Chemical changes

What is the pH scale

A measure of how alkaline or acidic a solution is

What numbers of the pH scale are acidic and what colour are they

• x < 7 → less than 7 so 0-6.9

• Red or yellow

What numbers of the pH scale are neutral and what colour are they

• 7

• Green

What numbers of the pH scale are alkaline and what colour are they

• x > 7 → greater than 7, 8-14

• Blue or purple

How do we measure pH

• use an indicator → group of chemical dyes that change depending on pH. E.g universal indicator (wide range indicator so can change colours over many pHs)

• pH probe → much more accurate as it doesn’t involve humans guessing shades and so more reliable

What is an acid

Any substance that forms aqueous solutions with a pH less than 7

What’s a base

Any substance with a pH greater than 7

What are alkaline

• subgroup of bases

• A base that dissolves in water to form a solution wit a pH greater than 7

• Form OH- ions in water → hydroxide ions

What do you get if you react an acid and base, what does it produce

• neutralisation reaction

• Products are a salt + water

• Products are neutral and have pH 7

What is the formula for sulfuric acid

H2SO4

What is the formula for hydrochloride acid

HCl

What’s the formula for nitric acid

HNO3

What’s the formula for sodium hydroxide (base)

NaOH

What’s the formula for calcium carbonate

CaCO3

What’s titration used for

find an unknown concentration of an acid or alkali

What equipment is needed for titration (R.P)

1. A pipette to accurately measure a certain volume of acid or alkali (25cm3)

2. a comical flask to contain liquid from pipette

3. A burette to add alkali or acid to conical flask

4. White tile to place conical flask on

What is the titration method

1. Use the pipette to add 25cm3 of alkali to a conical flask

2. Add a daw drops of indicator and put the conical flask on white tile

3. Fill burette with acid and note the starting volume

4. Slowly add the acid from the burette to the alkali in the conics, flask, swirling to mix

5. Stop adding acid when the end point is reached (when the acid has neutralised the alkali and the indicator changes colour)

6. Note the final volume reading and calculate how much acid you added in total

7. Repeat titration u til you get concord at results which means figures if acid that are within 0.10cm3 of each other

8. Use concordant results to calculate the mean column of acid required to neutralise the alkali

Why is it important to swirl the flask during titration

Even,y distribute the acid and ensure the colour change ensures as soon as they neutralisation takes place

Why is it important you place the conical flask on a white tile

So you can see the colour change happen as soon as possible

What colour change happens if you add an acid into an alkali in the flask

changes from colourless to pink

What colour change happens if you add a alkaline into a acid in the conical flask

Changes from colourless to purple

What are the 3 different type of indicators

• litmus

  • red in acidic solutions

  • Blue in alkaline solutions

• phenolphthalein

  • Colourless in acidic solutions

  • Pink in alkaline solutions

• Methyl orange

  • red in acidic solutions

  • Yellow in alkaline solutions

What is pH a concentration of

H+ ions

What do acids do to release H+ ions

Ionises

What’s a strong acid

An acid that ionises completely

• all the acid particles will dissociate / turn fully into products

What’s a weak acid

An acid that hasn’t fully ionised

• small amount dissociate (release H+ ions)

• Reversible reactions but as it doesn’t all dissociate / doesn’t fully turn into products, the equilibrium lies to the left

What does each decrease by one on the pH scale represent

• an increase of H+ ions by x 10 each time

How do you get an acidic solution with a low pH

Lots of H+ ions

Why can you only get a low pH of a weak acid in a very concentrated solution

• only few acid particles ionise and release H+ ions so would need a high concentration to get more H+ ions to create a low pH

What happens during a neutralisation reaction in terms of hydrogen ions and hydroxide ions.

The positive hydrogen ions from an acid react with the negative hydroxide ions from the alkali to form molecules of water

When an acid reacts with a metal oxide what is produced

Metal oxides + acid → salt + water

When an acid reacts with a metal hydroxide what is produced

Metal hydroxide + acid → salt + water

What happens when acids react with metal carbonates

acid + metal carbonate → salt + H2O + CO2

How do u make a soluable salt

React an acid with an insoluble base

What is the method to create soluble salt crystals from a acid base reaction

part 1 - obtain a solutions of the soluble salt we want

1. Put dilute acid in a beaker and gently heat with a Bunsen burner

2. Add the insoluble base a little at a time until it stops reacting, which means it’s in excess

3. Isolate the salt solution by filtering out the excess stolid base using filter paper and a funnel

   part 2 - Isco Kate the so,it me salt crystals from the solution

4. Heat the salt solution gently in a water bath until crystals start to form

5. Let the solution cool further, which will cause more crystals to precipitate

6. Filter out the soluble sat crystals using filter paper and funnel

What is the order of metals reactivity series

Most reactive :

• potassium

• Sodium lithium

• Calcium

  - - - - - - - -

• Magnesium

• Carbon

• Zinc

• Iron

  — - - - - - - - -

• Hydrogen

• Copper

What factor determines the reactivity of a metal

How easily the atoms of that element lose there outer electrons (so they have a full outer shell and become a positive ion )

What group of metal is the most reactive

Group 1 as it is easier for them to owe electrons as they only have 1 in their outer shell

If you react metal with an acid what does it form

Metals + acid → salt + H2

What happens when potassium reacts with acid

Most reactive so reacts vigorously

• could produce a flame as it releases lots of hydrogen which can catch fire

What happens when magnesium reacts with acid

Bubbles form on the surface

What is a displacement reaction

Where a more reactive metal displaces a less reactive metal

How do you measure the temperature change if the metals to test for reactivity

• the most reactive metals will produce the most heat

• need to make sure it’s a fair test by m

  • each metal has the same mass and surface area

  • The acid is the same type and has the same concentration each time

When you react metals and water what is formed

Metals + water → metal hydroxides + hydrogen

• Only the most reactive metals can do this (potassium, sodium, lithium, calcium)

What is oxidation

The process of gaining oxygen

What is reduction

The loss of oxygen

What happens when metals react with oxygen

Most metals are fairly reactive so react and form a metal oxide

What’s an example of unreactive metals

Gold

→ because you find them as pure metals

How do you get a pure metal

• reduction

• React the metal with carbon if it is less reactive as it displaces the metal and reacts with the oxygen, the metal loses oxygen so becomes reduced

• This produces CO2

What’s an ore

What is a redox reaction

When both oxidisation and reduction take place at the same time

What’s oxidisation in terms of electrons

Loss of electrons

What is reduction in terms of electrons

Gain of electrons

Where can redox reactions take place

• displacement reactions → can write as ionic equations

What’s an ionic equation

Only Show the reactions that take part in the reaction. → remove spectator

What’s the equipment for electrolysis

What’s an electrolyte

Liquid or solution that contains an ionic compound (ions that are free to move)

How do you get a molten liquid

Melt the insoluble product to get a liquid so ions are free to move

What are electrodes

Solid conductors

• generally made of metal or carbon

What is the positive electrode called

Anode

What’s the negative electrode called

Cathode

What is the battery used for in electrolysis

• Drives the flow of electrons

What does electrolysis mean

Splitting up ionic compounds into their pure elements with electricity

How does electrolysis work to separate lead bromide

• the wire provides a current

• Negative bromide ions will be attracted to the anode (opposites attract) and are discharged (becomes neutral). This causes the atoms to pair up and becomes bromine gas as it is oxidised→ the electrons from the bromide ions flow through to wire to the led ions where they will be reduced

• The positive led ions are attracted the negative cathode (opposite charges attract) and become discharged. The led then falls to the bottom and forms a layer of molten led. The ,ed ions are reduced

In electrolysis, what direction do electrons flow

From the anode to the cathode

Why are the electrodes normally made of inert carbon

Inert means it’s unreactive, so will not take place in a reaction

In electrolysis, why do the compounds need to be molten or dissolved

So the ions are free to flow and move around

Why is electrolysis not used to extract all metals

• expensive because it requires large amounts of electricity so if less reactive than carbon, reduction with carbon is used to displace the metals instead

Why is electrolysis used to extract aluminium

Aluminium is more reactive than carbon

• however Al is solid (ions are fixed → bad) and it’s often found within an ore called bauxite

  • therefore have to turn it into molten aluminium oxide before using electrolysis

1. Purify the aluminium oxide from the bauxite that was mined from the ground

2. As it has a high melting point, we mix aluminium oxide with cryolite to lower the melting point

3. Then melt Aluminium oxide to make them molten

1. The positive 3+ ions are attracted the cathode

2. The negative -2 oxygen ions are attracted to the anode electrode. The oxygen ions are oxidised. The oxygen molecules can firm as a pair and form a gas

3. Electrons travel through the wire to the cathode where the aluminium ions are reduced. The aluminium metal forms at the bottom of the beaker

What do you do to soluble compounds in electrolysis

Dissolve them in water to from the electrolyte

in aqueous solutions, what ions are also present in the electrolyte

• hydrogen ions H+

• Hydroxide ions HO+

What’s the rule at the cathode in electrolysis aqueous solutions

The ion of the least reactive element will be discharged ( if the metal is more reactive than hydrogen, hydrogen is the ion present at the cathode)

What’s the rule at the anode in aqueous electrolysis

If a halide is present, it will be discharged/ present. If not hydroxide will be discharged