Chapter 4. Controlling Chemical Reactions
Introduction: Examples of chemical reactions
• Camp fires
• Baking a cake
Activity 4A
Keeping in mind the three components of fire (heat, fuel, air), suggest
ways that you could slow down the rate of a burning camp fire.
4.1 Chemical Energy
Chemical energy is energy that is stored in the bonds between
atoms in a chemical compound.
The food you eat has chemical energy in them. The cells of your body
take these chemicals and use them in special, controlled reactions.
These reactions release energy to run the machinery of the cells an
ultimately, the body.
When speaking of reaction energy, there are two flavors:
Endothermic – (endo means ‘in’) where heat or energy is used in
order to make the reaction work.
• Baking a cake – thermal ← in
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• Frying an egg.
• Taking a picture – (how so?)
• Charging a battery
Exothermic – (exo means ‘out’) where energy is released as a result
of the reaction.
• Burning wood – thermal, light → out
• Explosions – thermal, light, mechanical →out
• Using a battery
Question: Why do explosions release mechanical energy and burning
wood doesn’t? Aren’t they both examples of burning?
Starting a chemical reaction
we know that in endothermic reactions we have to add heat in order
for the reaction to happen…
But…
…why do exothermic reactions like striking a match need a little bit of
energy to start the reaction going?
Because all chemical reactions are a result of
• First breaking chemical bonds
• Second making new bonds
The ‘start-up energy’ breaks bonds; the energy which comes out
comes from the reformed bonds.
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EXAMPLE: the match revisited and the energy hill
Energy and Light
Nearly all energy on earth originates with the sun. Plants absorb light
energy and transform it into chemical energy through the process of
photosynthesis. This is a very efficient process.
CO2 + H2O +
hν
→
light energy
C6H12O6
+ O2
chemical energy
Both plants and animals use glucose in a process called respiration
to run the machinery of their cells (basis for life). So sunlight is the
origin for all energy in almost every food chain in the world. Without
the sun we would die.
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Even fossil fuels come from the sun since they are just accumulations
of the energy from growth of plants and animals, heated and
pressurized.
Energy and the environment
…somehow we need to learn how to efficiently use the energy from
the sun to run the machinery of society.
Why?
Ways to Affect Reaction Rates
reaction rate – how fast or how slow (speed) that a reaction takes
place at.
Sometimes it is good to slow down a reaction rate –
• rusting of iron
• spoilage of food
Sometimes it is good to speed it up –
• cooking hamburgers - fast but not too fast. Why?
• headache tablet - fast but not too fast. Why?
In industry it is important to make some reactions go fast, keep others
slow and to stop some altogether: this is the work of industrial
chemists. They work out how to get the quickest results, the most
products with the highest purity with the least energy costs.
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Temperature
In endothermic reactions the reaction will go faster (rate increases) if
you add heat.
In exothermic reactions the reactions will go faster (rate increases) if
you cool it (remove heat).
Activity 4C Hot and Cold Alka Seltzer
Is this an endothermic or exothermic reaction?
Surface area
For two substances to react, they must come into close contact. The
greater the surface area, the greater the chance for close contact.
Consider the following:
Which has greater surface area?
a) 1 block of potato which is 2cm X 2cm X 2cm for a total volume of
8cm3
Or
b) 8 blocks of potato each is 1cm X 1cm X 1cm for a total volume of
8cm3?
Answer:
a) 1 cube X 6sides X (2cm X 2cm) = 24 cm2
b) 8 cubes X 6 sides X (1cm X 1 cm) = 48 cm2
Which will cook faster? Which will have the greatest reaction rate?
Concentration of a solution
The higher the concentration of a solution, the faster the reaction
proceeds.
Activity 4E Testing Predictions About Surface Area And Concentration.
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