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Calculating Reacting Masses. HIGHER LESSON. Learning Objectives. HIGHER LESSON. To explain what chemical equations tell us about chemical reactions To explain how we use equations to calculate masses of reactants and products. Starter Activity. HIGHER LESSON.

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learning objectives
Learning Objectives

HIGHER LESSON

  • To explain what chemical equations tell us about chemical reactions
  • To explain how we use equations to calculate masses of reactants and products
starter activity
Starter Activity

HIGHER LESSON

  • Balance the below equations:
  • Mg + O2 MgO
  • AgNO3 + MgCl2  AgCl + Mg(NO3)2

ANSWERS

  • 2Mg + O2 2MgO
  • 2AgNO3 + MgCl2  2AgCl + Mg(NO3)2
key question

To explain what chemical equations tell us about chemical reactions

Key Question

HIGHER LESSON

Why do we need to balance chemical equations?

DISCUSS 1 MINUTE

balanced equations

To explain what chemical equations tell us about chemical reactions

Balanced Equations

HIGHER LESSON

  • They tell us how much of each substance is involved in a chemical reaction

2H2 + O2 2H2O

2 moles of hydrogen react with 1 mole of oxygen to make 2 moles of water

  • This is useful because now we can use it to work out what mass of hydrogen and oxygen we need and how much water is made
2h 2 o 2 2h 2 o

To explain what chemical equations tell us about chemical reactions

2H2 + O2 2H2O

HIGHER LESSON

Masses in the equation

  • 2moles of hydrogen = 2x 2g = 4g
  • 1 mole of oxygen = 1 x 16g = 16g
  • 2 moles of water = 2 x 18g = 36g
slide7

To explain what chemical equations tell us about chemical reactions

Task

HIGHER LESSON

  • Balance and explain in terms of moles and mass what is happening in the reaction
  • Mg + HCl MgCl2 + H2
  • NaOH + Cl2  NaOCl + NaCl + H2O
answers

To explain what chemical equations tell us about chemical reactions

Answers

HIGHER LESSON

  • Mg + 2HCl  MgCl2 + H2

1 mole of magnesium reacts with 2 moles of hydrochloric acid to produce 1 mole of magnesium chloride and 1 mole of hydrogen.

1 mole of Magnesium = 24g 1 mole of magnesium chloride = 95g

2 moles of hydrochloric acid = 73g 1 mole of hydrogen = 2g

  • 2NaOH + Cl2  NaOCl + NaCl + H2O

2 moles of sodium hydroxide reacts with 1 mole of chlorine to produce 1 mole of sodium hypochlorite, 1 mole of sodium chloride and 1 mole of water

2 moles of sodium hydroxide = 80g 1 mole of sodium hypochloite = 74.5g

1 mole of chlorine= 71g 1 mole of sodium chloride = 58.5g

1 mole of water = 18g

how much

To explain how we use equations to calculate masses of reactants and products

How Much?

HIGHER LESSON

We need balanced chemical equations so we know how much of two chemicals to react together.

If we have too much of a reactant it will be wasted

If we have too little of a reactant not all of the other reactant will react

but first if you do not have
But first if you do not have…….
  • Write down every

equation from this triangle.

Or

  • Draw the triangle.
worked example 1

To explain how we use equations to calculate masses of reactants and products

Worked Example 1

HIGHER LESSON

If we have a solution containing 100g of sodium hydroxide, how much chlorine gas should we pass through the solution to make bleach?

2NaOH + Cl2  NaOCl + NaCl + H2O

NaOH = 40g Cl2 = 71g

So 100g of sodium hydroxide is 100 ÷ 40 = 2.5 moles

The chemical equation tells us that for every 2moles of sodium hydroxide we need one mole of chlorine.

So we need2.5 ÷ 2 = 1.25 moles of chlorine

So we’ll need 1.25 x 71g = 88.75g of chlorine to react with 100g of sodium hydroxide

STEP 1 : Balance the equation

STEP 2 : Work out the mass of 1 mole

STEP 3 : Work out moles of

the mass in the question

STEP 4 : Calculate the number of moles you need of the other reactant

STEP 5 : Multiply the number of moles needed by the mass of 1 mole substance

worked example 2

To explain how we use equations to calculate masses of reactants and products

Worked Example 2

HIGHER LESSON

You start with a solution containing 0.95g of magnesium chloride. You add silver nitrate. If all the magnesium chloride reacts, how much silver chloride could be made?

2AgNO3 + MgCl2  2AgCl + Mg(NO3)2

MgCl2 = 95gAgCl = 143.5g

So 0.95g of Magnesium chloride is 0.95g ÷ 95g = 0.01 moles

The chemical equation tells us that for every 1 mole of magnesium chloride we need 2 moles of silver chloride

0.01 x 2 = 0.02 moles of silver chloride

0.02 x 143.5g = 2.87g

slide13

To explain how we use equations to calculate masses of reactants and products

Task

HIGHER LESSON

  • Zinc oxide is heated with carbon to make zinc and carbon monoxide. How much zinc oxide do you need to make 130g of zinc?
  • A student adds 4.8g of magnesium to excess dilute hydrochloric acid. What mass of magnesium chloride would be made?
  • If you add 5.5g of sodium carbonate to excess dilute sulphuric acid, what mass of sodium sulphate would be made?

162g

19g

7.1g (rounded)

7.3g (un rounded throughout question)