Study Buddy: Energy in Chemical Changes
Terms
Chemical Change
Chemical Reaction
Law of Conservation
of Energy
Description
Intensive properties usually change; involves
bonds breaking and/or forming
Represents chemical changes symbolically
States: (s), (l), (g), (aq)
+: between 2 or more reactants or products
: shows direction of rxn; reactant  product
Heat: written as reactant/product or over arrow
Total energy of system cannot change, but it can
change form
Symbol (Unit) & Picture
+

Teacher Note: Equation is unbalanced
http://www.mhhe.com/physsci/chemistry/chang7/es
p/folder_structure/cr/m1/s2/
Stored – chemical potential E, thermal (kinetic) E
Transferred in/out – heat, light, sound
http://etnarufiati.blogspot.com/2009/09/calorimetry-task.html
Enthalpy change can be measured as difference
between chemical PE of reactants and products
Enthalpy change
Collision Theory
Dynamic equilibrium
Catalyst
∆H (J/mol or kJ/mol)
Endothermic: Chem PE of products > reactants;
+∆H
Exothermic: Chem PE of products < reactants;
-∆H
http://www.thinglink.com/scene/508706339329933314
To make or break bonds in a chemical reaction,
reactant particles have to collide with sufficient
energy and correct orientation for an effective
collision
http://collisiontheoryperiod4.blogspot.com/
Rate of forward and reverse reactions are equal;
overall amount of products and reactants are
constant; can be seen in simulations of closed
systems
http://uoitbiology12u2014.weebly.com/dynamic-equilibrium.html
Increase reaction rate by decreasing activation
energy and assisting orientation of reactants
http://www.chem4kids.com/files/react_catalyst.html
Representations:
1. Examine the reactions below to determine if they represent a chemical or a physical change
and whether they are endothermic or exothermic. Then draw a particle diagram.
a. H2O(l) + energy  H2O(g)
Physical change, endo
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∆
b. 2H2O(l)  2H2(g) + O2(g) c. H2 + Cl2 2HCl + 184 kJ/mol
Chemical change, endo
Chemical change, exo
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2. How are particle diagrams different for chemical and physical changes?
Chemical changes involve changing the way atoms are bonded to each other, but physical
changes only change how far apart the particles are from each other. In a physical change, the
particles are bonded the same way before and after the reaction.
The combustion of methane (CH4) in oxygen requires 634
kcal/mol to break the existing bonds and releases 828
kcal/mol when the new bonds are formed.
3. Label reactants, products and activation energy.
4. Write in energy terms and calculate the enthalpy of the
reaction. ∆H = -194 kcal/mol
5. Draw particle diagrams to represent the reactants and
products.
6. Does the graph represent an endothermic or
exothermic reaction? How do you know?
Exothermic reaction because the energy of the products
is less than the reactants, so energy has been released to
the surroundings.
7. Draw the effect of adding a palladium catalyst that could be used for methane combustion. Explain
what this catalyst would change in the reaction.
The catalyst would reduce the activation energy needed for the reaction. This means less energy
would be required in order for methane to combust.
8. Why do chemists carry out more reactions with gaseous or aqueous reactants than with solids?
Increased particle movement in gases and aqueous solutions allows more particle interactions
and more reactions to occur.
9. Explain the energy transformations and law of conservation of energy in the burning of coal.
C(s) + O2(g)  CO2(g) + 393.5 kJ
The reaction is exothermic and releases 393.5 kJ of energy to the surrounding for every mole of
coal. Energy released is KE in the forms of light and thermal energy and came from the PE of the
reactants. The law of conservation of energy is observed because the total energy in the reactants
is present in the products; some PE is in CO2, some energy is “lost” from the products and cannot
be used to do work.
10. Explain changes in reaction rate due to stirring, increasing temperature and increasing
concentration using the kinetic molecular theory and collision theory.
Stirring – Stirring increases the number of collisions between particles and causes more reactions
to occur faster
Increasing Temperature –Particles have more kinetic energy, are moving faster, collide more, and
react faster
Increasing Concentration – There are more particles per given volume, which are colliding more
often and react faster
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