Topic 3 - Chemical Changes✅

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What are acids in solutions sources of

Hydrogen ions

What are alkalis in solutions sources of

Hydroxide ions

What is the pH of a neutral solution

7

What is the pH of acidic and alkaline solutions compared to neutral solutions

• Acidic solutions = lower than 7

• Alkaline solutions = higher than 7

What the effect of acid and alkali for litmus solution

• Alkaline = blue

• Acidic = red

What is the effect of acid and alkali on blue litmus paper

• Stays blue in alkali

• Goes red in acidic

What is the effect of acid and alkali on red litmus paper

• Goes blue in alkaline

• Stays red in acidic

What the effect of acid and alkali for methyl orange

• Alkaline = yellow

• Acidic = red

What is the effect of acid and alkali for litmus solution

• Alkaline = blue

• Acidic = red

What the effect of acid and alkali for phenolphthalein

• Alkaline - pink

• Acidic = colourless

How does concentration of hydrogen ions relate to pH

The higher the concentration of hydrogen ions in acidic solution, the lower the pH

How does concentration of hydroxide ions relate to pH

The higher the concentration of hydroxide ions in alkaline solution, the higher the pH

How is hydrogen ion concentration proportionate to pH

• As H+ ion concentration increases by a factor of 10, the pH decreases by 1.

• e.g when H+ increases from 10 to 1000, pH decreases by 1

Core practical METHOD - investigate the change in pH on adding calcium hydroxide to hydrochloric acid

1. Add 25cm of dilute hydrochloric acid to a beaker

2. Add some universal indicator

3. Compare initial colour to a pH colour chart and record pH

4. Add calcium hydroxide one spatula at a time

5. Stir and record pH

6. Keep adding calcium hydroxide until the pH remains constant

How can you improve the accuracy of Core practical "investigate the change in pH on adding calcium hydroxide to hydrochloric acid"

• Use a volumetric pipette instead of a measuring cylinder

• Use a pH probe instead of universal indicator

Explain the term dilute

Lesser amount of substance in a given volume of solution

Explain the term concentrated

Larger amount of substance in a given volume of a solution

Explain the term strong acid

• Fully dissociates in aqueous solution

• (fully breaks down to release H+ ions)

Explain the term weak acid

• An acid that partially dissociates in aqueous solution

• (so less H+ ions are released)

What is a base

Any substance that reacts with an acid to form a salt and water only

What are alkalis

Soluble bases

General reaction of aqueous acids with metals

Acid + metal = salt + hydrogen

General reaction of aqueous acids with metal oxides

Metal oxide + acid = salt + water

General reaction of aqueous acids with metal hydroxides

Metal hydroxide + acid = salt + water

General reaction of aqueous acids with metal carbonates

Metal carbonate + acid = salt + water + carbon dioxide

Describe the test for hydrogen

• Place a light splint in a test tube of gas

• If the gas is hydrogen there will be a squeaky pop

Describe the test for carbon dioxide

• Bubble the gas through limewater

• If carbon dioxide is present, the limewater goes cloudy

What is a neutralisation reaction

When an acid and alkali neutralise each other to produce a salt and water

What is an acid-alkali neutralisation

A reaction in which hydrogen (H+) ions from the acid react with hydroxide (OH-) ions from the alkali, to form water

Explain why, if soluble salts are prepared from an acid and an insoluble reactant, an excess of the reactant is added

So the volume of acid reacts completely

Explain why, if soluble salts are prepared from an acid and an insoluble reactant, the excess reactant is removed

So you are left with just a salt and water

Reactant filtered out

Explain why, if soluble salts are prepared from an acid and an insoluble reactant, the solution remaining is only salt and water

• As the acid has reacted completely and reactant has been filtered out.

• (If a carbonate, carbon dioxide given off to the atmosphere)

Explain why, if soluble salts are prepared from an acid and a soluble reactant, a titration must be used

• Both reactants are soluble, so if one is in excess, you won't be able to remove it easily from the mixtures.

• So you need exact volumes, which can be measured using titration.

Explain why, if soluble salts are prepared from an acid and a soluble reactant, the acid and the soluble reactant are then mixed in the correct proportions

So there is no left over acid or alkali.

Explain why, if soluble salts are prepared from an acid and a soluble reactant, the solution remaining, after reaction, is only salt and water

Because the acid and alkali have completely neutralised.

Core practical METHOD - investigate the preparation of pure, dry, hydrated copper sulfate

1. Using a measuring cylinder, measure 25cm of sulfuric acid into a beaker

2. Place beaker into water bath

3. Add copper oxide to the acid, 1 spatula at a time, stirring between

4. Continue adding copper oxide until it is in excess

5. Put a piece of filter paper in a funnel over a beaker

6. Pour the solution through the funnel to remove excess copper sulfate

7. Pour the filtrate into an evaporating basin

8. Place on a tripod with gauze, and heat with a Bunsen burner

9. When all the water has evaporated, turn off the heat and leave to dry

10. Blue copper sulfate crystals will remain in the basin

Why is copper oxide added in excess for this core practical?

So that all of the sulfuric acid reacts

Describe the steps to carry out an acid-alkali titration

1. Wash burette using acid and water

2. Fill burette to 100cm

3. Add 25cm of alkali into a conical flask

4. Add some phenolphthalein as an indicator

5. Add acid from burette to the alkali until the phenolphthalein goes colourless

6. The titre (volume of acid needed to neutralise the acid) is the difference between the first and second reading on the burette

7. Repeat to gain more precise results

After carrying out an acid-alkali titration how do you prepare a pure, dry salt

• Warm the salt solution to evaporate the water

• Leave to dry

• Crystals form

Solubility rules for nitrates

All nitrates are soluble in water

Solubility rules for chlorides

Common chlorides are soluble in water, except silver and lead

Solubility rules for sulfates

Common sulfates are soluble in water except lead, barium and calcium

Solubility rules for carbonates

Common carbonates are insoluble except sodium, potassium, ammonium

Solubility rules for hydroxides

Common hydroxides are insoluble except sodium, potassium, ammonium

Solubility rules for common salts

All common sodium, potassium and ammonium salts are soluble in water

When do you know if a precipitate will be formed when named solutions are mixed together

• Use the solubility rules to determine if salts formed are soluble or insoluble

• Insoluble salts will form as precipitate

Describe the method used to prepare a pure, dry sample of an insoluble salt

• mix the two solutions needed to form the salt

• filter the mixture using filter paper, which the insoluble salt will be left on

• wash the salt using distilled water

• leave the salt to dry on filter paper

• (water will evaporate, speed this process up by drying it in an oven)

What are electrolytes

· Ionic compounds that are molten or dissolved in water

· When an ionic substance is melted or dissolved in water the ions are free to move

What is electrolysis

· passing an electric current through a solution to cause a chemical reaction that splits the substance into its elements.

· Passing a current through a solution means that the solution can be broken down into elements

Explain the movement of ions during electrolysis

· The positively charged ions (cations) moved to the negatively charged electrode (cathode).

· Negatively charged ions (anions) move to the positive electrode (anode)

What happens during the electrolysis of copper chloride solution

Cathode

• Cu⁺ ions go to the cathode

• Cu is produced

Anode

• Cl^- ions go the anode

• Cl₂ is produced

What happens during the electrolysis of sodium chloride

Cathode

• H⁺ ions go to the cathode

• H₂ is produced

Anode

• C^l- ions go to the anode

• Cl² is produced

What happens during the electrolysis of sodium sulfate

Cathode

• H⁺ ions go the cathode

• H₂ is produced

Anode

• OH^- ions go to the anode

• O₂ is produced

What happens during the electrolysis of water acidified with sulfuric acid

Cathode

• H⁺ goes to the cathode

• H₂ is produced

Anode

• OH^- goes to the anode

• O₂ is produced

What happens during the electrolysis of molten lead bromide

Cathode

• Pb²⁺ goes to the cathode

• Pb is produced

Anode

• Br^- goes to the anode

• Br₂ is produced

Predict the products of electrolysis of other ionic compounds in the molten state

· If the element is in a molten state, you have to identify which ions are present in the ionic compound

· Then you can work out which ions go to the cathode (positive)

· And which ones go to the anode (negative)

Write half equations for the anode and cathode

• Cathode X⁺ + e^- → X

• Anode X^- → X + e^-

What is OILRIG

· Oxidation is loss of electrons

· Reduction is gain of electrons

Which OILRIG occurs at the anode and which at the cathode and why

· Cathode = reduction because electrons are gained to obtain a neutral charge

· Anode = oxidation as there is a loss of electrons to obtain a neutral charge

Explain how electrolysis can be used to purify copper

Carry out electrolysis with an anode made of impure copper and a cathode made of pure copper Use copper sulfate as the solution During electrolysis, Cu2+ ions move from the anode to the cathode, where they gain electrons and are discharged as pure copper Cathode will gain mass and anode will lose mass

Core practical METHOD - electrolysis with inert electrodes

1. Pour copper sulfate solution into a beaker to the half way point

2. Place two inert electrodes into the beaker

3. Attach to a power supply using crocodile clips and wires

4. Fill 2 test tubes with copper sulfate solution and place over electrodes

5. Turn on power

6. Record observations

7. Use a glowing splint to test for gas that has been collected in the test tubes

Diagram for electrolysis with inert electrodes

Results for inert electrodes (observation)

Negative electrode

• Pink solid forms

Positive electrode

• Bubbles of colourless gas

Results for inert electrodes (gas test)

Negative electrode

• None

Positive electrode

• Gas relights a glowing splint - so oxygen is produced

Core practical METHOD - electrolysis with copper electrodes

1. measure the mass of copper electrode and attach to the negative power supply

2. Measure mass of another copper electrode and attach to positive power supply

3. Pour copper sulfate solution into a beaker and place electrodes in the beaker

4. Turn on the power supply for 20 minutes

5. Remove each electrode and wash in distilled water

6. Measure the mass of both electrodes again

7. Repeat the steps with fresh electrodes and copper sulfate solution, but using a different current

Diagram for electrolysis with copper electrodes

Results for electrolysis with copper electrodes

• The mass gained by the negative electrode is equal to the mass lost by the positive electrode

• Mass gained is proportional to current