IB DP1 Chemistry
Energetics
Why do chemical reactions get hot or cold?
Topic 5: Energetics (8 hours)
5.1 Exothermic and endothermic reactions
5.1.1 Define the terms exothermic reaction,
endothermic reaction and standard enthalpy change of
reaction ( ∆HÖ ) .
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.
5.2 Calculation of enthalpy changes
5.2.1 Calculate the heat energy change when the
temperature of a pure substance is changed.
5.2.2 Design suitable experimental procedures for
measuring the heat energy changes of reactions.
5.2.3 Calculate the enthalpy change for a reaction using
experimental data on temperature changes, quantities
of reactants and mass of water.
5.2.4 Evaluate the results of experiments to determine
enthalpy changes.
5.3 Hess’s law
5.3.1 Determine the enthalpy change of a reaction that
is the sum of two or three reactions with known
enthalpy changes.
5.4 Bond enthalpies
5.4.1 Define the term average bond enthalpy.
5.4.2 Explain, in terms of average bond enthalpies, why
some reactions are exothermic and others are
endothermic.
Thermochemistry
 Study of energy changes during chemical reactions
 Heat, light, mechanical energy …..
mix
ammonium
nitrate and
water
burn ethanol
See
Think
Wonder
Why do chemical reactions get hot (or cold)?
 What is the difference between energy and enthalpy?
 What is the difference between temperature and heat?
 How much heat does it take to increase the temperature of a
substance?
 What is the difference between enthalpy change and standard
enthalpy change?
 How is calorimetry used to measure enthalpy changes?
 How is standard enthalpy change calculated from a temperature
change?
Exothermic and endothermic reactions
Ammonium nitrate and water
Iron and oxygen
Cold pack
A. The temperature of the cold pack decreases during the
reaction
B. The temperature of the cold pack increases during the
reaction
C. The cold pack transfers cold to the person touching it
D. Heat is taken in from the person to the cold pack
E.
The cold pack has less energy and lower enthalpy after it is
used
Exothermic reactions examples
NaOH(s) + H2O  NaOH(aq) + heat
Exothermic
HCl + NaOH  NaCl + H2O + heat
Neutralisation
Wood + O2  CO2 + H2O + heat
Combustion
Endothermic reaction example
Ba(OH)2(s) + 2 NH4SCN(s) + heat 
Ba2+(aq) + 2 SCN-(aq) + 2 H2O(l) + NH3(aq)
Endothermic
Endothermic or exothermic?
In chemical reactions bonds break and form
different amounts of energy are in the bonds before and after the
reaction
 Exothermic reaction: less energy is in the bonds after the reaction:
heat is produced
 Endothermic reaction = energy is needed
Distribution of speeds of particles
number of
particles
speed
Temperature- average KE per particle
higher average speed  higher
temperature
more particles at same average
speed  same temperature
Distribution of speeds
Enthalpy, H
 Energy stored in chemical bonds of reactants (in Joules)
 PE and KE of particles + energy to make space for substance
Image:
https://en.wikipedia.org/wiki/File
:Ammonium_Nitrate.jpg
Exothermic reaction
CH4 + 2 O2  CO2 + 2H2O + heat
Energy rich
Energy poor
DH = (Energy poor) – (Energy rich)  negative value
Exothermic reactions: DH < O  more stable products
Endothermic reactions DH > O  more reactive products.
Exothermic reactions
Enthalpy, J
reactants
∆H
products
reaction
coordinate
Endothermic reactions
Enthalpy, J
products
∆H
reactants
reaction
coordinate
Standard enthalpy change of a reaction
∆H° to compare reactions
in kJ/mol
measured at 298K and 1atm
Calculating standard enthalpy changes
2 Mg + O2  2 MgO
Exothermic
DH = -1202 kJ/mol
 The amount of energy released when 0.6 g of Mg is burnt?
Mg
m
0.6 g
M
24.3 g/mol
n
0.025 mol
 1202*0.025 = 30 kJ
Calculating standard enthalpy change
What is the enthalpy change in kJ per mole if 45kJ are given out
when 0.8g of methane is burned?
What is the enthalpy change in kJ per mole if 1.6g of methanol is
used to heat 200mL water from 20C to 38C?
Using temperature to calculate ∆Hᶱ
Heat energy = mass x specific heat capacity x temperature change
Q = mc∆T
How much energy is in a cracker?
 1408kJ per 100grams
 7 grams per cracker
 How much will a cracker increase the temperature of 100mL
of water when we burn it?
Calculating uncertainties
How do you calculate uncertainties when:
 adding or subtracting variables?
 multiplying or dividing variables?
Calculate the standard enthalpy change for the reaction
between hydrochloric acid and sodium hydroxide
To calculate the uncertainty in the standard enthalpy for
the reaction between zinc and copper sulphate...
 absolute uncertainty in temperature change =
 percentage uncertainty in temperature change =
 percentage uncertainty in mass of liquid =
 percentage uncertainty in specific heat capacity =
 pecentage uncertainty in enthalpy =
 percentage uncertainty in number of moles =
 total percentage uncertainty =
 total absolute uncertainty in standard enthalpy =
Neutralization- acid-base reaction
Write word and chemical equations for the following reactions:
 hydrochloric acid + sodium hydroxide
 sulphuric acid + sodium hydroxide
 nitric acid + potassium hydroxide
Standard enthalpy of neutralization
H+ (aq) + OH-(aq)  H2O(l)
standard enthalpy change of neutralization for a strong acid and
base is -55.90 kJ/mol
Bond enthalpy
The enthalpy change when one mole of bonds is formed in the
gaseous state:
X (g) + Y(g)  X-Y(g)
Forming bonds is exothermic (negative ∆H)
Breaking bonds is endothermic (positive ∆H)
Calculate a theoretical standard enthalpy
of combustion of methane
Enthalpy of combustion
Bond
Average bond
enthalpy kJ/mol
C-C
347
C=O
746
C-H
413
O=O
498
O-H
464
C-O
358
Image:
https://en.wikipedia.org/wiki/Burning
How does the theoretical standard enthalpy of combustion depend
on the number of carbon atoms for the alkanes?
 Compare your answers with experimental values.
Standard enthalpy of combustion of alkanes
C no. n
1
2
3
4
5
6
7
8
name
methane
ethane
propane
butane
pentane
hexane
heptane
octane
alkane
CH4
C 2H 6
C 3H 8
C4H10
C5H12
C6H14
C7H16
C8H18
ΔH
-890
-1560
-2219
-2877
-3509
-4163
-4817
-5470
name
alcohol
ΔHcomb
methanol
CH3OH
-726
ethanol
CH3CH2OH
-1367
propan-1-ol
CH3(CH2)2OH
-2021
butan-1-ol
CH3(CH2)3OH
-2676
pentan-1-ol
CH3(CH2)4OH
-3329
hexan-1-ol
CH3(CH2)5OH
-3984
heptan-1-ol
CH3(CH2)6OH
-4638
Ammonia, NH3
 Draw a Lewis diagram of the molecule
 State the bond angles and shape
 State whether the N-H bonds are polar, and whether the
molecule is polar
 Calculate the standard enthalpy of formation from data
booklet data
Standard enthalpy change of formation
ΔHfᶱ
the standard enthalpy change when a compound is formed from
its elements
 CH4 -74.4kJmol-1
 CO2 -393.5kJ/mol-1
 H2O -285.8kJ/mol-1
Formation of ammonia
N2 + 3 H2  2 NH3
N N triple bond
944kJmol-1
H-H
436 kJmol-1
N-H
388 kJmol-1
DH = (cost of bond breaking) – (gained for bond forming) =
( 944 + 3*436) – (2*3*388) = -76 kJmol-1
Exothermic
Hess’s Law
 conservation of energy applied to chemistry
 total enthalpy change is the same whatever the route of a
chemical reaction
Combustion of carbon to form Carbon dioxide
 C + ½ O2  CO
DH1 = -110kJmol-1
 CO +½ O2  CO2 DH2 = -283kJmol-1
 C + O2  CO2
DH3 = DH1+DH2 =
Plan investigation…
Links
 Ionic bonding
http://www.teachersdomain.org/asset/lsps07_int_ionicbondi
ng/
 Covalent bonding
http://www.teachersdomain.org/asset/lsps07_int_covalentbo
nd/