C4: Chemical calculations

Mass of products =

Mass of reactants

Law of conservation of mass

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

When are ionic compounds formed?

When metals react w non-metals

Ions

Charged atoms

What ions do metals form?

Positive

Rules for ions (metals)

Charge on ion = group no in periodic table

What ions do transition metals form?

Positive- but can form diff ions

State the formula for these non metal ions

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

Form positive ions, not -

Formula of ionic compounds

Charges on ionic compounds cancel out → overall charge is 0

Overall charge on ionic compounds

0

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

Drop and swap method

Calcium hydroxide formula

Balanced chemical equation

No. of atoms on LHS = RHS

Relative atomic mass

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

Abundance

How common an isotope is

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

Relative formula mass (Mr)

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

Key facts about relative formula mass (Mr)

• No units

• Doesn’t involve big numbers

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

Sum of Mr products

Percentage by mass

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

Percentage by mass equation

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

1 mole of that atoms of that element

What are chemical amounts measured in?

Moles (mol)

1 mole of a substance contains?

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

Number of moles equation

Mr mol carries mass

Number of moles of a compound equation

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.

Balance this equation

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

• Atoms

• Molecules

Avogadro’s constant

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

Value of Avogadro’s constant

6.02 × 10²³ per mole

Calc no of moles of atoms in 1 molecule of H2O

3 moles of atoms

How to calculate no. of atoms in a compound?

1. Calc no. of moles

2. Multiply by 6.02 × 10²³

Calc no of atoms in 1 mole of HCl

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

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

Calc no. of atoms in 54g of H2O

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

1. Balance equation

2. Calc moles

3. Use molar ratio to find mass

4 × 100 = 400

Limiting reactant

Reactant that is completely used up

Why is it called the limiting reactant?

It limits the amount of products formed

Excess reactant

Reactant not fully used up

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

How to find the limiting reactant?

1. Calc no of moles

2. Find molar ratio

3. Compare to balanced equation

Find the limiting reactant

LR = HCl

• 1:1 ratio of reactants

• 1 mol : 0.25 mol

Concentration

Mass of a solute in a given volume of solution

Solute

Chemical dissolved in a solvent

Unit of concentration

• g/dm3

• mol/dm3

Decimeter

Litre (1000ml)

Concentration equation

• conc = mass / vol

• conc = mol / vol

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

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

cm3 divide 1000 = dm3

Yield

Amount of product obtained from a chemical reaction

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

Percentage yield equation

100% yield

All reactants end up as products

Percentage yield

Actual yield compared with the max theoretical amount as a %

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

No

• Can’t create atoms

• Due to law of conservation of mass

Percentage yield can’t be?

Greater than 100%

Calculate % yield

20%

Theoretical yield

Max amt of product expected from the reaction

Actual yield

Amt of product actually obtained from the reaction

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

Atom economy (utilisation)

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

• Determines efficiency of a reaction

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

• Sustainable development

• Economic reasons

How do reactions with high atom economy increase sustainability?

Resources not wasted

How are reactions with high atom economy economical?

Minimize production of unwanted products → money saved

Atom economy equation

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

• FE = from equation

Key feature of atom economy

Big numbers count

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

What do reactions often produce + why is this bad?

Unwanted side products

• Waste of money

What do scientists do with unwanted side products?

Find a use for them

• Eg in other reactions → money not wasted

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

Concentration (moles)

No. of moles in a given vol of solution

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

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

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

• Use moles

• CNV

0.71 (2dp)

Equal amounts of moles of gas occupy?

The same volume under the same conditions of temp + pressure

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

24 dm3

Room temp + room pressure

• 20°C

• 1 atmosphere

Volume of gas equation

No. of moles x 24 = vol of gas

120 dm³

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

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