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MAY 1, 2016







CHEMISTRY C2 TOPICS 5&6
Topics:
- Endothermic/Exothermic reactions
- Rates of reaction
- Collision Theory
- Chemical Calculations



ELLIOT HUNT



Exothermic Reactions
When we observe the temperature in a chemical reaction to RISE, this is because the
reaction has GIVEN OUT HEAT energy to surroundings

Examples:
- Combustion
- Respiration
- Neutralization
- Metal and acid
- MOST



Endothermic Reactions

When we observe the temperature in a chemical reaction to FALL, this is because the
reaction has TAKEN IN HEAT energy from
surroundings

Examples:
- Photosynthesis
- Thermal Decomp
- Dissolving

- Strange reactions


























Explaining Endothermic and Exothermic Reactions

Breaking Chemical Bonds
- Breaking chemical bonds is the first step of any reaction
- It requires energy
- It is an ENDOTHEMRIC process (draws in heat to break bonds)



HEAT IN
Reactants

Energy needed to overcome bonds

Making Chemical Bonds
- The second step of any reaction
- Releases heat – EXOTHERMIC



Un bonded atoms

HEAT OUT








Un bonded atoms


Products




Whether a reaction is endothermic or exothermic depends on the difference between the
energy needed to break bonds and the energy released when new bonds form
...

• in exothermic reactions the reactants are higher than the products
• in endothermic reactions the reactants are lower than the products


Exothermic







- Reactants have chemical

energy

Less Stable


- Energy is given off



- Products have less energy

than the reactants





More Stable







Means “delta H” means Change in energy
...











= -




Endothermic






Delta h is positive because

products have more energy
Less Stable

than reactants








More Stable







- If less energy is released when new bonds form than is required to break them
overall, energy is being taken in so is endothermic





Burning methane (exothermic)
CH4 + O2 = CO2 + H2O
















=+



Calculating Bond Energy change

Reaction energy change can be calculated using average bond energies, the amount of
energy required to break a bond
...


-

The collision must have enough energy to break bonds in order for reaction to occur
...

The more successful collisions there are, the greater rate of reaction


Concentration
Concentrated bleach will remove stains faster than dilute bleach because there are more
acid particles in the solution therefore more collisions






Surface Area
Powered calcium carbonate will react faster than lump calcium carbonate because only top
particles can collide, by increasing surface area more particles are exposed to react
...
When warm, particles also collide with more energy
...
Because the magnesium’s has a higher surface area meaning more particles are
exposed to react with acid and available to collide
...





Catalysts
-


A catalyst is a substance that speeds up a reaction without getting used up in the
process
...

Different catalysts work in different ways, but most decrease the reactions activation
energy (energy needed for reaction to start)



Everyday catalyst
- Many are transmission elements or metal compounds
...
As more reactant is used
less is avalible to collide
...
Ar is the mass number of atom
...

The Ar of carbon is 12 and the Ar of oxygen is 16
...

Example 2:Find the Mr of sodium oxide (Na2O)
...

So the Mr of sodium oxide is (23 x 2) + 16 = 62
...
/ "0","%& =
×100%
A@

Example:

Water, H2O

RFM (Mr) : 1+1+16 = 18

E×F
Percentage of compound mass hydrogen: ×100 = 11
...
8%

Percentage Yield


Yield means the amount of useful product obtained from a chemical reaction
...
#"$&L$'0 QL"0M
RS
e
...
Found from mass in g
...


Method:
- Write down mass of each element
- Divide mass by atomic mass of element
- Dived by the smallest number to give ratio
- Use ratio to obtain formula



Reacting mass Calculations


If you know the balanced equation of a reaction, you can calculate masses of products and
reactants from one known mass

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