Ch 4 REVIEW –
Controlling Chemical Reactions - ANSWERS
Sc 9
1
1. Describe the difference between an exothermic and endothermic reaction.
An exothermic reaction will release energy (so you will feel heat) while an
endothermic reaction will absorb energy (so it might feel cold to the touch).
2. Give at least two examples of each type of the above reactions.
Endothermic reactions: photosynthesis, CHARGING batteries, cooking food, “cold
packs”, producing an image on film, etc.
Exothermic: burning anything, explosions, USING batteries, cellular respiration,
etc.
3. Where does the energy go (or come from) when it is released or used in a
chemical reaction? The energy is trapped in the bonds that hold the chemical
compounds together. When energy is released, it is being released from the
bonds of the compound.
4. What molecule do we use for energy? Where is the energy stored in this
molecule? Where, initially, did this energy come from? Our bodies use glucose
for energy. The energy that we use is released from the bonds that hold the
carbon, hydrogen and oxygen together in the glucose molecule. We release that
energy when we do cellular respiration.
Initially, the energy was trapped in the bonds of the glucose molecules by
PLANTS. They trapped the energy from the SUN.
5. Why doesn’t paper just burn up right away? There is lots of oxygen to combine
with… Most reactions do not occur spontaneously. Whether they are
endothermic OR exothermic reactions, most reactions need a bit of
ACTIVATION energy to get the reaction started. This energy is used to start
breaking apart the bonds of the reactants, so they can rearrange themselves in
the reaction and make the products. Once the initial activation energy is
provided (like striking a match), a combustion reaction like burning paper will
continue on it’s own.
6. Approximately how much energy does a peanut release per gram? How much for
a normal 500g bag? A normal set of peanuts release about 12000 J (12 kJ) of
energy per gram. Therefore, a 500 gram bag would release about 6000kJ of
energy. (12kJ x 500 = 6000kJ)
7. How could you measure how much energy is in a potato chip? You could use the
same method that we used in the lab to measure the energy from a peanut. You
would burn the chip under a beaker of water and measure how much the
temperature increased as it burned. Then you calculate the amount of energy
that was released using the formula E = mass x heat capacity x temperature
change.
Ch 4 REVIEW –
Controlling Chemical Reactions - ANSWERS
Sc 9
2
8. What does rate of reaction mean? It is the speed at which new products are
formed in a chemical change.
9. What 4 factors can affect the rate of a chemical reaction, and how is the reaction
affected by each one of the factors?
i)
temperature – if you increase the temperature, you will increase the
reaction rate until the maximum rate is obtained. If you decrease the
temperature, the rate will also decrease until it stops.
ii)
Concentration: if you increase the concentration of the reactants,
you will increase the reaction rate until maximum is achieved.
Decreasing the concentration of the reactants decreased the rate.
iii)
Surface Area: Increasing the surface area of the reactants increased
the reaction rate until maximum is achieved. Decreasing the surface
area of the reactants also decreases the reaction rate.
iv)
Presence of a catalyst. Adding a catalyst for the reaction will
increase the reaction rate until the catalyst is working at it’s full
potential. Taking away some catalyst will reduce the rate of the
reaction.
10. What does concentration mean? How would you increase the concentration of
your orange juice? Concentration is the amount of solute (the solid part of the
solution) dissolved in a certain amount of solvent (the liquid part of the
solution). It is a measure of how “strong” the solution is. If you wanted to
increase the concentration of your orange juice, you would have to add more
orange pulp, or take away some of the water (dehydrate it!).
11. What is an enzyme? How do they work? An enzyme is a biological catalyst.
They are proteins. They help chemical reactions by reducing the amount of
activation energy they need to get started. Enzymes are never used up in the
reaction, they are just there to help out. Therefore, they can be used over and
over again. That is why you only need a small amount of enzymes to do a very
large job – they just keep working until the reactants are all gone.
.
12. Why is it so important for us to maintain the conditions that will make our
enzymes run effectively? We depend on our enzymes to control ALL the
chemical reactions in our body. If we can’t perform chemical reactions, we
DIE. INSTANTLY. Therefore, we spend a whole lot of energy keeping the
conditions that make our enzymes happy – 37.5 oC, not too acidic, not too basic
etc.
13. What is surface area? How would you increase the surface area of a particular
reactant? Surface area is the space that is in contact with the outside. It is the
area that covers the outside of the object. It’s “surface”. To increase the surface
area of an object, you have to make smaller pieces of the object – grind it up, or
crush it into a powder.
14. For combustion to occur properly, what reactant must be available?
Ch 4 REVIEW –
Controlling Chemical Reactions - ANSWERS
Sc 9
3
You must have a good supply of oxygen, the main reactant (other than the fuel) in
a combustion reaction.
15. What happens to a burning piece of wood if you only supply it with carbon
dioxide? If you only have carbon dioxide, the combustion reaction will stop. The
fire will go out, since it needs oxygen to continue.
16. What happens if you raise the temperature of your body too much? Why?
When you raise the temperature of your body too much, you “cook” your enzymes.
Since enzymes are proteins, they undergo a chemical change just like a piece of
meat would do at higher temperatures. Once you “cook” your enzymes, they don’t
work any more. If you cook too many of your enzymes, you die, since you can no
longer perform the chemical reaction essential to life. Of course, if you continue to
raise the temperature of your body, it will burn, which also would not be pleasant.
17. What is an alloy? An alloy is a mixture of metals in one sample. An example of
an alloy is brass – copper and zinc. They make alloys to increase the strength of
some metals, or to help resist corrosion of the metal “Stainless Steel” is an alloy
that does not corrode (rust) as easily as iron. It will “stain” less than iron, but is
will still “ stain” – otherwise they would call it stain-free steel!
18. What is corrosion? Why is the corrosion of iron so much more destructive than
the corrosion of copper or aluminium? Corrosion a chemical reaction between
metal and some other reactant. It will use up the metal, destroying the strength
of the object and it’s properties. Iron corrodes “rusts”, much more quickly than
copper or aluminium, since the product of iron corrosion - rust – actually
increases the rate of the corrosion reaction.
However, the product of corrosion for both copper and aluminium creates a film or
coating on the surface of the metal, and that coating prevents any further corrosion
underneath it.