2. What you need to be able to do and understand
Energetics in Chemical Reactions Interpret data obtained from
Relate the terms exothermic and experiments concerned with speed of
endothermic to the temperature reaction.
changes observed during Describe the application of the above
chemical reactions. factors to the danger of explosive
Demonstrate understanding that combustion with fine powders (e.g.
exothermic and endothermic flour mills) and gases (e.g. mines).
changes relate to the Describe and explain the effects of
transformation of chemical temperature and concentration in
energy to heat (thermal terms of collisions between reacting
energy), and vice versa. particles (concept of activation
energy will not be examined).
Speed of Reactions Define catalyst as an agent which
Describe the effect of increases rate but which remains
concentration, particle unchanged.
size, catalysis and temperature
on the speeds of reactions. Redox
Describe a practical method for Define oxidation and reduction in
investigating the speed of a terms of oxygen loss/gain, and
reaction involving gas evolution. identify such reactions from given
Devise a suitable method for information.
investigating the effect of a given Define redox in terms of electron
variable on the speed of a transfer, and identify such reactions
reaction. from given information.
3. Endothermic and Exothermic Reactions
During a chemical reaction there is always an energy change
Exothermic: give out energy (heat, light or sound)
reactants → products + energy
The total energy is the same but the products have
lower energy than the reactants.
Endothermic: take in energy
reactants + energy → products
The total energy is the same but the products have
higher energy than the reactants since energy was
taken in.
A song to help you remember the
difference:
http://www.youtube.com/watch?v
=XgiCn1IpvzM
4. Which is which- Endothermic or Exothermic?
• A cold pack (for sport injuries)
• Endothermic
• The combustion of fuels
• Exothermic
• Firing a cannon
• Exothermic
• Cooking a steak
• Endothermic
• Cellular respiration
• Exothermic
• Photosynthesis
• Endothermic
5. Is this an exothermic or endothermic reaction?
Remember energy is measured in kilojoules (kJ)
CaCO3(s) → CaO(s) + CO2(g) the energy change is +178kJ
The + sign shows that energy is taken in
This is an endothermic reaction
Fe(s) + S(s) → FeS(s) the energy change is -100kJ
The - sign shows that energy is given out
This is an exothermic reaction
A little fun
6. Rates of Reactions
A rate is a measure of how fast or slow something is. Reactions can be slow or
fast.
Some fast reactions include: How can you measure the rate of
• Fireworks reaction?
• Wood burning
• Florescent light bulbs In general, to find the rate of a reaction, you
should measure:
Some slow reactions include:
• Rust forming on a car • the amount of a reactant used up/unit
• Food rotting time
• Cellular respiration OR
• the amount of a product produced/unit
time
A song to introduce rates of
reactions
http://www.youtube.com/watch?v=X
X9Xo6zm_kM
7. Factors that change reaction rates
The rate of a reaction depends on how many successful collisions there are in a
given unit of time
If we want to increase/decrease the number of
collisions what would we change?
• Temperature
• Concentration
• Surface area (particle size)
More animations on The Collision Theory:
We will use this animation to http://www.kentchemistry.com/links/Kinetics/FactorsAf
change some variables to see the fecting.htm
effect on the product
http://www.kscience.co.uk/animati
ons/collision.swf
8. Effect of concentration or temperature on the rate of a reaction
If the concentration or temperature of a reactant is increased the
reaction goes faster.
When the reactants have a higher temperature, they move faster, thereby increasing
the chance of a collision.
When the reactants have a lower temperature, they move slower, thereby decreasing
the chance of a collision.
When the reactants are less concentrated there is less chance of a collision.
When the reactants are more concentrated there is more chance of a collision.
An animation on temperature:
Why does this graph have a plateau?
http://www.ltscotland.org.uk/high
ersciences/chemistry/animation
s/collision_theory.asp
9. Effect of surface area (size of particle) on the rate of a reaction
If the surface area of a reactant is increased the reaction goes
faster.
Remember a collision involves surfaces.
Particles that are smaller have more surfaces to react with.
When the reactants are smaller there is more chance of a collision.
When the reactants are larger there is less chance of a collision.
Why does this graph
have a plateau?
10. Effect of a catalyst on the rate of a reaction.
If a catalyst is added the reaction goes faster.
A catalyst is a substance that speeds up a chemical reaction but remains chemically
unchanged itself.
Catalysts lower the activation energy needed to run
the chemical reaction – so it goes faster. The
reactants are able to react in a way that requires
less energy. This means that more collisions now
have enough energy to be successful
Enzymes are
biological catalysts
This animation give a good basic overview of enzymes A cool video showing
http://www.northland.cc.mn.us/biology/biology1111/ani catalysts!
mations/enzyme.swf
11. labs:
The effect of temperature on rate of reaction
The effect of particle size of rate of reaction
How would you design a lab to investigate
either one of these using steradent tablets?
You have 10 min to discuss
this with your neighbour
then you have 15 min to put
something in writing.
What do you think happens
when you dissolve an alka
setlzer tablet in space?
After which time we will
do one of them.
12. What happens when the reaction goes too fast?
BOOM!!!
There have been many examples when DUST (i.e. high surface area) of
flour, sugar, wood, or wheat explode. The dust from all of these can catch
fire and burn. If the reaction goes too fast you can get an explosion.
In mines methane and other flammable gasses can also have explosive
results if they are in high enough concentrations.
13. Oxidation and reduction – REDOX
- it’s all about oxygen
Oxidation: oxygen is gained
Iron is being oxidised to form iron(III) oxide- this is how rust forms.
Fe(s) + O2(s) →Fe2O3(s)
Fe gained oxygen
Reduction: oxygen is lost
If you heat magnesium oxide and carbon you get magnesium metal and
carbon monoxide. Magnesium oxide was reduced to magnesium metal.
MgO(s) + C(s) →Mg(s) + CO(g)
MgO lost oxygen
Two examples: which is which?
1- 2PbO2(s) → 2PbO(s) + O2(g) PbO2 lost oxygen
2- N2(g) + O2(g) → 2NO(g) N gained oxygen
14. But in reality…
Oxidation is accompanied by reduction.
For example, in the extraction of iron from its ore:
reduction So the reaction is
Iron(III)oxide lost oxygen called REDOX
Fe2O3 + 3CO → 2Fe + 3CO2
Carbon monoxide gained oxygen
oxidation
Because they always occur together we can
now define oxidation and reduction a bit
differently:
Oxidation is a loss of electrons. OIL RIG
Reduction is a gain of electrons. OxidationIs Losing
Reduction Is Gaining
IMPORTANT
15. REDOX in pictures When magnesium is burned, magnesium oxide
is formed. Magnesium has clearly oxidised
2Mg(s) + O2(g) → 2MgO(s) (gained an oxygen) but oxidation and reduction
always occur together. So oxygen was
reduced. But how does oxygen lose oxygen?
oxidation We need to look at the electrons.
2 electrons transfer
Mg2+ O2-
giving
Mg O Mg O
During the reaction each magnesium atom loses two electrons and each oxygen atom
gains two. Therefore magnesium was oxidized (loss of electrons) and oxygen was
reduced (gain of electrons).
Writing the half-equations:
Oxidation: 2Mg → 2Mg2+ + 4e-
Reduction: O2 + 4e- → 2O2-
16. REDOX without oxygen
Remember any reaction in which
electrons are transferred is a REDOX
2Na(s) + Cl2(g) → 2NaCl(s) reaction.
electron transfer
giving Cl-
Na+
Na Cl Na Cl
During the reaction each sodium atom loses one electron and each chlorine atom
gains one. Therefore sodium was oxidized (loss of electrons) and chlorine was
reduced (gain of electrons).
Writing the half-equations:
Oxidation: 2Na → 2Na+ + 2e-
Reduction: Cl2 + 2e- → 2Cl-
17. Have you learned anything?
Let’s check out this reaction rate online
simulation
http://phet.colorado.edu/en/simul
ation/reactions-and-rates
