Chapter 5
Tiers 1,2 & 3
TIER 1:REMEMBER
DEFINE:
•Energy
•Joule
•Temperature
•Heat
•Calorimetry
•Calorimeter
•Specific heat capacity
•Endothermic reactions
•Exothermic reactions
•Standard enthalpy change of reaction
•Average bond enthalpy
ENERGY:
• is the measure of the ability to do work.
•it comes in many forms
•Heat
•Light
•Sound
•Electricity
•Chemical energy
JOULE is the unit for energy. It is defined as the heat required to raise the
temperature of 1 g of water by 1oC
TEMPERATURE and HEAT
HEAT is the measure of the total energy in a given amount of a substance
TEMPERTURE is the measure of the average kinetic energy of a substance
CALORIMETRY
Calorimetry is the quantitative measurement of the heat required or evolved
during a chemical process.
A calorimeter is an instrument for measuring the heat of a reaction during a
well defined process. The following diagram depicts a constant volume or
'bomb' calorimeter (left) and a much simpler, but less accurate “coffee cup”
calorimeter (right) often use in the laboratory.
SPECIFIC HEAT CAPACITY
The specific heat capacity of a substance (liquid or solid)
•is defined as the amount of heat needed to raise the unit mass of a
substance by a single degree of temperature.
•it is generally symbolized by displaying a 'C' or a 'c' depending upon
how the heat is measured.
•the common unit is J g-1 oC-1 which is Joule per gram per degree Celsius
CHEMICAL ENERGY
CHEMICAL ENERGY is energy that is released or absorbed
during a chemical reaction.
ENDOTHERMIC REACTIONS
•involve reactions where the energy is absorbed
•the temperature of the reaction decreases
•a cold pack used for sports injuries use endothermic
reactions
EXOTHERMIC REACTIONS
• involve reactions where the energy is released.
•the temperature of the reaction increases
•examples include combustion and neutralization reactions
•a heat pack used for sports injuries or to keep warm use
exothermic reactions
STANDARD ENTHALPY CHANGE OF REACTION
Is a change that occurs in a system when one mole of matter is transformed
by a chemical reaction under standard conditions. The symbol is ΔHƟ and
the unit is kJ dm-3
STANDARD CONDITIONS FOR ENTHALPY CHANGES ARE:
•A temperature of 298K or 25oC
•A pressure of 101.3 kPa (1 atm)
•Concentrations of 1 mol dm-3 for all solutions
•All the substances in their standard states
AVERAGE BOND ENTHALPY
Average bond enthalpy:
• also called bond energy
• is the energy needed to break a chemical bond.
• the unit is also kJ dm-3 measured at 298K or 25oC.
•enthalpy changes can also be calculated directly from bond enthalpies
•The limitation to using average bond enthalpies is that it can only be
used if all the reactants and products are in the gaseous state.
•If water were a liquid product there would be even more heat
evolved since the enthalpy change of vaporization would also
need to be considered
•The values have been obtained by considering a number of
similar compounds. In practice, the energy of a particular bond
would be slightly different under different circumstances due to
the affects of neighboring atoms
TIER 2:UNDERSTAND
•Distinguish between heat and temperature
•Distinguish between the system and the
surroundings in a chemical reaction
•State that combustion and most neutralization
reactions are exothermic processes
TEMPERATURE and HEAT
Since heat is the measure of the total energy in a given amount of a
substance whereas temperature is the measure of the average kinetic
energy of a substance, heat depends on the amount of a substance but
temperature does not.
Sometimes heat is calculated by the equation:
q = cmΔt where q is heat, c is specific heat capacity, m is mass & Δt is
change is temperature.
q is in Joules, c is in J g-1 oC-1, m is in g and Δt is in oC
SYSTEM VS SURROUNDING
In chemistry, the system is referred to as the area of
interest which is most times the actual reaction and
the surroundings is considered everything else.
surrounding
system
EXOTHERMIC
Energy is given off by
the system to the
surroundings
surrounding
system
ENDOTHERMIC
Energy is absorbed
by the system from
the surroundings
EXAMPLES OF EXOTHERMIC REACTIONS
•Combustion reactions of fuels
•Adding concentrated acid to water
{neutralization}
•Burning of a substance
•Adding water to anhydrous
•Copper(II) sulfate
•The thermite reaction
•Reactions between lime and aluminum
• Corrosion of metals
•Respiration
•Decomposition of vegetable matter into
compost
An exothermic thermite reaction using
iron(III) oxide. The sparks flying
outwards are globules of molten iron
trailing smoke in their wake.
NEUTRALIZATION REACTIONS
Often, neutralization reactions are exothemic (the
enthalpy of neutralization). For example, the reaction of
sodium hydroxide and hydrochloric acid or adding
concentrated acid to water {neutralization}
However, forms of endothermic neutralization do exist,
such as the reaction between sodium bicarbonate(baking
soda) and acetic acid (vinegar)
Neutralization of sodium
hydroxide (analyte) with
hydrochloric acid (titrant)
using phenolphthalein as the
indicator
TIER 3:APPLY
•Apply the relationship between temperature
change, enthalpy changes and the classification of
a reaction as endothermic or exothermic.
•Explain in terms of average bond enthalpies,
why some reactions are exothermic and others are
endothermic
EXOTHERMIC VS ENDOTHERMIC REACTIONS
EXOTHERMIC
ENDOTHERMIC
•if the bonds in the products are
stronger than the bonds in the
reactants, heat is given off to the
surroundings
•the enthalpy change for the
products are less than that of the
reactants
•ΔH is negative
•if the bonds in the reactants are
stronger than the bonds in the
products, heat is absorbed by the
system
•the enthalpy change for the
products are greater than that of
the reactants
•ΔH is positive
EXOTHERMIC VS ENDOTHERMIC REACTIONS
The quantity and the sign of ΔH can be derived from the equation:
ΔH = ΔH(products) + ΔH(reactants )
EXOTHERMIC REACTIONS: ΔH(products) < ΔH(reactants ) THEREFORE ΔH IS NEGATIVE
ENDOTHERMIC REACTIONS: ΔH(products) > ΔH(reactants ) THEREFORE ΔH IS POSITIVE