C4: Chemical calculations

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86 Terms

1

Mass of products =

Mass of reactants

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2

Law of conservation of mass

No atoms are lost or made during a chemical reaction so mass of the products = mass of the reactants

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3

When are ionic compounds formed?

When metals react w non-metals

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4

Ions

Charged atoms

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5

What ions do metals form?

Positive

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<p>Rules for ions (metals)</p>

Rules for ions (metals)

Charge on ion = group no in periodic table

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7

What ions do transition metals form?

Positive- but can form diff ions

<p>Positive- but can form diff ions</p>
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8
<p>State the formula for these non metal ions</p>

State the formula for these non metal ions

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9

Which non-metal ions are diff compared to others and why?

Form positive ions, not -

<p>Form positive ions, not -</p>
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Formula of ionic compounds

Charges on ionic compounds cancel out → overall charge is 0

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11

Overall charge on ionic compounds

0

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12

How to work out the charges of ions in an ionic compound?

Drop and swap method

<p>Drop and swap method</p>
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13
<p>Calcium hydroxide formula</p>

Calcium hydroxide formula

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14

Balanced chemical equation

No. of atoms on LHS = RHS

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15

Relative atomic mass

The average mass of the isotopes of that element weighted to take into account the abundance of each isotopes

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Abundance

How common an isotope is

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17

Formula for Ar (based on percentage abundance of isotopes)

((mass no of isotope 1 x percentage abundance of isotope 1) + (mass no of isotope 2 x percentage abundance of isotope 2)) / 100

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18

Relative formula mass (Mr)

Sum of relative atomic masses of the atoms in the no.s shown in the formula

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Key facts about relative formula mass (Mr)

  • No units

  • Doesn’t involve big numbers

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20

In a balanced chemical equation, sum of the Mr of reactants =

Sum of Mr products

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21

Percentage by mass

Shows what % of Mr is due to 1 of the elements in a compound

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22

Percentage by mass equation

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23

Relative atomic mass (Ar) of any element in grams =

1 mole of that atoms of that element

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24

What are chemical amounts measured in?

Moles (mol)

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25

1 mole of a substance contains?

The same no. of particles / atoms / molecules / ions as 1 mole of any other substance

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<p>Number of moles equation</p>

Number of moles equation

Mr mol carries mass

<p>Mr mol carries mass</p>
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27

Number of moles of a compound equation

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How to balance an equation using moles?

  1. Calc no. of moles of each

  2. Write mole no. in front of the respective chemical

  3. Simplest ratio → divide all mole no.s by the smallest no.

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29
<p>Balance this equation</p>

Balance this equation

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30

The no of … in 1 mole of C is the as the no of … in 1 mole of CO2

  • Atoms

  • Molecules

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31

Avogadro’s constant

No. of atoms, molecules or ions in a mole of a given substance

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Value of Avogadro’s constant

6.02 × 1023 per mole

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33
<p>Calc no of moles of atoms in 1 molecule of H2O</p>

Calc no of moles of atoms in 1 molecule of H2O

3 moles of atoms

<p>3 moles of atoms</p>
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How to calculate no. of atoms in a compound?

  1. Calc no. of moles

  2. Multiply by 6.02 × 10²³

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35
<p>Calc no of atoms in 1 mole of HCl</p>

Calc no of atoms in 1 mole of HCl

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36
<p>What does this equation show?</p>

What does this equation show?

1 mole of Mg reacts w 2 moles of HCl to make 1 mole of MgCl2 + 1 mole of H2

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37
<p>Calc no. of atoms in 28g of Lithium</p><p>Ar of Li = 7</p>

Calc no. of atoms in 28g of Lithium

Ar of Li = 7

  1. Calc moles- 28 / 7 = 4

  2. 4 × 6.02 × 10²³

  3. 2.408 × 1024

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38
<p>Calc no. of atoms in 54g of H2O</p>

Calc no. of atoms in 54g of H2O

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39

How to calc masses of reactants + products in chemical reactions?

  1. Balance equation

  2. Calc moles

  3. Use molar ratio to find mass

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40
term image

4 × 100 = 400

<p>4 × 100 = 400</p>
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41

Limiting reactant

Reactant that is completely used up

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42

Why is it called the limiting reactant?

It limits the amount of products formed

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Excess reactant

Reactant not fully used up

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44

In chemical reactions why is an excess of 1 reactant used, rather than exact amounts?

To ensure all the other reactant is used

  • If solid → can filter excess

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45

How to find the limiting reactant?

  1. Calc no of moles

  2. Find molar ratio

  3. Compare to balanced equation

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46
<p>Find the limiting reactant</p>

Find the limiting reactant

LR = HCl

  • 1:1 ratio of reactants

  • 1 mol : 0.25 mol

<p>LR = HCl</p><ul><li><p>1:1 ratio of reactants</p></li><li><p>1 mol : 0.25 mol</p></li></ul>
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47

Concentration

Mass of a solute in a given volume of solution

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Solute

Chemical dissolved in a solvent

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49

Unit of concentration

  • g/dm3

  • mol/dm3

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50

Decimeter

Litre (1000ml)

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51

Concentration equation

  • conc = mass / vol

  • conc = mol / vol

<ul><li><p>conc = mass / vol</p></li><li><p>conc = mol / vol</p></li></ul>
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52

How to increase conc of a solution?

  1. Increase mass of solute, same vol of solution

  2. Same mass of solute, decrease vol of solution

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53

How do you get from cm^3 to dm^3

cm3 divide 1000 = dm3

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54

Yield

Amount of product obtained from a chemical reaction

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55

Why isn’t it always possible to achieve a 100% yield

  1. Reversible reaction doesn’t go to completion

  2. Some product lost when it is separated from the reaction mixture

  3. Some reactants react in ways diff to the expected reaction

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56

Percentage yield equation

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57

100% yield

All reactants end up as products

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Percentage yield

Actual yield compared with the max theoretical amount as a %

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Can a yield greater than 100% be achieved- why / why not?

No

  • Can’t create atoms

  • Due to law of conservation of mass

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60

Percentage yield can’t be?

Greater than 100%

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61
<p>Calculate % yield</p>

Calculate % yield

20%

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62

Theoretical yield

Max amt of product expected from the reaction

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Actual yield

Amt of product actually obtained from the reaction

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<p>Which reaction should be chosen to make ethanol and why?</p>

Which reaction should be chosen to make ethanol and why?

1st → efficient, high atom economy

  • All products in reactant end up as useful product

  • No waste products

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Atom economy (utilisation)

A measure of the amt of starting materials that end up as useful products

  • Determines efficiency of a reaction

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Why is it important to use reactions with high atom economy?

  • Sustainable development

  • Economic reasons

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68

How do reactions with high atom economy increase sustainability?

Resources not wasted

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How are reactions with high atom economy economical?

Minimize production of unwanted products → money saved

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70

Atom economy equation

Mr of desired products (FE) / Sum of Mr of all reactants (FE)

  • FE = from equation

<p>Mr of desired products (FE) / Sum of Mr of all reactants (FE)</p><ul><li><p>FE = from equation</p></li></ul>
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71

Key feature of atom economy

Big numbers count

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Can atom economy be greater than 100%- why / why not?

No

  • Can’t create atoms

  • Law of conservation of mass

  • Mass of reactants = mass of products

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73

What do reactions often produce + why is this bad?

Unwanted side products

  • Waste of money

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What do scientists do with unwanted side products?

Find a use for them

  • Eg in other reactions → money not wasted

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Factors affecting if a particular reaction pathway is chosen to produce a product?

  1. Atom economy

  2. Yield

  3. Rate

  4. Equilibrium position

  5. Usefulness of by-products

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76

Concentration (moles)

No. of moles in a given vol of solution

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77

How to get conc (g/dm3) from conc (mol/dm3)

Conc (g/dm3) = Conc (mol/dm3) x Mr

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78

How to use the conc of 1 solution to determine the conc of another?

  • Use moles

  • CNV

<ul><li><p>Use moles</p></li><li><p>CNV</p></li></ul>
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79
term image

0.71 (2dp)

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80

Equal amounts of moles of gas occupy?

The same volume under the same conditions of temp + pressure

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81

Volume of 1 mole of any gas at room temp + pressure

24 dm3

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82

Room temp + room pressure

  • 20°C

  • 1 atmosphere

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83

Volume of gas equation

No. of moles x 24 = vol of gas

<p>No. of moles x 24 = vol of gas</p>
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84
term image

120 dm³

<p>120 dm³</p>
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85

Why may some reactions involving a gas appear to break the law of conservation of mass?

  • Reaction in conical flask

  • Gas produced → gas escapes into air

  • So mass reading of products less than reactants

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86

Examples of reactions that appear the break the law of conservation of mass?

  1. Metal react w O2

    • Mass metal oxide greater than mass of the metal

  2. Thermal decompositions of metal carbonates

    • CO2 produced + escapes into atmosphere

    • So metal oxide only solid product

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