Reactions

Level 1

What happens during a reaction

During a chemical reaction, chemical substances mix and react with each other to form new chemical species 
The starting substances called reactants transform (so their quantity decreases) into new substances called products(so their quantity increases).

The transformation is summarized with a  word equation :

Reactant 1 + Reactant 2 → Product 1 + Product 2

Example: Simple chemical transformation

BEFORE (Reactants)
 
 
→
AFTER (Products)
 
 
Ready to start

  Example : 

Methane reacts with oxygen to form carbon dioxide and water.

Methane + Oxygen → Carbon dioxide + Water

In this reaction, the quantity of methane and oxygen decreases during the reaction because they react with each other and transform. Conversely, the quantity of carbon dioxide and water increases, because the reaction produces them.

Note: Reactants are always to the left of the arrow and products to the right.

From the word equation to the reaction equation

The word equation can be converted into a reaction equation by replacing each species with its chemical formula.

  Example:
Word equation: 

Methane + Oxygen → Carbon dioxide + Water

Reaction equation: 

CH4 + O2 → CO2 + H2O

The physical state can be specified: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water after each formula (if needed)

  Examples: 

  • CH4 (g) + O2(g) → CO2 (g) + H2O (g)
  • HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

Lavoisier's Law

« Nothing is lost, nothing is created, everything is transformed » 

The matter present at the beginning of the reaction is found entirely at the end of the reaction. 

For this to happen, the atoms present in the molecules must not disappear during chemical reactions. Thus, the number of atoms of each kind must be the same before and after the reaction. This is the conservation of atoms

Therefore, it happens that for certain reactions, several similar molecules react at the same time, or several similar molecules are produced.
To express this, we adjust / balance the reaction equation

How to balance?

  1. Count the atoms on both sides.
  2. Add coefficients in front of the molecule formulas.
  3. Never modify the subscripts in the formulas.
  4. Check and simplify if possible.

Example (combustion of methane): 1 molecule of methane reacts with 2 molecules of dioxygen to form 1 molecule of carbon dioxide and 2 molecules of water.

CH₄ + 2 O₂ → CO₂ + 2 H₂O
REACTANTS
CH₄
O₂
PRODUCTS
CO₂
H₂O

Atom count
Initial
Reactants
C: 0
H: 0
O: 0
Products
C: 0
H: 0
O: 0
Final
Reactants
C: 0
H: 0
O: 0
Products
C: 0
H: 0
O: 0
Add reactants to start

Other examples:

  • Combustion of dihydrogen
    • Dihydrogen reacts with oxygen to form water.
      • H2 + O2 → H2O   (unbalanced: 2 H and 2 O  ==> 2 H and 1 O)
        • 2 H2 + O2 → 2 H2O (balanced:  2 H and 2 O  ==> 2 H and 2 O  Conservation of atoms is maintained.)
  • Sodium reacts with chlorine gas to form sodium chloride
    • Na + Cl2 → NaCl (unbalanced:  1 Na and 2 Cl ==> 1 Na and 1 Cl)
      • 2 Na + Cl2 → 2 NaCl (balanced: 2 Na and 2 Cl ==> 2 Na and 2 Cl Conservation of atoms is maintained.)

 

Link to mass

When a chemical reaction occurs:

Reactants → Products

  • Stoichiometric coefficients indicate the proportions in which substances react or are formed.
  • However, the mass of the reacted reactants must be equal to the mass of the products (Lavoisier's Law)

  Example: Carbon combustion reaction:
$C$$ + $$O_2$$ \rightarrow $$CO_2$

  • Coefficients: $1$ $C$$ + 1 $$O_2$$\rightarrow 1 $$CO_2$
    • indicates 1 carbon atom reacts with 1 molecule of O2 and forms 1 molecule of CO2
  • Mass: 12 g of carbon + 32 g of dioxygen → 44 g of carbon dioxide
    • Depending on the molecules, the masses are different, since the molecules are different.
    • But the total mass of the reactants and the products are the same

At the microscopic level

A chemical reaction corresponds to a reorganization of atoms: bonds break and new ones form. The atoms themselves do not disappear.

The limiting reactant

The limiting reactant is the one that is consumed first and prevents the reaction from continuing.

  Example: Combustion of methane
 $CH_4 + 2 O_2 → CO_2 + 2 H_2O$, 
if we have 5 $CH_4$ and only 6 $O_2$, the reaction can only occur 3 times: $O_2$ is limiting.

The limiting reactant: Observe how a reactant runs out first

Choose a scenario:
CH₄ + 2 O₂ → CO₂ + 2 H₂O
CH₄ (reactant)
C
H
H
H
H
3
O₂ (reactant)
O
 
O
6
CO₂ (product)
O
 
C
 
O
0
H₂O (product)
H
 
O
 
H
0
Number of reactions performed:
0
 

Conditions for a reaction

  • The reactants must be in contact.
  • Sometimes, energy must be provided (heat, spark).
  • A catalyst can speed up the reaction without being consumed.