Chapter 6: Observing Chemical Change
Chemistry – the study of matter and how matter changes
Physical property – a characteristic of matter that can be observed without changing the
chemical identity of the matter
Examples: melting point, boiling point, color, hardness, texture, luster, flexibility, solubility,
and conductivity
Equation: H2O (s) → H2O (l)
Chemical property – a characteristic of matter that can be observed only by changing the
chemical identity of the matter
Examples: burning, tarnishing, rusting, oxidation, reduction, and decomposition
Equation: 2 H2O (g) → 2 H2 (g) + O2 (g)
Chemical changes occur when bonds break and new bonds form, in other words, when a
chemical reaction occurs
Evidence for a chemical reaction
There are two main kinds of changes that can be observed
Changes in energy: endothermic (absorbs heat) or exothermic (releases heat) reactions
Formation of new substances
Indicators of a chemical change:
Beware of physical changes like:
Heat or light given off
Dissolving acids is very exothermic
Evolution of a precipitate
Crystals form upon cooling or evaporation
Evolution of a gas
Boiling is not a chemical change
Color change
Dichroism in cordierite
Odor changes
Vaporization of mixtures at differing temperatures
Chemical equation – a shorthand method of representing a chemical reaction
Structure of an equation
Reactants – substances used at the beginning to start a chemical reaction
Products – new substances that are generated by a chemical reaction
Example: Reactant + Reactant → Product + Product
BaCl2 + Na2SO4 → BaSO4 + 2 NaCl
Read as: Barium chloride plus sodium sulfate yields barium sulfate and sodium chloride
Chemical equations can fall into three categories
Word equation: names of reactants and products combined with symbols to represent a
chemical reaction
Formula equation: use of correct formulas and symbols to represent a chemical reaction
Balanced equation: addition of coefficients to a formula equation to represent a chemical
reactions
Because atoms are neither created nor destroyed in a chemical reaction but rather are
rearranged, balanced equations must accurately represent the conservation of matter
Open system – matter can enter or escape to the surrounding environment
Closed system – matter can neither enter from nor escape to the surrounding environment
Balancing chemical equations
It is best to balance reactions in a step-by-step process
Change the sentence into a word equation
Write correct formulas for each reactant and product – do not alter correct formulas
Use coefficients in front of formulas to balance the equation – conservation of matter
Example: Balance the equation for the addition of zinc metal to silver nitrate which reacts to
form zinc nitrate and silver metal.
Word equation:
zinc + silver nitrate → zinc nitrate + silver
Formula equation:
Zn + AgNO3 → Zn(NO3)2 + Ag
Balanced equation: Zn + 2 AgNO3 → Zn(NO3)2 + 2 Ag
If you have trouble balancing an equation (the last step), use atom tracking to help you
Track atoms by making a list of the atoms and polyatomic ions (if ions do not change):
Formula equation: Zn + AgNO3 → Zn(NO3)2 + Ag
Zn
Ag
NO31–
Add numbers that total the number of atoms of each element for products and reactants:
Formula equation: Zn + AgNO3 → Zn(NO3)2 + Ag
1 Zn 1
1 Ag 1
1 NO31– 2
Adjust the number of nitrate ions on the reactant side:
Formula equation: Zn + 2 AgNO3 → Zn(NO3)2 + Ag
Recalculate the atom tracking:
Formula equation: Zn + 2 AgNO3 → Zn(NO3)2 + Ag
1 Zn 1
2 Ag 1
2 NO31– 2
Correct the number of silver atoms in the product side:
Formula equation: Zn + 2 AgNO3 → Zn(NO3)2 + 2 Ag
Recalculate the atom tracking:
Formula equation: Zn + 2 AgNO3 → Zn(NO3)2 + 2 Ag
1 Zn 1
2 Ag 2
2 NO31– 2
The tracking shows the equation is now balanced:
Balanced equation: Zn + 2 AgNO3 → Zn(NO3)2 + 2 Ag
Classifying chemical reactions
In order to complete chemical equations before balancing, it is helpful to classify the reaction
and match it to its general equation
Type of Reaction
General Form
Examples
1. Synthesis
A + Y → AY
2 H2 + O2 → 2 H2O
2. Decomposition
AY → A + Y
H2CO3 → CO2 + H2O
3. Single-displacement
A + BY → AY + B
Zn + CuSO4 → Cu + ZnSO4
4. Double-displacement AX + BY → AY + BX
AgNO3 + NaCl → NaNO3 + AgCl
5. Combustion
CH + O2 → H2O + CO2 CH4 + 2 O2 → CO2 + 2 H2O
Example: Write a balanced equation for the reaction of iron(II) sulfide with hydrogen chloride.
Word equation: iron(II) sulfide + hydrogen chloride →
Identify type:
AX
+
BY
→ double replacement
AX
+
BY
→
AY
+
BX
Finish type:
Word equation: iron(II) sulfide + hydrogen chloride → iron(II) chloride + hydrogen sulfide
Formula equation:
FeS
+
HCl
→
FeCl2 +
H2S
H2S
Balanced equation:
FeS
+
2 HCl
→
FeCl2 +
Controlling chemical reactions
Activation energy – the minimum amount of energy needed to start a chemical reaction
All chemical reactions need a certain amount of activation energy to bet started
The activation energy is needed to break the bonds in the reactants (always an endothermic step)
before new bonds can form the products (always an exothermic step)
If the activation energy is greater than the
If the activation energy is less than the
energy released when the products are
energy released when the products are
formed the overall reaction will be an
formed the overall reaction will be an
endothermic reaction
exothermic reaction
Rates of chemical reactions can be controlled by changing factors such as:
Surface area
Increasing the surface area of the reactants increases the number of collisions of particles in a
reaction so the speed of the reaction will increase
Temperature
Svante Arrhenius wrote a rate equation that shows reaction rates near room temperature will
(approximately) double for every 10° C increase in the temperature
Concentration
Increasing the concentration (number of particles in a given volume) of the reactants increases the
number of collisions of the particles and so the rate of the reaction will increase
Catalysts
Catalysts increase the rate of a reaction by lowering the activation energy which means more
particle collisions will have enough energy to react
Catalysts are not permanently changed by a reaction and so are not considered reactants
Enzymes are biological catalysts that usually affect only one specific chemical reaction
Inhibitors
Inhibitors slow reactions by combining either temporarily or permanently with one of the reactants
Inhibitors include preservatives added to food to prevent them from spoiling
Fire and fire safety
Combustion equation – CH4 + 2 O2 → CO2 + 2 H2O
From the equation, it is clear that combustion requires fuel and oxygen (air)
So why doesn’t methane gas start burning in air as soon as it escapes?
It requires enough heat to supply the activation energy
Once a fire starts, the heat from the flame will supply the activation energy
Combustion actually requires three things: Fuel
Oxygen
Heat
From: https://en.wikipedia.org/wiki/Fire_triangle
Controlling fire
How does water put out fires?
A layer of water prevents oxygen from reaching the fuel
Evaporating water cools the fuel removing activation energy
Use a cover to smother a grease fire in a skillet
Why should you never use water on a grease fire? (Grease floats and the fire will spread.)
What can you use on a grease fire? (Baking soda forms CO2 when heated.)
Fire prevention and safety
Common sources of home fires
Small heaters
Flammable liquids near pilot lights for furnace or water heater
Cooking
Electrical fires
Cigarettes – start the fires that cause the most deaths
Prevention
Keep flammables away from flames (water heaters and furnaces)
Use smoke detectors on every floor
Keep matches and candles out of reach of children
Have a fire extinguisher in the kitchen
Keep baking soda handy near the stove