Chemistry A level OCR A

Chemical vs physical properties of isotopes

What is relative isotopic mass?

Relative isotopic mass - The mass of an atom of an isotope compared to 1/12 the mass of a carbon-12 atom.

What is relative atomic mass?

Relative atomic mass (A_r) - The weighted mean mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom.

What is relative molecular mass?

Relative molecular mass (M_r) - The average mass of a molecule compared to 1/12 the mass of a carbon-12 atom.

What is relative formula mass?

Relative formula mass - Used for ionic compounds. It is calculated by adding up the A_r values of all the ions in one formula unit.

What does a mass spectrum do?

Key facts about plotting a mass spectrum?

A mass spectrum plots the relative abundance of ions against their mass-to-charge ratio (m/z).

In a molecular sample in a mass spectrum, how do you find the molecular mass?

It is the right most significant peak.

Another peak beyond this may be caused by a different isotope in the molecule in trace amounts.

What is the empirical formula?

This formula gives the simplest whole number ratio of atoms of each element present in a compound.

Charge of an electron?

1.6 × 10^-19

Fact about any gas?

One mole of any gas at room temperature and pressure will occupy 24 dm³

Ideal gas equation?

What is the gas constant?

8.3145 J mol^-1 K^-1

What is a theoretical yield?

The theoretical yield of a chemical reaction is the maximum mass of product that could be produced, assuming the reaction goes to completion with no loss of product.

What is actual yield?

Key fact and points about it?

What is percentage yield?

What can be said about its value?

Percentage yield is a measure of how efficient a reaction is, indicating how close the actual yield is to the theoretical maximum.

What is atom economy?

1st reaction type and its facts in relation to atom economy.,

Addition reactions

Two (or more) reactants combine to form a single product.

Since all the reactants are used in making the product, the atom economy is always 100%

2nd reaction type, and its facts in relation to atom economy.

Substitution reaction

Some atoms are displaced between reactants, generating at least 2 products: the desirable product and one by-product.

Think reducing copper with carbon.

Atom economy of such reactions are (always) less than 100%.

Advantages of a high atom economy?

What are acids?

They are species that give up protons (H⁺)

What are bases?

They are species that accept protons (H⁺)

How is ammonia a base?

It does not produce hydroxide ions directly, but it does stimulate water to do so:

NH_3+H_2O_{}\leftrightarrow NH_4^{+}+OH^{-}

Then:

NH_4^{+}+OH^{-}+HCl_{}\to NH_4Cl+H_2O

But since it starts and ends with water, that part can be removed:

NH_{3\left(aq\right)}+HCl_{\left(aq\right)}\to NH_4Cl_{\left(aq\right)}

What does the strength of an acid / alkali depend on?

If they fully ionise / dissociate in solution.

A strong acid will fully ionise / dissociate in solution, the reaction is not reversible:

HCl_{}\to H^{+}+Cl^{-}

A weak acid will partially ionise / dissociate in solution, this reaction is reversible:

CH_3OOH\leftrightarrow CH_3OO^{-}+H^{+}

4 Acid reactions with metals and their compounds

What is titration?

It is an analytical method used to determine the concentration of a solution of a known volume by adding a solution with a known concentration to it until neutralisation occurs.

How do you do a titration?

How to read a burette.

The volume of the burette should be recorded to the nearest half division (nearest 1cm³).

There will be a meniscus, a curved surface of the liquid, caused by adhesive forces between the walls and the liquid.

Readings should be taken from the bottom of the meniscus.

What indicators should you use in titration?

What else is important?

Methyl Orange - This changed yellow to red when acid is added to an alkali.

Phenolphthalein - Turns from pink in alkaline to colourless in neutral.

There should be a white tile under the conical flask to ensure easy readings.

How to prepare accurate known concentrations.

1. Measure the mass of the solute using a mass balance

2. Dissolve it in distilled water in a beaker

3. Transfer it to a volumetric flask of a specific volume

4. Rinse the beaker several times, adding the rinsings to the flask.

5. Using a pipette, top up the flask to the measurement line.

6. Invert the flask to allow for thorough mixing of the solution.

Oxidation number rules (no oxygen or fluorine)

1. Pure element → 0

2. Ions → solve like an equation

3. The more electronegative element will have a negative oxidation number.

Oxidation number of Oxygen

Oxide = -2

Peroxide = -1

Superoxide = -0.5

The 2 OPs keep having after I deal with them.

If it is bonded to fluorine → +2

Oxidation number of Fluorine.

F₂ → 0

Bonded → -1

Oxidation state of Hydrogen

Bonded with a non-metal → +1

Bonded with a metal → -1

Electronegativity series

What else can Electronegativity be used for?

To determine the bond type:

Oxygen + Another element in an ion makes…

-ate, e.g.

Sulphate

Chlorate

What is oxidation?

What is reduction?

Oxidation Is Loss (of electrons)

Reduction Is Gain (of electrons)

(Use oxidation numbers to figure this out)

What is an oxidising agent?

What is a reducing agent?

An oxidising agent accepts electrons, and is itself reduced.

A reducing agent donated electrons, and is itself oxidised.

(Use oxidation numbers to figure this out)