Conservation of Mass Noadswood Science, 2016

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

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.

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

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

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

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

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

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)

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

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

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!

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!

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!

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!

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

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

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

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

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

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

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

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

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

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

Practice Questions – Application 127 + 32 = 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

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