Section 4 b: energetics
Exothermic and endothermic reactions
4.10 recall that chemical reactions in which heat energy is given out are described as exothermic and
those in which heat energy is taken in are endothermic
Exothermic reactions



a reaction which releases (=gives out) heat energy (could also be light, electrical energy, sound,.);
observation: shows a rise in temperature!! e.g. a warm test tube means an exothermic reaction.
as energy is released to the surroundings the temperature of the surroundings increases which is
what we can observe and measure;
exothermic reactions need a small amount of energy to get started but once it has started it goes
on and does not need to be given any energy anymore as it provides its own;
most reactions are exothermic;
important exothermic reactions:




burning of fossil fuels like coal, oil and natural gas = oxidation of fuel to form carbon dioxide and
water:
o fuel + oxygen  carbon dioxide + water

respiration = release of energy from food:
o


sugar + oxygen  carbon dioxide + water
neutralisation reactions
electrochemical in a cell: converts chemical energy into electrical energy.
Endothermic reactions

reaction which absorbs or takes in energy from the surroundings usually in the form of heat or
light;
as the system removes energy from the surroundings, the temperature of the surroundings
decreases so: observation: it feels cold;
needs a continuous input of energy; when you stop heating or giving light reaction stops!!
important endothermic reactions:





photosynthesis: needs light!!!! No light  no reaction! (=photochemical reaction)
decomposition reactions eg electrolysis or thermal decomposition of carbonates
surroundings
endothermic
exothermic
SYSTEM
SYSTEM
= indicates flow of heat energy
Section 4b energetic
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Energy level diagram
4.14 represent exothermic and endothermic reactions on a simple energy level diagram
Energy changes taking place during reactions can be shown in energy level diagrams.
The horizontal lines show us how much energy the reactants (= the amount of energy of the system
before the reaction) and products (amount of energy after the reaction has finished) have.
exothermic reaction
endothermic reaction
energy
energy
reactants
products
reactants
products
Study the energy level diagrams and complete the following sentences using the words below:
more
absorbed
higher
products
released
more
less (2)
system
In an exothermic reaction:

the reactants are at a ________ energy level than the products;

the difference in height between the two levels is the energy that is ________________;

the system has _________energy after the reaction than it had before;

the surroundings have now ______ energy
In an endothermic reaction:

the ______________ are at a higher energy level than the reactants;

the difference in height between the two levels is the energy that is ______________;

the _______________ has more energy after the reaction than it had before;

the surroundings have now __________ energy.
Section 4b energetic
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Measuring ΔH
4.11 describe simple calorimetry experiments for reactions, such as combustion, displacement,
dissolving and neutralisation in which heat energy changes can be calculated from measured
temperature changes
4.12 calculate molar enthalpy change from heat energy change
4.13 understand the use of ΔH to represent molar enthalpy change for exothermic and endothermic
reactions
ΔH = the amount of heat energy or enthalpy absorbed or released for 1 mole of a substance; it is
called the molar enthalpy change.
The amount of energy released or absorbed by a reaction can be measured experimentally using the
expression below.
T (C) x
ΔH =
mass of H2O (g) x 4.2J/g C.
number of moles
ΔH is measured in J/mol or kJ/mol.




ΔH = molar enthalpy change
T = temperature increase or decrease caused by the reaction – this is what we can
measure;
mass of H2O = mass of water in grams;
4.2J/g C = specific heat capacity of water;
Enthalpy of combustion of a fuel
The following apparatus can be used.
The following measurements are taken (raw data collected):
1. Mass of cold water (g)
2. Temperature of the water before and after burning the
fuel (C)
3. Mass of the fuel before and after burning(g)
The following processing of the raw data needs to be done:
1. Calculate the temperature change
2. Use T (C) x mass of H2O (g) x 4.2J/g C to
calculate the heat energy absorbed by the water.
3. Calculate the mass change of the fuel
4. Calculate number of moles of the fuel burned
5. Calculate the molar enthalpy change.
(image from http://www.docbrown.info/page07/delta1Hc.htm on 23/06/2010)
Evaluation
List some experimental errors.
Section 4b energetic
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Example of calculation (from http://www.docbrown.info/page07/delta1Hc.htm
Volume of water in calorimeter (cm3)
Mass spirit burner + ethanol before burning(g)
Mass spirit burner + ethanol after burning(g)
Temperature at start (C)
Temperature at end (C)
on 23/06/2010)
100
18.62
17.14
18
89
Calculations


amount of energy released
o temperature rise = 89 - 18 = 71oC (exothermic, heat energy given out).
o mass of fuel burned = 18.62-17.14 = 1.48g.
o heat absorbed by the water = heat released by fuel = mass of water x shcwater x temperature
change
= 100 x 4.2 x 71 = 29820 J (for 1.48g)
for 1 mole of ethanol:
o Mr(ethanol) = 46 (H=1, C=12, O=16)
o therefore 1.48g ethanol = 1.48/46 = 0.03217 mol
o scaling up to 1 mole of ethanol burned gives 29820 x 1 / 0.03217 = 926951 J
Answer: molar enthalpy of combustion of ethanol = ΔHc(ethanol) = -927 kJ/mol
Enthalpy change of reactions such as dissolving, neutralisation or displacement
A simple calorimeter is used.
(image from http://www.docbrown.info/page07/delta1Hc.htm on 23/06/2010)
The following measurements are taken (raw data collected):
1. Temperature of the water or solution before and after (highest temperature) the reaction
2. Amount of substance used either as a mass or a concentration and volume
The following processing of the raw data needs to be done:
1.
2.
3.
4.
Calculate the temperature change
Use T (C) x mass of H2O (g) x 4.2J/g C to calculate the enthalpy change
Calculate number of moles of the substance used
Calculate the molar enthalpy change
Evaluation
List some experimental errors
Section 4b energetic
4|Page
Remember that if the reaction is exothermic ΔH must have a negative sign whilst if it is endothermic is
must have a positive sign.
Making and breaking bonds
4.15 recall that the breaking of bonds is endothermic and that the making of bonds is exothermic
4.16 use average bond energies to calculate the enthalpy change during a simple chemical
reaction
During any chemical reaction the particles are re-arranged. For this to happen, bonds within the
reactants need to be broken. When bonds are broken new bonds can be made.
Energy must be supplied to break old bonds (endothermic process)
Energy is released when new bonds form (exothermic process)
Whether a reaction is exothermic or endothermic depends on the amount of energy that is needed to
break the old bonds and the amount of energy.
If in a reaction more energy is released for making new bonds than what is needed to break the old
ones, the reaction is exothermic and H will have a negative value.
In an endothermic reaction more energy is needed to break the bonds than the amount released
when new ones are made. H = positive.
If we know how much energy is needed to break bonds and how much energy is released when new
bonds are made we can calculate the overall enthalpy change and find out if the reaction is
endothermic or exothermic.
We can carry out such calculations by using the bond energies.
The bond energy of a covalent bond is the amount of energy needed to break 1 mole of that bond; it
is also the amount of energy released when that bond is made.
Example: calculate the enthalpy change for the following reaction:
CH4 (g) + 2O2 (g)

CO2 (g) + 2H2O (g)
H of reaction = sum of bond energies of reactants - sum of bond energies of products

Bonds broken = (4 x (C-H) + 2 x (O=O)) = (4 x 413) + ( 2 x 498) = 2648 kJ

Bonds made = (2 x (C=O) + 4 x (H-O))= (2 x 746) + ( 4 x 464) = 3338 kJ
H of reaction = 2648 kJ - 3348 kJ
= - 700 kJ
Exercises
Use the following bond energies to complete the exercises below.
bond
H-H
C-H
O-H
C=O
Cl - Cl
C–C
C=C
Section 4b energetic
bond energy kJ/mol
436
413
464
746
243
348
612
bond
H - Cl
N-H
O =O
N N
Br - Br
H - Br
bond energy kJ/mol
432
391
498
945
190
366
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1
H2 (g) + Cl2 (g)  2 HCl (g)
2
N2 (g) + 3 H2 (g)  2 NH3 (g)
3
2 H2 (g) + O2 (g)  2 H2O (g)
4
CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)
5
H2 (g) + Br2 (g)  2 HBr (g)
6
C2 H6 (g)  H2 (g) + C2H4 (g)
Energy level diagram for the first reaction in the exercise above.
We can show the above also in an energy level diagram like the exothermic reaction shown below
involving the formation of water.
or the energy level diagram below which shows us another way of how we can calculate the amount
of energy released in a reaction.
Section 4b energetic
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