Introducing Chemical Reactions

What is an element?

A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler

What type of formula is a metallic element?

Always an empirical formula and cannot be made any simpler

What are the formulae of non-metallic elements?

• non-metallic elements in group 0 exist as individual atoms, so their formula is the same as their symbol

• many other non-metallic elements exist as molecules and giant structures, so their formulae are not always empirical

Which elements must be written as molecules as they exist in nature as two atoms joined together?

H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂

What is a compound?

A pure substance made of two or more elements chemically combined and which cannot be separated by physical means

What number of compounds are there?

unlimited

What does the chemical formula of a compound tell you?

The ratio of atoms in a compound

How must you write the chemical formula for ionic compounds?

• the symbol of the metal is always written first

• always an empirical formula

Why is the chemical formula for ionic compounds always an empirical formula?

Ionic compounds are giant structures containing millions of ions so the formula used is the simplest possible ratio of the ions in the structure

Can ionic compounds not contain metals?

Give an example

In these cases what is written first?

• occasionally

• ammonium chloride, NH₄Cl, which contains positive ammonium ions and negative chloride ions

• we write the positive ion first

Why are roman numerals often used to indicate the charge of transition metals?

The charges on the ions can vary

How are roman numerals used in the compounds formed from transition elements?

Give an example of this

• to show the charge (or oxidation state) of metal ions

• in copper (II) oxide, the copper ion has a charge of 2+ whereas in copper(I) nitrate, the copper has a charge of 1+

Are there some non-metal positive ions?

If so, name 2

• yes

• NH4+ (ammonium)

• H+ (hydrogen)

What suffix is used for non-metals with a negative charge in groups 5-7?

Give 4 examples

• ‘ide’

• nitride, chloride, bromide, iodide

What are polyatomic or compound negative ions?

Negative ions made up of more than one type of atom

Compound ions: what is the chemical formula and charge of ammonium?

NH₄⁺

Compound ions: what is the chemical formula and charge of hydroxide?

OH⁻

Compound ions: what is the chemical formula and charge of nitrate?

NO₃⁻

Compound ions: what is the chemical formula and charge of sulfate?

SO₄²⁻

Compound ions: what is the chemical formula and charge of carbonate?

CO₃²⁻

Compound ions: what is the chemical formula and charge of hydrogen carbonate?

HCO₃⁻

Compound ions: what is the chemical formula and charge of phosphate?

PO₄³⁻

When balancing equations to get the same number of atoms of each element on either side, what law does it relate to?

Law of Conservation of Mass

When thinking about state symbols, what is the general rule for metal compounds unless they are in a solution?

What about ionic compounds unless they are in a solution?

• will always be solid, although there are a few exceptions

• ionic compounds will usually be solids

What can non metal compounds be?

What does it depend on?

• solids, liquids or gases

• depends on chemical structure

What do precipitates formed in solutions count as?

Solids

What is aqueous?

dissolved in water

What are state symbols for?

For showing which physical state each substance is in

What happens to ionic compounds in aqueous solutions?

Give an example as a symbol equation

• They dissociate into their ions, meaning they separate into the component ions that formed them

• hydrochloric acid and potassium hydroxide:

HCl (aq) →  H⁺ (aq) + Cl^-(aq) 

KOH (aq)  → K⁺ (aq)  + OH^- (aq) 

Write a step by step method to writing the ionic equation for the reaction of aqueous chlorine and aqueous potassium iodide

Step 1: Write out the full balanced equation:

2KI (aq) +  Cl₂ (aq) → 2KCl (aq) + I₂ (aq)

Step 2: Identify the ionic substances and write down the ions separately

2K⁺ (aq) + 2I^- (aq) +  Cl₂ (aq) → 2K⁺ (aq) + 2Cl^- (aq) + I₂ (aq)

Step 3: Rewrite the equation eliminating the ions which appear on both sides of the equation (spectator ions ) which in this case are the K⁺ ions:

 2I^- (aq) +  Cl₂ (aq) → 2Cl^- (aq) + I₂ (aq)

What is a half equation for?

How would you do them for the reaction between sodium and chlorine?

• Representing what happens to one of the reactants in a chemical reaction

• 2Na (s) + Cl₂ (g) → 2NaCl (s)

  • This reaction can be split into two half equations which require electrons to balance them

  • The sodium reaction involves the sodium losing an electron

  Na → Na⁺ +  e^-

  • The chlorine reaction involves the chlorine molecule gaining two electrons

  Cl₂ + 2e^- → 2Cl^-

  • Notice that the half equations do not have to have the same coefficients as the overall equation, but they do need to be balanced in the atoms and charges  

What are chemical amounts measured in?

Moles

What does one mole of a substance contain?

What was the previous definition of a mole?

What was the revised edition in 2018 by the International union of Pure & Applied Chemistry?

• The same number of the stated particles, atoms, molecules, or ions as on mole of any other substance

• the number of atoms present in exactly 12.00g of carbon-12

• one mole contains exactly 6.02214076 × 10²³ elementary entities

What is the number from the previous card called?

What is it called when used with units of mol^-1 ?

What should the constant be rounded to for practical purposes?            

• the Avogadro number

• the Avogadro Constant

• 6.02 x 10²³ mol^-1

Why do we use mol^-1 as the unit for Avogadro’s Constant?

Give 3 examples

• Because this is the number of entities per mole of a substance

• One mole of sodium (Na) contains  6.02 x 10²³  atoms of sodium

• One mole of hydrogen (H₂) contains  6.02 x 10²³  molecules of hydrogen

• One mole of sodium chloride (NaCl) contains  6.02 x 10²³ formula units of sodium chloride

What is the mass of one mole of a substance known as?

What is it the same for in with an element?

And for a compound?

• the molar mass

• the relative atomic mass written in grams

• the relative formula mass or relative molecular mass in grams

How do we find the mass of one atom of an element?

What’s the equation?

We divide the molar mass of that element by the Avogadro constant

• mass of 1 atom or molecule = (molar mass) / (avogadro constant)