Transcript snc2p u2l10 word equations and conservation of mass

Word Equations
Lesson 10

A word equation is a way of representing a
chemical reaction: it tells you what reacts
and what is produced. Word equations are
an efficient way to describe chemical
changes, to help chemists recognise
patterns, and to predict the products of a
chemical reaction.
Writing word equations

They are written in a particular order.
Reactants (what you start with) are always
on the left side of the arrow and products
(what you make) are always on the right side
of the arrow. Multiple reactants or products
are separated by a + sign.
Word Equations
examples.

Silver nitrate + copper  silver + copper (II) nitrate
Reactants
Products
 Hydrogen + Oxygen  water vapour
Reactants
Products
The Conservation of Mass
In a chemical reaction, the total mass of the
reactants is always equal to the total mass of
the products.
 This tells us a few things.

Atoms do not change in a reaction. The
molecules that they form can be changed but
the atoms themselves are not.
 Mass cannot be destroyed. If it could we could
use E = MC2 to create energy

Example
 Methane + oxygen  water + carbon dioxide

Skeleton equations +
Balancing Equations
A representation of a chemical reaction
where the formulas of the reactants are
connected to the formulas of the products by
an arrow.
 CH4 + O2  H2O + CO2
 This however does not demonstrate the Law
of Conservation of Mass. The law states that
the mass of the products will equal the mass
of the reactants.

Combustion of methane
(the above reaction)
Type of atom
Reactants
Products
C
1
1
H
4
2
O
2
2+1=3

We can’t change the formulas of the
products or reactants so the only thing we
can do is change the number of molecules
instead of their formulas.
CH4 + O2 + O2  H2O +H2O + CO2
 = CH4 + 2O2  2H2O + CO2
 Now the chemical equation is balanced and
the mass of the reactants will equal the
mass of the products.

Type of atom
Reactants
Products
C
1
1
H
4
4
O
4
2 + 1 + 1= 4
Steps to balancing an
equation
Step 1
 Write the word equation of the reaction
 Aluminum + bromine  aluminum bromide
Step 2
 Write the skeleton equation by replacing
each name with a correct formula.
 Al +Br2  AlBr3
Steps to balancing an
equation
Step 3
 Count the number of atoms on each side of
the equation
Type of atom
Reactants
Products
Al
1
1
Br
2
3
Steps to balancing an
equation
Step 4
 Multiply each of the formulas by the appropriate
coefficients to balance the number of atoms.
Start out by picking the element with the
most number of atoms and try to balance it
first. We will start with Bromine. The 2 and 3
will be balanced if we multiply the reactant
side by 3 which would give it 6 Br, and
multiply the product side by 2 to give us 6 Br.
Now we have 2 Al products which need to
be balanced so we add a 2 to the Al on the
reactant side.
 2Al + 3Br2  2AlBr3

More examples
on the board

Hydrogen gas + Chlorine gas  hydrogen
chloride


Sodium + chlorine  sodium chloride

Nitrogen + hydrogen  ammonia (hydrogen
nitride)
 N2 + H2  NH3 = N2 + H2  NH3


Complete the worksheet and hand in.