Lesson 9: Chemical Reactions

Objectives:

  • to describe the evidence for chemical changes
  • to differentiate between endothermic and exothermic chemical reactions
  • be able to translate word equations to balanced chemical equations

Timeline:

  • 2 classes

Advance:

How do we know a chemical reaction has taken place? In class, we will be doing a demonstration. In preparation for that, come up with some physical evidence that would indicate a chemical reaction. Watch this youtube clip. What forms of energy are released in the reactions shown?

Content:

Chemical reactions occur when two or more substances react, and form new substances. We call the initial substances the reactants, and the new substances the products. All reactions involve a change in energy, like a temperature change, a light, a sound, or a current (electricity). We can also have chemical changes producing odours, colour changes, a gas being produced, or the formation of a solid (also called a precipitate). If one or more of these things have happened –  you have a chemical reaction!

Let’s talk more about the energy changes in reactions. When a reaction releases energy, we call it exothermic (think explosion). For example, anything that is on fire is an exothermic reaction, because we can feel the heat, and see the light coming from it. Another example is cellular respiration. Our cells burn sugar to produce energy. Other reactions require energy in order to get started. We call these endothermic. One way to tell if a reaction is endothermic is if you can feel the container get cold. An example of an endothermic reaction is splitting water into hydrogen and oxygen. In ancient times, water was considered an element, because they couldn’t split it. It took the invention of electricity before they figured out it was made of two elements. Photosynthesis is another endothermic reaction. Plants use the sun’s energy to combine water and carbon dioxide into sugar.

Remember, in every reaction, energy is conserved. This means that the energy is neither created, nor destroyed. It’s transferred from one form to another. This is otherwise known as the Law of Conservation of Energy. Energy is needed to break chemical bonds, and energy is released when new bonds are formed.

For example: Energy is needed to break the bond between hydrogen and oxygen in a water molecule. This is endothermic. It takes more energy to break the bonds than is released by forming new bonds.

energy + H2O H2 + O2

Energy is released when hydrogen and oxygen form a new bond to create water. This is exothermic. It takes less energy to break the bonds than is released by forming new bonds.

H2 + O2 H2O + energy

This is the idea behind the hydrogen cell battery.

Writing Chemical Equations: We can represent chemical equations kind of like a math equation. For example:

Take the simple reaction: if you add hydrocloric acid to sodium hydroxide, you produce water and table salt.

We can represent this with chemical symbols instead of words.

HCl + NaOH –> H2O + NaCl

Note:

  • we use the element symbols from the periodic table
  • we use plus signs (+) to separate compounds
  • we use an arrow ( –> ) to show what is produced
  • reactants are on the right, products are on the left
The last thing we need to do, is add states after each compound. We use (s) for solid, (l) for liquid, (g) for gas, and (aq) for a solution dissolved in water (aqueous)
So:
HCl(aq) + NaOH(aq) –> H2O(l) + NaCl(aq)
Balancing Chemical Equations:
Writing a chemical equation is one step, but we might need to know how much of each element will react. The way we do that is by balancing the equation. Since we know that mass is conserved (mass can’t be created nor destroyed), we know that everything we have the reactions, we must have in the products. Let’s check our example reaction above:
Reactants Products
H = 2 H = 2
Cl = 1 Cl = 1
Na = 1 Na = 1
O = 1 O = 1
Ok, well that one was pretty easy, it was already balanced. Let’s try a harder one:
Solid aluminum sulphate mixes with aqueous potassium nitrate producing aqueous aluminum nitrate and solid potassium sulphate
We would write it out like this:

Al2(SO4)3(s) + KNO3(aq) –> Al(NO3)3(aq) + K2SO4(s)

Ok let’s recall a few things:
  • We must make sure each compound is formed according to our ionic compound rules
  • We put brackets around polyatomic ions to show that it’s one unit
  • When counting how many of each element there are, we must multiply the number outside and to the right of the bracket by each atom inside. So for example, in aluminum sulfate there are 2 aluminums, 3 sulphurs and 12 oxygens. You can think of it visually like this:

aluminum sulphate

Let’s compare how many atoms we have on the products side to the reactants side:

Products Reactants
Al = 2 Al = 1
SO4 = 3 SO4 = 1
K = 1 K = 2
NO3 = 1 NO3 = 3
As you can see, they are unequal, so the equation is unbalanced. We fix this by adding coefficients to the beginning of the compound that is unequal. The coefficient means you multiply each element by that number, equaling how many there are in total.
Let’s multiply our aluminum on the reactants side by 2:
Al2(SO4)3(s) + KNO3(aq) –> 2Al(NO3)3(aq) + K2SO4(s)
This means we now have 2 aluminums and 6 (remember to multiply by the number outside of the brackets) NO3’s. Our table now looks like this:
Products Reactants
Al = 2 Al = 2
SO4 = 3 SO4 = 1
K = 1 K = 2
NO3 = 1 NO3 = 6
Next, let’s balance our sulphates. If we put a coefficient of 3 before K2SO4(s) our formula becomes:
Al2(SO4)3(s) + KNO3(aq) –> 2Al(NO3)3(aq) + 3K2SO4(s)
The table looks like this:
Products Reactants
Al = 2 Al = 2
SO4 = 3 SO4 = 3
K = 1 K = 6
NO3 = 1 NO3 = 6
Moving along, we can balance our potassiums. We can put a coefficient of 6 in front of KNO3(aq)
Now our formula is:
Al2(SO4)3(s) + 6KNO3(aq) –> 2Al(NO3)3(aq) + 3K2SO4(s)
A final check of our table confirms it, we’ve balanced this equation!
Products Reactants
Al = 2 Al = 2
SO4 = 3 SO4 = 3
K = 6 K = 6
NO3 = 6 NO3 = 6
Phew, that was a lot of work, but eventually this will become easier and you can leave the tables behind. It just takes a little practice.

Self Check:

Turn these statements into balanced chemical equations, including the states.

1. Gaseous propane reacts with oxygen to form carbon dioxide and water.

2. Liquid sulfuric acid reacts with liquid sodium hydroxide to form water and sodium sulfate.

3. Copper metal reacts with aqueous silver nitrate to form aqueous copper (II) nitrate and silver metal.

Balance the following chemical equations:

4. H2(g) + O2(g) –> H2O(l)

5. CH4(g) + O2(g) –> CO2(g) + H2O(g)

6. Fe(NO3)3(aq) + MgBr(s) –> Fe2Br3(s) + Mg(NO3)2(aq)

If you need more practice, you can go to this website. There are a lot of good practice equations, with answers included!

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