1.
2.
3.
4.
5.
What are the standard units of energy?
What units of energy are mostly used in
Chemistry?
What is another term for heat energy of
a reaction?
How can changes in heat energy of
reactions be detected?
Give a biochemical reaction which
releases heat energy.
Natural
Energetics
Objectives (Ref SGp28)
5.1.1 Define the terms exothermic reaction, endothermic reaction and standard enthalpy
change of reaction (∆Hө)
5.1.2 State that combustion and neutralization are exothermic processes.
5.1.3 Apply the relationship between temperature change, enthalpy change and the
classification of a reaction as endothermic or exothermic.
5.1.4 Deduce, from an enthalpy level diagram, the relative stabilities of reactants and
products, and the sign of the enthalpy change for the reaction.
Enthalpy is the heat energy of a reaction.
Only changes in enthalpy can be
measured.
Standard enthalpy change (∆Hө) “is the
heat energy transferred under standard
conditions”
 pressure of 101.3 kPa (1 atm)
 temperature 298 K (25 ◦C)
Reactants → Products
Hreactants > Hproducts , ∆H = Hp – Hr so,
∆H is negative, heat energy is given out so
the temperature of the container increases.
This can be shown in an
Energy Level Diagram.
SPONTANEOUS REACTION – THE
REACTANTS ARE UNSTABLE
REACTION REQUIRING INITIAL HEATING
FIRST – THE REACTANTS ARE STABLE
E.g. HCl + NaOH → NaCl + H2O, ∆H -ve
1. Write 2 more and draw an Energy Level
Diagram for each.
E.g. CH4 + 2O2 → CO2 + 2H20, ∆H -ve
2. Write 2 more with an Energy Level
Diagram for each.
Reactants → Products
Hreactants < Hproducts , ∆H = Hp – Hr so,
∆H is positive ,
heat energy is taken in so the temperature of
the container decreases.
This can be shown in an
Energy Level Diagram.
G&D p137 for a further example
IN CLASS
G+D P137 Q1-3
AT HOME
P137 Ex 5.1 Q4+5 due
Monday 20 January 2014.
CAN WE?

Define the terms exothermic reaction,
endothermic reaction and standard
enthalpy change of reaction (∆Hө)

State that combustion and neutralization
are exothermic processes.

The classification of a reaction as
endothermic or exothermic.

Deduce, from an enthalpy level diagram,
the relative stabilities of reactants and
products, and the sign of the enthalpy
change for the reaction.
NEXT TIME
Calculations on Enthalpy.