Chemical Reactions and Dynamics

The Law of Conservation of Mass and Balancing Equations

One of the most important rules in chemistry is the law of conservation of mass. This law states that mass is conserved during a chemical reaction in a closed system because atoms are neither created nor destroyed; they are simply rearranged.

Because of this law, every chemical equation must be balanced. A balanced equation ensures that the number of atoms for every element is identical on both the reactant side and the product side.

How to Balance an Equation

To balance an equation, you must adjust the coefficients (the large numbers in front of formulas) rather than the subscripts (the small numbers within a formula). Changing a subscript changes the identity of the substance itself.

Worked Example: Balancing Magnesium Oxide

1. Unbalanced: $Mg + O_2 \rightarrow MgO$
   • Left side: 1 Mg, 2 O
   • Right side: 1 Mg, 1 O (Oxygen is unbalanced)
2. Add a coefficient to the product: $Mg + O_2 \rightarrow 2MgO$
   • Right side: 2 Mg, 2 O
   • Left side: 1 Mg (Magnesium is now unbalanced)
3. Add a coefficient to the reactant: $2Mg + O_2 \rightarrow 2MgO$
   • Check: Left (2 Mg, 2 O) = Right (2 Mg, 2 O). Balanced!

Chemical Formulae: Subscripts and Coefficients

To read chemical equations effectively, you must master chemical formulae, which use symbols and numbers to represent the composition of a substance.

• Subscripts: Small numbers written below and to the right of an element symbol. They indicate how many atoms of that specific element are in the formula. For example, in $H_2O$, the subscript 2 means there are 2 hydrogen atoms.
• Coefficients: Large numbers placed in front of a chemical formula. They multiply the entire substance. For example, $3H_2O$ contains 6 hydrogen atoms ($3 \times 2$) and 3 oxygen atoms ($3 \times 1$).
• Brackets: When a subscript is placed outside a bracket, it multiplies every element inside those brackets. For example, $Ca(OH)_2$ contains 1 Calcium, 2 Oxygen, and 2 Hydrogen atoms.

Reaction Dynamics: Rates of Reaction and Collision Theory

The rate of reaction describes how quickly reactants are consumed or how quickly products are formed. Some reactions, like combustion, are nearly instantaneous, while others, like the rusting of iron, occur very slowly.

Collision Theory

Why do some reactions happen faster than others? Collision theory explains that for a reaction to occur, reactant particles must collide with:

1. Sufficient energy (to overcome the activation energy).
2. Correct orientation (hitting each other in the right way).

A collision that meets both these criteria is called a successful collision. The more successful collisions that occur per second, the faster the reaction rate.

Factors Affecting Reaction Rate

Several factors can increase the frequency of successful collisions:

• Temperature: Increasing temperature causes particles to move faster. This leads to more frequent collisions and ensures a greater proportion of particles have enough energy to react.
• Concentration: Increasing the concentration of a solution means more reactant particles occupy the same volume, increasing collision frequency.
• Surface Area: Breaking a solid into smaller pieces (like powdering a tablet) exposes more particles to the other reactant, increasing the frequency of collisions.
• Catalysts: A catalyst provides an alternative reaction pathway with a lower activation energy. This allows more collisions to be successful without the catalyst being permanently consumed.

Exam Tip: Writing Strong Explanations When asked to explain why a reaction is faster (e.g., why powder reacts faster than a solid), use the Factor $\rightarrow$ Particle Explanation $\rightarrow$ Effect structure.

• Weak: "It's faster because of surface area."
• Strong: "Powdering the substance increases its total surface area, exposing more particles to the reactant. This increases the frequency of successful collisions per second, which increases the reaction rate."