Measurement, Mass & Equations (AQA GCSE Chemistry Combined Science): Revision Notes
Balanced equations and masses
What are balanced equations?
A balanced equation shows what happens in a chemical reaction. The particles in the reactants get rearranged to make the products. Understanding this fundamental concept is essential for mastering chemical reactions.
No atoms are ever lost or made during a chemical reaction. This is the foundation of all chemical equation balancing.
Chemical reactions simply involve the rearrangement of existing atoms - they never create or destroy atoms. This principle guides everything we do when working with chemical equations.
Worked Example: Hydrogen and Chlorine Reaction
Consider the reaction: H₂ + Cl₂ → 2HCl
- One molecule of hydrogen reacts with one molecule of chlorine
- This makes two molecules of hydrogen chloride
- All the atoms are still there, just rearranged:
- Left side: 2 H atoms + 2 Cl atoms = 4 atoms total
- Right side: 2 H atoms + 2 Cl atoms = 4 atoms total
Law of conservation of mass
This is a fundamental law in chemistry that governs all chemical reactions. It states that matter cannot be created or destroyed in a chemical reaction - only rearranged.
Law of Conservation of Mass
No atoms are lost or made in any reaction.
Therefore: The total mass of reactants = the total mass of products
We can express this mathematically using relative formula masses:
This law is our primary tool for checking if equations are balanced properly and for calculating masses in chemical reactions.
Working with relative formula masses
To calculate masses in reactions, you need to know the relative atomic masses. These values are fundamental to all mass calculations in chemistry.
Common Relative Atomic Masses
- Hydrogen (H) = 1
- Carbon (C) = 12
- Oxygen (O) = 16
- Chlorine (Cl) = 35.5
- Calcium (Ca) = 40
These values are usually provided in exam questions, but it's helpful to memorise the common ones.
Let's apply this knowledge to verify that our previous equation is properly balanced:
Worked Example: Mass Verification for H₂ + Cl₂ → 2HCl
Step 1: Calculate mass of reactants
- H₂ = (2 × 1) = 2
- Cl₂ = (2 × 35.5) = 71
- Total mass of reactants = 2 + 71 = 73
Step 2: Calculate mass of products
- 2HCl = 2 × (1 + 35.5) = 2 × 36.5 = 73
Step 3: Compare
- Reactants: 73
- Products: 73
- The masses are equal, confirming the equation is balanced!
When mass appears to change
Sometimes during experiments, it looks like mass has been lost or gained in a reaction. This apparent change occurs due to gases entering or leaving the reaction system, but the total mass of all substances involved always remains constant.
Common Reason for Apparent Mass Change
Gases escape from the reaction
- If a gas like CO₂ is produced and escapes, the container becomes lighter
- The mass hasn't really been lost - the gas has just left the container
- If you could capture all the products, the total mass would equal the reactants
This is particularly common in reactions involving carbonates, where carbon dioxide gas is produced.
Worked Example: Calcium Carbonate and Hydrochloric Acid
Reaction: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
What happens:
- Calcium carbonate (solid) reacts with hydrochloric acid (liquid)
- Products include calcium chloride (solid), water (liquid), and carbon dioxide (gas)
- The CO₂ gas escapes into the air
- The container becomes lighter, but no mass is actually lost from the universe
Calculating mass changes
You can predict and calculate how much mass will be lost when gases escape from a reaction. This is useful for planning experiments and understanding results.
Worked Example: Calculating Mass Loss
Question: If 50g of CaCO₃ reacts completely, how much mass will the container lose?
Step 1: Find the relative formula masses
- CaCO₃ = 40 + 12 + (3 × 16) = 100
- CO₂ = 12 + (2 × 16) = 44
Step 2: Check the balanced equation ratio
- CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
- The equation shows a 1:1 ratio of CaCO₃ to CO₂
Step 3: Calculate the mass of CO₂ produced
- If 100g of CaCO₃ produces 44g of CO₂
- Then 50g of CaCO₃ produces: of CO₂
Step 4: Determine mass loss
- The container will lose 22g of mass when the CO₂ escapes
This calculation method works for any reaction where gases are produced or consumed.
Key Points to Remember:
- Balanced equations show that atoms are rearranged, never created or destroyed
- Total mass of reactants always equals total mass of products
- Use relative atomic masses to calculate formula masses
- Mass might appear to change if gases escape or are absorbed from the air
- Always verify your calculations by checking that both sides of the equation have the same total mass