Balanced equations and masses (AQA GCSE Chemistry): Revision Notes
Balanced equations and masses
What are balanced equations?
A balanced equation shows how particles in reactants get rearranged to make products. This is a key idea in chemistry. No atoms are lost or made during a chemical reaction - they just get rearranged into new substances.
Chemical Equation Example: Hydrogen and Chlorine
H₂ + Cl₂ → 2HCl
- One hydrogen molecule reacts with one chlorine molecule
- They make two hydrogen chloride molecules
- The equation is balanced because we have the same number of each type of atom on both sides
Law of conservation of mass
The Law of Conservation of Mass
No atoms are lost or made in a reaction - this means the total mass stays the same in all chemical reactions.
The law of conservation of mass tells us something important about chemical reactions:
- No atoms are lost or made in a reaction
- This means the total mass stays the same
- The mass of all reactants = the mass of all products
We can write this as:
Calculating relative formula masses
To work with balanced equations and masses, you need to calculate relative formula masses (Mr).
Steps to Calculate Mr:
- Look up the relative atomic mass for each element
- Multiply by the number of atoms of each element
- Add up all the values
Worked Example: Finding Mr of HCl (hydrogen chloride)
- H = 1, Cl = 35.5
- Mr of HCl = 1 + 35.5 = 36.5
Using balanced equations to predict mass changes
You can use balanced equations to work out how much product you'll get from a certain amount of reactant.
Worked Example: Calcium carbonate + hydrochloric acid
The reaction is: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Step 1: Calculate the Mr values
- Mr(CaCO₃) = 40 + 12 + (3 × 16) = 100
- Mr(CO₂) = 12 + (2 × 16) = 44
Step 2: Use the balanced equation ratio
- The equation shows 1:1 ratio of CaCO₃ to CO₂
- So 100g of CaCO₃ would make 44g of CO₂
Step 3: Scale down for the actual amount
- If we use 50g of CaCO₃
- We get: 50g ÷ 100g × 44g = 22g of CO₂
Why does mass sometimes appear to decrease?
In some reactions, the mass of your reaction container might go down. This doesn't break the law of conservation of mass!
Common Misconception: Decreasing Mass
When mass appears to decrease in a reaction, it's usually because one of the products is a gas that escapes from the reaction container.
What's happening:
- One of the products might be a gas that escapes
- For example, CO₂ gas bubbles away in the calcium carbonate reaction
- The atoms are still there - they've just left the container
- If you could capture all the gas, the total mass would be the same
Key Facts for Your Exam:
- Always check equations are balanced before doing calculations
- Use relative atomic masses given in the exam paper
- Remember the 1:1:1 ratio rule - if the equation shows equal numbers of molecules, you can work out masses directly
- Gas escaping explains why mass appears to decrease in some reactions
Key Points to Remember:
- Balanced equations show that atoms get rearranged, not lost or made
- The law of conservation of mass means total mass stays the same in reactions
- You can calculate how much product you'll get using relative formula masses and balanced equations
- If mass appears to decrease, it's usually because a gas has escaped
- Always use the ratios shown in the balanced equation for your calculations