1. Conservation of Mass
Noadswood Science, 2016

2. Conservation of Mass
Friday, February 22, 2019
To understand conservation of mass, and to be able to balance equations

3. Precise Learning
The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
This means that chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.
Students should understand the use of the multipliers in equations in normal script before a formula and in subscript within a formula.

4. Chemical Reactions
What are the signs that a chemical reaction has taken place?
There are usually some obvious changes during a chemical reaction, including: -
A change in colour
A gas coming off (you may see fizzing or bubbling)
A change in temperature (the reaction mixture may get hotter)
A solid may be formed when two solutions are mixed together

5. Chemical Reactions
New substances are formed by chemical reactions – when elements react together to form compounds their atoms join to other atoms via chemical bonds
For example: -
Iron and sulfur react together to form a compound called iron sulfide
Mixture of powered iron (grey) + sulfur (yellow)
Mixture is heated
A chemical reaction occurs, forming iron sulfide

6. Chemical Reactions
New substances are formed by chemical reactions – when elements react together to form compounds their atoms join to other atoms via chemical bonds
Chemical bonds involve electrons from the reacting atoms – bonds can form when: -
Electrons are transferred from one atom to another, so that one atom gives electrons and the other takes electrons
Electrons are shared between two atoms

7. Chemical Formula
The chemical formula of a compound shows how many of each type of atom join together to make the units that make the compound up
In iron sulfide every iron atom is joined to one sulfur atom, so we show its formula as FeS
In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na2O
In carbon dioxide, every carbon atom is joined to two oxygen atoms, so we show its formula as CO2

8. Chemical Formula
Carbon dioxide (CO2) – every carbon atom is joined to two oxygen atoms
What do the following compounds contain (what are the ratios of the atoms): -
Na2SO4
Fe(OH)3

9. Chemical Formula
Na2SO4 – a unit of sodium sulfate contains two sodium atoms, one sulfur atom and four oxygen atoms joined together
Fe(OH)3 – a unit of iron (III) hydroxide contains one iron atom, three oxygen atoms and three hydrogen atoms (the brackets show that the 3 applies to O and H)

10. Balancing Equations
When elements are joined to cause a chemical reaction, no atoms are made or lost during the process - but at the end of it they are joined differently from the way they were at the start
This means that the mass of the substances at the start (reactants) is the same as the mass of the substances at the end (products)
Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide
Reactants  Products

11. Magnesium + Oxygen  Magnesium oxide
Symbol Equations
Symbol equations – this shows the atoms on both sides
Magnesium + Oxygen  Magnesium oxide
2Mg + O2  2MgO

12. Balancing Equations
There must always be the same number of atoms on both sides – equations are balanced by putting a number in front of the formulae where needed…
Cu + O2  CuO
The formulae are all correct but the numbers of some atoms do not match up on both sides – the formula cannot be changed, only numbers may be added in front of them
See if you can balance the equation – try one type of atom at a time!

13. Balancing Equations
Find an element which does not balance and pencil in a number to try and sort it out – if this creates another imbalance pencil in another number etc…
Cu + O2  CuO
In the above equation there are more O atoms on the left than on the right (2O on the left but only 1O on the right) – to correct this add more O on the right: -
Cu + O2  2CuO
This has now caused too many Cu atoms on the right hand side (2Cu on the right but only 1Cu on the left) – to correct this add more Cu on the left: -
2Cu + O2  2CuO
Having changed this we now have a fully balanced equation!

14. Balancing Equations
There must always be the same number of atoms on both sides – equations are balanced by putting a number in front of the formulae where needed…
H2SO4 + NaOH  Na2SO4 + H2O
The formulae are all correct but the numbers of some atoms do not match up on both sides – the formula cannot be changed, only numbers may be added in front of them
See if you can balance the equation – try one type of atom at a time!

15. Balancing Equations
Find an element which does not balance and pencil in a number to try and sort it out – if this creates another imbalance pencil in another number etc…
H2SO4 + NaOH  Na2SO4 + H2O
In the above equation there are more H atoms on the left than on the right (3H on the left but only 2H on the right) – to correct this add more H on the right: -
H2SO4 + NaOH  Na2SO4 + 2H2O
This has now caused too many H and O atoms on the right hand side (4H and 6O on the right but only 3H and 5O on the left) – to correct this add more O and H on the left: -
H2SO4 + 2NaOH  Na2SO4 + 2H2O
Having changed this the Na has balanced itself – this is now a fully balanced equation!

16. Conservation Of Mass
In any reaction the total mass of products is the same as the total mass of the reactants
Magnesium oxide + Hydrochloric acid  Magnesium chloride + Water Mg O H Cl
1 x magnesium, 1 x oxygen, 2 x hydrogen & 2 x chlorine atoms
1 x magnesium, 1 x oxygen, 2 x hydrogen & 2 x chlorine atoms

17. Conservation Of Mass
In any reaction the total mass of products is the same as the total mass of the reactants
Methane + Oxygen  Carbon dioxide + Water C H O
1 x carbon, 4 x hydrogen, & 4 x oxygen
1 x carbon, 4 x hydrogen, & 4 x oxygen

18. Experiments
How could you design a practical to test the conservation of mass hypothesis - in any reaction the total mass of products is the same as the total mass of the reactants?
You are going to complete 2 experiments looking at how mass remains the same from the start of the reaction to when it finishes
It is vital you measure precisely the mass of reactant(s) and product(s)
*In some reactions it may appear the mass has increased / decreased – this is due to the addition of atoms (e.g. oxygen has bound from the atmosphere) / loss of atoms to the atmosphere (e.g. carbon dioxide has been produced and released into the atmosphere).
*For this reason it is vital you consider how you would collect gas which may be formed / control gas which may be added to the experiment

19. Write a word and balanced symbol equation for the reaction
Practical 1
Complete the following practical, ensuring you identify the mass of reactants and products (remember, we want to see if there was any change in mass)!
Measure out 50ml of sodium hydroxide (NaOH) into a beaker (find its mass)
Measure out 50ml of copper sulfate (CuSO4) into a beaker (find its mass)
Mix the two liquids – what is the final mass
Write a word and balanced symbol equation for the reaction

20. Write a word and balanced symbol equation for the reaction
Practical 2
Complete the following practical, ensuring you identify the mass of reactants and products (remember, we want to see if there was any change in mass)!
Measure out 50ml of copper sulfate (CuSO4) into a beaker (find its mass)
Take 2g of magnesium (Mg) and add this to the copper sulfate
What is the final mass?
Write a word and balanced symbol equation for the reaction

21. Copper sulfate + Hydrochloric acid  Copper chloride + Sulfuric acid
Reactions
Practical 1 – copper sulfate + hydrochloric acid
Copper sulfate + Hydrochloric acid  Copper chloride + Sulfuric acid
CuSO4 + 2HCl  CuCl2+ H2SO4

22. Magnesium + Copper sulfate  Magnesium sulfate + Copper
Reactions
Practical 2 – magnesium + copper sulfate
Magnesium + Copper sulfate  Magnesium sulfate + Copper
Mg + CuSO4  MgSO4 + Cu

23. Conservation of Mass and Mr
By adding up the relative formula masses of the substances on each side of a balanced symbol equation, you can see that mass is conserved: the total Mr of the reactants is equal to the total Mr of the products
2Li + F2  2LiF
Mr of reactants = (2 x 7) + (2 x 19) = 52
Mr of products 2 x (7 + 19) = 52

24. Practice Questions State the law of conservation of mass
State the relationship between the sum of the relative formula masses of the products and the sum of the relative formula masses of the reactants in a reaction
Give an example of a reaction where the mass of the products in the reaction container would be greater than the mass of the reactants

25. Answers
During a reaction no atoms are made or destroyed, so the mass of the products is the same as the mass of the reactants
The masses will be the same
The reaction between a metal and oxygen in an unsealed container

26. Practice Questions – Application
= 159g
The mass will decreases (carbon dioxide will be produced)
Before the reaction calcium is in the reaction vessel, but oxygen is in the air so the mass of the oxygen isn’t accounted for. When the calcium and oxygen react the total mass increases as the oxygen has bound to the calcium

27. Answers State the law of conservation of mass
State the relationship between the sum of the relative formula masses of the products and the sum of the relative formula masses of the reactants in a reaction
Give an example of a reaction where the mass of the products in the reaction container would be greater than the mass of the reactants