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Summary Thermochemistry form 4
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Thermochemistry form 4 Enthalpy change Heat change Exothermic reaction Endothermic reactions Chemistry notes Form four
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THERMOCHEMISTRY
Introduction
Energy is the capacity to do work.
There are many/various forms of energy like heat, electric, mechanical, and chemical
energy.
There are two types of energy:
1. Kinetic Energy(KE) ;the energy in motion.
2. Potential Energy(PE); the stored/internal energy.
Energy is neither created nor destroyed but can be transformed from one form to the
other(interconvertible).
Chemical and physical processes take place with absorption or
production of energy mainly in form of heat.
The study of energy changes that accompany physical and chemical reaction/changes is called
Thermochemistry.
Physical and or chemical reaction/changes that involve energy changes are called
thermochemical reactions.
The SI unit of energy is the Joule(J).
The Joule(J) is defined as the:
Quantity of energy transferred when a force of one newton acts through a distance
of one metre.
Quantity of energy transferred when one coulomb of electric charge is passed
through a potential difference of one volt.
All thermochemical reactions should be carried out at standard conditions of 298K /25°C
temperature101300Pa/101300N/m2 /760mmHg/1 atmosphere pressure.
Exothermic and endothermic processes/reactions
Reactions / processes that take place with evolution/production of energy are called
exothermic while others take place with absorption of energy are called endothermic.
Exothermic reactions cause arise in temperature.
Endothermic reactions cause a fall in temperature.
Example of exothermic and endothermic reactions
,1. Dissolving Potassium nitrate(V)/ammonium chloride crystals
Procedure:
Measure 20cm3 of water in a beaker.
Determine and record its temperature T1.
Put about 1.0g of Potassium nitrate(V) crystals into the beaker.
Stir the mixture carefully and note the highest temperature rise /fall T2.
Repeat the whole procedure by using ammonium chloride in place of Potassium nitrate
(V) crystals.
results
Temperture (oC) Using Potassium Using Ammonium
nitrate(V) crystals chloride crystals
T 2 (Final temperature) 21.0 23.0
T 1 (Initial temperature) 25.0 26.0
Change in temperature(T2 –T1) 4.0 3.0
Initial(T1) temperature of dissolution of both potassium nitrate(V) crystals and
ammonium chloride crystals is higher than the final temperature(T2)
Change in temperature(T2 –T1) is not a mathematical “-4.0” or “-3.0” .
Dissolution of both potassium nitrate(V) and ammonium chloride crystals is an
endothermic process because initial(T1) temperature is higher than the final
temperature(T2) thus causes a fall/drop in temperature.
2 . Dissolving concentrated sulphuric(VI) acid/sodium hydroxide crystals
Procedure:
Measure 20cm3 of water in a beaker.
Determine and record its temperature T1.
Carefully put about 1.0g/four pellets of sodium hydroxide crystals into the beaker.
Stir the mixture carefully and note the highest temperature rise/fall T2.
Repeat the whole procedure by using 2cm3 of concentrated sulphuric(VI) acid in place
of sodium hydroxide crystals.
CAUTION:
a. Sodium hydroxide crystals are caustic and cause painful blisters on contact with
skin.
b. Concentrated sulphuric (VI) acid is corrosive and cause painful wounds on
contact with skin.
Results
Temperture (oC) Using Sodium Using Concentrated
hydroxide pellets sulphuric(VI) acid
T 2 (Final temperature) 30.0 32.0
T 1 (Initial temperature) 24.0 25.0
Change in temperature(T2 –T1) 6.0 7.0
, Initial (T1) temperature of dissolution of both concentrated sulphuric (VI) acid and
sodium hydroxide pellets is lower than the final temperature (T2).
Dissolution of both Sodium hydroxide pellets and concentrated sulphuric (VI) acid
is an exothermic process because final (T2) temperature is higher than the initial
temperature (T1) thus causes a rise in temperature.
At the same temperature and pressure, heat absorbed and released is called
enthalpy/heat content (H).
Energy change is measured from the heat content/enthalpy of the final and initial
products(ΔH(delta H).
Example
Energy change in heat content ∆H = Hfinal – Hinitial
For chemical reactions: ∆H = Hproducts – Hreactants
In exothermic reactions, the heat contents of the reactants is higher than the heat contents
of products (-ΔH)
In endothermic reactions, the heat contents of the reactants is less than/lower than the heat
contents of products (+ΔH)
Example of energy level diagram for endothermic reaction
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