Intensive Chemistry
Day 3: Chemical Reactions
Katy Johanesen
Ph.D. Candidate, USC
Department of Earth Sciences
Chemical reactions
• A process where original chemical substances
(reactants) transform to new chemical
substances (products)
Reactant 1 + reactant 2 ↔ product A + product B
• Chemical reactions are often written as
chemical equations (using element symbols
from the periodic table)
HCl + NaOH ↔ NaCl + H2O
Chemical reactions
• New substances often have different properties than the
original substances
• HCl is sour acid and can dissolve metals, NaOH is bitter base; NaCl
is salty in taste and unreactive
• Substances can be solid (s), liquid(l), or gas(g)
• Sometimes we represent this with letters in parentheses
• Zn (s) + 2HCl (aq) = ZnCl2 (s) + H2 (g)
• Substances in water solution are noted (aq)
• Reactions can be reversible
• 2H2O ↔ 2H2 + O2
• With electricity, water can convert to hydrogen and oxygen gas
• With fire, hydrogen and oxygen gas can form water
Charge balancing
• During all chemical reactions, electric charges
must balance to equal 0 net charge
• Zn + 2HCl  ZnCl2 + H2
• Every reactant and product has 0 charge
• While H is 1+, Cl is 1-, HCl = 0 charge
• Let’s try it. Don’t worry about balancing the number
or atoms yet
HF (aq) + SiO2 (s)  Si?F ? (g)+ H2O (l)
Conservation of mass
• Mass in = mass out.
•
no new matter is created or destroyed
• Volumes and densities of reactants and products
may change, but masses on both sides of the
reaction are always equal
•
•
In other words, you should have the same number of
each type of atom on both sides of the equation
Gases tend to leave the area of reactions, making it look
like matter disappears
_HF (aq) + _SiO2 (s) _SiF 4 (g) + _H2O (l)
Conservation of mass
• Mass in = mass out.
•
no new matter is created or destroyed
• Volumes and densities of reactants and products
may change, but masses on both sides of the
reaction are always equal
•
•
In other words, you should have the same number of
each type of atom on both sides of the equation
Gases tend to leave the area of reactions, making it look
like matter disappears
4 HF (aq) + SiO2 (s) SiF 4 (g) + 2H2O (l)
Breaking down a reaction equation
• We write full reaction equations for simplicity:
• Zn (s) + 2HCl (aq) = ZnCl2 (s) + H2 (g)
• But in reality, we form ions in water first:
• Hydrochloric acid is made by dissolving hydrogen chloride gas in
water:
• HCl (g) + H2O (l) = H3O+ (aq) (simplified as H+) + Cl- (aq)
• A more detailed equation could be stated as:
• Zn + 2H3O+ + 2Cl- = ZnCl2 + H2 + 2H2O
• Products and reactants do not = 0 charge, but helps to explain what is
happening (water is not usually written in the equation, it is implied it
is there by the (aq) )
Half-reaction equations
• Some materials can change electric charge in reactions
• Zinc metal (all metals) has zero charge (Zn0)
• When acids dissolve metals, the electrons from the metals transfer to
cancel out positive charge (H+) from acids
• Zn0 + 2H+ = Zn2+ + H2 0 (remember: 0 – [-1] = +1)
• This can be written has 2 “half-reactions”:
• Zn0 - 2e- = Zn2+
• 2H+ + 2e- = H2 0
• We do not usually see electrons in a final equation because
they do not have a charge of 0 – and they ALWAYS cancel out:
• [Zn0 - 2e-] + [2H+ + 2e-] = Zn2+ + H2 0
Simplify your life…
• You can see how complex a chemical reaction
really is:
[Zn0 - 2e-] + [2H3O+ + 2e-] + 2Cl- = ZnCl2 + H2 + 2H2O
Zn + 2HCl = ZnCl2 + H2
What’s going on in this reaction?
http://www.youtube.com/watch?v=OXxyrcPdmpA
_CaCO3(s) +_HCl(aq) → _CaCl
_?(aq)2(aq)
+ _CO
+ _CO
_H
+2_H
O(l)2O(l)
2(g) +
2(g)
Energy in chemical reactions
• Reactions may be spontaneous or may involve
the input of energy (activation energy)
• Combustion of gasoline to CO2 needs a spark plug
• Spontaneous reactions don’t require activation energy
• Exothermic vs. endothermic
– Exothermic = reaction that releases energy in the
form of heat (system gets hotter)
– Endothermic = reaction that absorbs energy
(heat) (system gets colder)
Bond energy
• Breaking bonds in chemical compounds
requires energy
– When you break the bond, energy is absorbed
– When you form a bond, energy is released
– The amount of energy it takes to break that bond
is exactly the same as the amount of energy
released when that bond is formed. This value is
called the bond energy. Different bonds have
different energies.
The Thermite Reaction
Fe2O3 + 2Al ↔ Al2O3 + 2Fe + heat
Exothermic
Enough heat to melt the Fe metal!
Also requires high activation energy
http://www.youtube.com/watch?v=o8gapa8ibK0
• The reverse of this
reaction is why we should
recycle
It takes large amounts of heat
to break bonds with Al
Exothermic Reaction
• K(s) + H2O(l)  KOH(aq) + H2 (g)+ heat
• H2 is released as a gas, so remaining solution
becomes a base (depleted in H+ ions)
http://www.youtube.com/watch?v=hiueYVhFTlk&feature=related
• Phenolphthalein turns pink in bases
• H2 is flammable!
• http://www.youtube.com/watch?v=uixxJtJPVXk
(a good video for alkali metal reactivity)
Endothermic Reaction
• Citric Acid + Sodium Bicarbonate
H3C6H5O7(aq) + 3 NaHCO3(s) + heat → 3 CO2(g) + 3H2O(l) +
Na3C6H5O7(aq)
A more comlicated, but visually appealing endothermic reaction:
http://www.youtube.com/watch?v=MyAzjSdc3Fc
Is this reaction spontaneous, or does it have an
activation energy?
Why?
• Energy of bonds broken ≠ Energy of new
bonds formed (enthalpy)
IF energy of bonds broken > energy of new bonds formed
What happens?
IF energy of bonds broken < energy of new bonds formed
What happens?
Bond Energy
Single Bond Energies (kJ/mol) at 25°C
H
C
N
O
S
F
Cl
H
C
N
O
S
F
Cl
Br
I
436
414
389
464
339
565
431
368
297
347
293
351
259
485
331
276
238
159
222
—
272
201
243
—
138
—
184
205
201
201
226
285
255
213
—
153
255
255
—
243
218
209
193
180
Br
I
http://chemistry.about.com/od/chartstables/a/bondenergytable.htm
151
Endo- or Exothermic?
H2 (g) + Cl2 (g) → 2 HCl (g)
What is the change in Enthalpy?
Red cabbage as a pH
indicator
• Contains flavin, an
anthocyanin group compound
• Chemical bonds are H+ and
OH- sensitive
If you add and subtract H+ and
OH-, you change the bonding
configuration in the molecule to
absorb different colors of light
anthocyanin group
http://www.erowid.org/archive/rhodium/chemistry/equipment/pictures/ph-cabbage.jpg
Thank You
• Please email me with questions!
johanese@usc.edu
• I can help with Chemistry, Earth Science, or Physics.