Fe and common salt.

 

ENDOTHERMIC REACTION

Those reactions in which heat energy is absorbed are called endothermic reaction.

During endothermic reaction, heat energy is taken in from the surrounding to the system because total internal energy of the reactant is less than the total energy of the products.

It usually breaks the bond.

 

FOR EXAMPLE

Reactant + Heat        →        Product

H₂O   + ΔH                →        H₂ + ½ O₂       ΔH= +286 K.J/mol

½ N₂ + ½ O₂               →        NO                  ΔH= +90 K.J/mol

 

 

ENTHALPY FOR ENDOTHERMIC REACTION

If enthalpy of product is greater than enthalpy of reactants than ∆H will be positive and heat is absorbed.

H₂O                 →        H₂ + ½ O₂      ΔH= +286 K.J/mol

 

∆H = H₂ – H₁,

where ∆H= change of enthalpy,

H₁= heat content of reactant,

H₂= heat content of product

 

 Q. Distinguish between exothermic reaction and endothermic reaction.

 

Exothermic reaction	Endothermic reaction
Heat is released	Heat is absorbed or gained
Represent by negative sign	Represent by positive sign
Usually bond formation occurs	Usually bond dissociates
Total energy of the system decreases	Total energy of the system increases
Combustion, addition, neutralization, single displacement reactions are the examples.	Decomposition reaction is the example.

 

Q. What is Enthalpy?

ENTHALPY

The energy possesses by the substance is called heat content of substance or enthalpy.

It represents by ΔH.

ΔH= ( H₂-H₁)

Change of enthalpy = Enthalpy of Product – Enthalpy of reactant

 

MEASUREMENT OF ENTHALPY

Exothermic and endothermic reaction can be detected by knowing the temperature of vessel before and after the reaction:

·        Rise in temperature indicates the exothermic reaction

·        Fall in temperature indicates the endothermic reaction

·        ∆H value can be determined by Calorimeter.

 

SIGN OF ENTHALPY

1.      If enthalpy of product is greater than the enthalpy of the reactant then the sign of ΔH will be positive.

2.      If enthalpy of product is smaller than the enthalpy of the reactant then the sign of ΔH will be negative.

 

NEUTRALIZATION

When acids react with base then salt and water is formed, the process is called Neutralization.

 

REACTION
HCl + NaOH  →   NaCl + H₂O