Mass, particles, balanced Learning Objective To be able to: Write balanced equations and use them to explain the conservation of mass Key Words: Mass, particles, balanced

Balancing Equations and Conservation of Mass Saturday, 25 May 2019 Balancing Equations and Conservation of Mass Date and title in books. You need a pen, pencil & ruler. Starter Activity – 5 minutes Exam question

Explain the conservation of mass Target 9-1 Identify the conservation of mass (F/H) Target 4-9 Write balanced symbol equations (H) Target 5-9 Explain the conservation of mass Extension: Application of Knowledge Target 8/9 Explain the conservation of mass using a balanced symbol equation Learning Outcomes

Conservation of Mass New substances are made during chemical reactions However, the same atoms are present before and after reaction. They have just joined up in different ways. Because of this the total mass of reactants is always equal to the total mass of products. This idea is known as the Law of Conservation of Mass. Reaction but no mass change

Conservation of Mass There are examples where the mass may seem to change during a reaction. Eg. In reactions where a gas is given off the mass of the chemicals in the flask will decrease because gas atoms will leave the flask. If we carry the same reaction in a strong sealed container the mass is unchanged. Gas given off. Mass of chemicals in flask decreases Mg HCl Same reaction in sealed container: No change in mass 11.71

Activity What is the conservation of mass law? Two hydrogen atoms react with one oxygen atom. What happens to these atoms? What new substance is made? Two oxygen atoms reacts with one carbon atom

What have you learnt? Can you apply your knowledge about the conservation of mass law? Lets find out!

Chemical formulae – what is the ratio of atoms? The name of a chemical can also be written as a formula. A formula uses chemical symbols and numbers to show the ratio of atoms of each element present in the chemical. Name of salt Formula Ratio of atoms in salt calcium chloride CaCl2 Ca Cl 1 2 sodium sulphate Na2SO4 Na S O 2 1 4 magnesium sulphate MgSO4 Mg S O 1 1 4 magnesium nitrate Mg(NO3)2 Mg N O 1 2 6 calcium sulphate CaSO4 Ca 1 S 1 O 4

What is the ratio of atoms in each salt? Name of salt Formula Ratio of atoms in salt sodium chloride NaCl 1 Na 1 Cl magnesium chloride MgCl2 1 Mg 2 Cl potassium nitrate KNO3 1 K 1 N 3 O potassium sulphate K2SO4 2 K 1 S 4 O calcium nitrate Ca(NO3)2 1 Ca 2 N 6 O

Writing equations using chemical formulae The names of chemicals in a word equation can be replaced with chemical formulae. When a chemical reaction is written using chemical formulae, the equation must be balanced. zinc hydrogen hydrochloric acid zinc chloride Zn + HCl → ZnCl2 + H2 2 1 Zn 1 H, 1 Cl 2 H, 2 Cl 1 Zn, 2 Cl 2 H Write down the formula for each chemical. Count the number of atoms on each side of the equation. Where needed, adjust the numbers in front of the formula so that the numbers of atoms on each side are balanced.

Writing and balancing chemical equations Use chemical formulae to write a balanced chemical equation for the reaction between lead and hydrochloric acid. lead hydrochloric acid lead chloride hydrogen Pb + HCl → PbCl2 + H2 2 Use chemical formulae to write a balanced chemical equation for the reaction between calcium oxide and hydrochloric acid. calcium hydrochloric acid calcium chloride hydrogen Ca + HCl → CaCl2 + H2 2

Your Task Can you write and balance equations? Lets find out!

True or False Quiz! Atoms can be destroyed in a reaction The atoms you start with in a reaction are the atoms you end with Atoms cannot be rearranged into different compounds All atoms are the same Some mass is lost during a reaction as it becomes a gas