IB Chemistry Sub-topic R2.1 Notes

Chemical reactions

Within chemistry, we look at substances undergoing chemical reactions, reactions that form new substances by breaking bonds in the reactants and forming bonds in the products. This causes an energy change in the system but conserves mass and particles. The aim for each reaction is to reach the highest possible desired product yield.

Balancing Equations 

The first step in considering any chemistry problem is to make sure a chemical reaction is balanced. This means that in every chemical reaction the number of atoms of every element in the reactants must be the same as in the products. 

In order to balance an equation, coefficients can be changed or added in front of certain species. Balanced equations also include state symbols in brackets behind species. These include: (s), (l), (g) and (aq), as shown below:

CaCO_{3 (s)} + HNO_{3 (aq)} → Ca(NO₃)_{2 (aq)} + CO_{2 (g)} + H₂O _(l)

An effective order for balancing equations is as follows:

1. Balance the metallic elements.
2. Balance the next element that only occurs once on each side.
3. Balance the rest of the elements (if required).

At the end, the balanced equation becomes:

CaCO_{3 (s)} + 2HNO_{3 (aq)} → Ca(NO₃)_{2 (aq)} + CO_{2 (g)} + H₂O _(l)

This is only a rough guide and difficult equations will require practice to master!

Basics of reactions

Now that you know how to balance an equation, let's cover the basics of reactions by breaking down our equation.

CaCO_{3 (s)} + 2HNO_{3 (aq)} → Ca(NO₃)_{2 (aq)} + CO_{2 (g)} + H₂O _(l)

1. Reactants - the species undergoing the reaction. In this case, these are CaCO₃ and HNO₃.
2. Products - the species produced at the end of the reaction. In this case, these are Ca(NO₃)₂, CO₂, and H₂O.
3. Molar ratio - this is the ratio between any two species in the reaction. For example, the molar ratio of CaCO₃ to HNO₃ is 1:2 and the molar ratio of CaCO₃ and Ca(NO₃)₂ is 1:1.

As you can see, the molar ratio thus guides how much of each species is involved in the reaction. In real life, this will not always occur, either accidentally or purposefully - meaning that actual amounts of reactants will not follow the molar ratio.

This means each reagent takes one of two forms:

1. Limiting reagent - this is the reagent with the moles leading to the smallest molar ratio.
2. Excess reagent - naturally, this is any other reagent, which has more moles than the smallest molar ratio.