C1.1 Chemical Reactions And Balancing Equations

What makes a chemical reaction occur?

Reactants ‘react’ together to make Products.

A + B                        →      C + D (A and B are reactants)    (C and D are products)

When different substances (elements or compounds) react together, reactant atoms ‘break’ their chemical bonds and make new chemical bonds with other reactant atoms to form new substances (called products).

What is a chemical bond? A chemical bond between atoms is a special type of attractive force that is generated by the outermost (valence) electrons of the atoms. The attractive force (‘chemical bond’) holds the atoms close together to make a molecule of 2 or more ‘joined’ atoms. Importantly, atoms can only transfer (donate or accept) or share their outermost (valence) electrons with other atoms to form these chemical bonds. This is explained in greater detail here.

Can you predict the type of compounds made from metal and non-metal reaction combinations?

In general, metals do not chemically combine with metals, and the following rules apply for the remaining combinations:

Metals combine with non-metals Outermost (valence) electrons are transferred from metal to non-metals Metals lose electrons to form positive ions Non-metals gain electrons to form negative ions Positive and negatively charged ions are held together by ionic bonds (strong electrostatic force) to form ionic compounds that have no overall charge (neutral)             Non-metals combine with non-metals Outermost (valence) electrons are shared between non-metals A pair of electrons shared between non-metal atoms forms a single covalent bond between the atoms. Covalent bonds can be formed between identical atoms (e.g. H2, hydrogen) or non-identical atoms (e.g. hydrogen and oxygen in H2O, water). Molecules containing covalent bonds are neutral or charged ions. e.g. NH3, H2O are neutral covalent molecules e.g. NH4+, CO32-, SO42- are ions containing covalent bonds

e.g. NaCl is an ionic compound

A chemical equation shows the chemical reaction that has occurred between reactants (left-hand side) to form the products (right-hand side).

Equations can be written for any combination of reacting atoms, compounds, ions, molecules and free radicals.

All chemical symbol equations are balanced with respect to the quantity of each element (and charge if ions are involved).

Physical state of chemicals are stated in parentheses after the chemical symbol: (s)   solid (l)    liquid (g)   gas (aq)  aqueous solution (this means dissolved in water).

Law of conservation of mass

As no atoms are lost or made during a chemical reaction, the mass of the products equals the mass of the reactants, meaning that, the number of atoms for each element is not changed by a chemical reaction.

Try out this BBC Bitesize simulation showing how mass is conserved in a chemical reaction (click here)

Example of a balanced chemical equation:

2HCl + 2Na → 2NaCl + H2 Reactants               Products

This balanced chemical equation indicates:

• Two hydrogen atoms (contained inside two HCl molecules) react to form two hydrogen atoms (contained inside one molecule of hydrogen gas, H2)
• Two chlorine atoms (contained inside two HCl molecules) react to form two chlorine atoms (contained inside two molecules of NaCl)
• Two sodium atoms react to form two sodium atoms (contained inside two molecules of NaCl)

Balancing chemical equations (word and symbol)

Chemical reactions can be represented by word equations or by symbol equations.

Symbol equations provide more information than word equations as they show how many atoms of each element combine together and their exact ratio of combination.

The golden rule with balancing chemical equations is that you are only allowed to insert a whole number to multiply the number of molecules of a particular reactant or product: this whole number is positioned on the left side of the molecule you want to multiply.  You are not allowed to change the chemical formula of any reactant or product molecule.

Worked examples of how to balance a chemical equation

EXAMPLE 1

Magnesium  +  Hydrochloric acid → Magnesium Chloride  + Hydrogen gas Mg             +   HCl                    →  MgCl2                      +  H2

STEP 1: Count how many atoms of each element are present on each side of the equation

Mg: 1                                       →  Mg: 1 H:   1                                             H:  2 Cl:   1                                            Cl: 2

STEP 2: Identify if any atoms are unbalanced, and if found, attempt to correct the imbalance

Here, both H and Cl atoms are unbalanced. I will therefore start by inserting a 2 in front of HCl to double the number of H and Cl atoms on the left-hand side of the equation.

Mg             +   2HCl                    →  MgCl2                     +  H2

STEP 3: Check the new atom count and identify if any atoms are unbalanced, and if found, attempt to correct the imbalance

New atom count: Mg: 1                                       →  Mg: 1 H:   2                                             H:  2 Cl:   1                                            Cl: 2

STEP 4: New atom count shows all atoms are balanced. This is now a balanced chemical equation!

In this example, balancing was achieved in STEPS 1 TO 4. But chemistry is never that easy ! In some cases, you will need to repeat STEP 3 again and again, inserting different whole numbers to multiply the number of molecules of a reactant or product, and then re-check the atom count to see if you achieve atom balance.

EXAMPLE 2

Methane gas + Oxygen gas  →  Carbon  Dioxide  + Water

CH4              +  O2             →   CO2                   + H2O

STEP 1: Count how many atoms of each element are present on each side of the equation

C: 1                                 →   C: 1 H: 4                                       H: 2 O: 2                                       O: 2+1=3

STEP 2: Identify if any atoms are unbalanced, and if found, attempt to correct the imbalance

Here, both H and O atoms are unbalanced. I will therefore start by inserting a 2 in front of H2O to double the number of H and O atoms on the right-hand side of the equation.

CH4              +  O2             →   CO2                   + 2H2O

STEP 3: Check the new atom count and identify if any atoms are unbalanced, and if found, attempt to correct the imbalance

C: 1                                 →   C: 1 H: 4                                       H: 2×2=4 O: 2                                       O: 2+2=4

Here, only O atoms are unbalanced. I will therefore insert a 2 in front of O2 to double the number of O atoms on the left-hand side of the equation.

CH4              +  2O2             →   CO2                   + 2H2O

STEP 3 AGAIN: Check the new atom count and identify if any atoms are unbalanced, and if found, attempt to correct the imbalance

C: 1                                 →   C: 1 H: 4                                       H: 4 O: 2×2=4                               O: 2+2=4

STEP 4: New atom count shows all atoms are balanced. This is now a balanced chemical equation!

Interactive simulations for balancing equations  

To become good at balancing chemical symbol equations you need to practise. Try out the simulations below !

Balancing Equations for GCSE Chemistry. This resource was produced by the University of Birmingham.

Boardworks simulations (click here). Also download free Balancing equations powerpoint from Boardworks (weblink click here or download click here).

Footprints science simulation (click here)

Footprints science simulation (click here)

Phet simulation (click here)

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