Chemical Reactions and the Law of Conservation of Mass (HSC SSCE Chemistry): Revision Notes

Chemical Reactions and the Law of Conservation of Mass

Introduction to chemical reactions

Chemistry involves two major aspects: understanding the structure of substances and studying the permanent changes that occur when materials are mixed together. These permanent changes are known as chemical reactions.

Chemical reactions are happening all around us constantly. When you light a gas burner on a stove, you're initiating a chemical reaction. Cooking food involves chemical reactions. Cars move because of chemical reactions occurring in their engines. Many everyday materials are produced through chemical reactions, including the iron and steel in vehicles and appliances, the copper in water pipes and electrical wiring, and the aluminium in window frames and aeroplanes.

Understanding mass conservation through investigation

A fundamental principle of chemical reactions can be demonstrated through a simple experiment designed to test whether mass is conserved during a chemical reaction.

Investigation: Mass in chemical reactions

Aim: To determine whether mass remains constant during a chemical reaction.

Materials required:

• Calcium chloride (dried)
• Sodium hydrogen carbonate
• $10~\text{mL}$ measuring cylinder
• Water
• Universal indicator in dropper bottle
• $2$ teaspoons
• Glass vial with flat bottom
• Resealable plastic storage bag
• Electronic balance

Procedure:

1. Add one teaspoon of calcium chloride to the plastic bag
2. Add one teaspoon of sodium hydrogen carbonate to the bag
3. Place $10~\text{mL}$ of water and four drops of universal indicator solution into the glass vial
4. Carefully stand the glass vial upright in the bag, ensuring it won't fall over
5. Seal the bag completely
6. Weigh the sealed bag and record the mass
7. Invert the bag to allow the indicator solution to mix with the solid chemicals
8. Observe and record what happens
9. Reweigh the sealed bag and record the final mass

Analysis questions:

• How can you tell that a chemical reaction is occurring? Look for visual changes such as colour changes, gas production (bubbling), or temperature changes
• Why is it important to use a sealed bag? The sealed system prevents any products from escaping, particularly gases
• Would the results differ if the bag wasn't sealed? If the bag were open, any gases produced would escape, causing an apparent loss of mass

The law of conservation of mass

The investigation demonstrates a fundamental principle in chemistry called the law of conservation of mass.

This fundamental law has two important implications for all chemical reactions:

Mass is conserved

The total mass of all products formed in a reaction equals the total mass of all reactants that were consumed. This can be expressed as:

$\text{total mass of products} = \text{total mass of reactants}$

This means that if you start with $100~\text{g}$ of reactants, you will end up with exactly $100~\text{g}$ of products, even though the substances have changed form.

The number of atoms of each element is conserved

During a chemical reaction, atoms are neither created nor destroyed; they are simply rearranged into different combinations. For example, in the reaction between zinc and hydrochloric acid to form zinc chloride and hydrogen gas:

• The number of chlorine atoms in the products equals the number in the reactants
• The number of zinc atoms remains the same
• The number of hydrogen atoms is conserved

Although new substances are formed during chemical reactions, no new atoms appear and no atoms disappear. New compounds are created by rearranging the atoms from the original substances (the reactants) into different configurations.

Chemical equations: Representing reactions

Chemists use equations to describe precisely what happens during chemical reactions. There is a systematic process for writing these equations correctly.

Word equations

The first step in representing a chemical reaction is to write a word equation. This requires knowing all the reactants (starting materials) and all the products (substances formed). For example, when solid phosphorus reacts with chlorine gas, it forms liquid phosphorus trichloride. The word equation is:

$\text{phosphorus} + \text{chlorine} \rightarrow \text{phosphorus trichloride}$

In this equation, phosphorus and chlorine are the reactants, while phosphorus trichloride is the product. The arrow means "reacts to form" or "yields".

Symbol equations

After writing the word equation, we convert it into a symbol equation (also called a chemical equation). This involves replacing the words with chemical symbols and formulae. To do this successfully, you need to know or be able to work out the correct formulae for all substances involved.

For the phosphorus and chlorine reaction, the symbol equation is:

$\text{P} + \text{Cl}_2 \rightarrow \text{PCl}_3$

Balanced equations

Because mass and the number of atoms are conserved in chemical reactions, a correct chemical equation must have the same number of atoms of each element on both sides of the arrow. We achieve this by placing coefficients (numbers) in front of the formulae until the atoms balance.

For the phosphorus reaction, the balanced equation becomes:

$2\text{P} + 3\text{Cl}_2 \rightarrow 2\text{PCl}_3$

Let's verify this is balanced:

• Phosphorus: $2$ atoms on the left, $2$ atoms on the right ✓
• Chlorine: $3 \times 2 = 6$ atoms on the left, $2 \times 3 = 6$ atoms on the right ✓

State symbols

The final step is to add state symbols to indicate the physical state of each substance:

• $(s)$ for solid
• $(l)$ for liquid
• $(g)$ for gas
• $(aq)$ for aqueous solution (dissolved in water)

The complete balanced equation with state symbols is:

$2\text{P}(s) + 3\text{Cl}_2(g) \rightarrow 2\text{PCl}_3(l)$

This equation reads: "Two atoms of solid phosphorus react with three molecules of chlorine gas to form two molecules of liquid phosphorus trichloride."

Another example: Hydrogen and chlorine reaction

Let's work through another example step by step. Hydrogen gas reacts with chlorine gas to form hydrogen chloride gas.

This diagram shows the molecular representation of the reaction, illustrating how one molecule of hydrogen ($\text{H}_2$) combines with one molecule of chlorine ($\text{Cl}_2$) to produce two molecules of hydrogen chloride ($\text{HCl}$).

Worked example: Balancing a decomposition reaction

Let's work through a more complex example: the decomposition of copper nitrate to form copper oxide, nitrogen dioxide, and oxygen gas.

Step-by-step guide to writing chemical equations

To write a complete, balanced chemical equation, follow these systematic steps:

1. Write a word equation that includes all reactants and all products
2. Write the formulae for all substances involved (this creates an unbalanced equation)
3. Balance the equation by placing coefficients in front of symbols or formulae:
   • Start with atoms that occur in only one place on each side of the equation
   • Save elements that appear in multiple compounds for later
   • Check your work by counting atoms of each element on both sides
4. Add state symbols: $(s)$, $(l)$, $(g)$, $(aq)$

Remember!