Chemical Changes

Which ions make aqueous solutions acidic?

Hydrogen ions (H+)

Which ions make aqueous solutions alkaline?

Hydroxide ions (OH-)

What is the pH scale?

The pH scale ranges from pH 0 to pH 14 and measures the acidity or alkalinity of a solution.

What are pH ranges for acids and alkalis? What the pH of a neutral solution?

Acid - Less than pH 7 (pH 1 is strongest).

Neutral - pH 7.

Alkali - Greater than pH 7 (pH 14 is strongest).

What can be used to measure pH?

Universal indicator; pH probe

What colour is phenolphthalein in acid and alkali?

Acid - Colourless;

Alkali - Pink

What colour is methyl orange in an acid and an alkali?

Acid - Red;

Alkali - Yellow

What colour is blue litmus paper in an acid and an alkali?

Acid - Turns red;

Alkali - Stays blue

What colour is red litmus paper in an acid and an alkali?

Acid - Stays red;

Alkali - Turns blue

Suggest a problem with using universal indicator to test the pH of a solution

The colour is matched to a chart so it is subjective and may vary between people;

It does not give an exact pH value

Acid X has a pH of 1. What can you say about the concentration of hydrogen ions in acid X? (higher only)

There is a high concentration of hydrogen ions; The lower the pH, the higher the concentration of H+ ions

Alkali Y has a pH of 8.5. What can you say about the concentration of hydroxide ions in alkali Y? (higher only)

There is a low concentration of hydroxide ions; The lower the pH of an alkali, the lower the concentration of OH- ions

If pH decreases by one unit, what happens to the concentration hydrogen ions? (higher only)

The hydrogen ion concentration increases by a factor of 10

What is a neutralisation reaction? During an acid-alkali neutralisation reaction, what happens?

A reaction between an acid and a base; H+ ions react with OH- ions to form water

What is the ionic equation for a neutralisation reaction?

H+(aq) + OH-(aq) → H2O(l)

What do the terms concentrated and dilute mean when talking about acid? Is this the same as strong/weak acids? (higher only)

Concentrated acids have more moles per unit volume than dilute acids;

This is not the same as strength, which refers to complete or partial dissociation

An acid only partially dissociates in water. What can be said about the strength of the acid? (higher only)

Weak acid

What is a base?

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

What is an Alkali?

Alkalis are soluble bases

What are the products when an acid reacts with a metal?

Salt and hydrogen;

acid + metal → salt + hydrogen

What are the products when an acid reacts with a metal oxide?

Salt and water;

acid + metal oxide → salt + water

What are the products when an acid reacts with a metal hydroxide?

Salt and water;

acid + metal hydroxide → salt + water

What are the products when an acid reacts with a metal carbonate?

Salt, water and carbon dioxide;

acid + metal carbonate → salt + water + carbon dioxide

Why are metal oxides normally bases rather than alkalis?

They are usually insoluble; alkalis are soluble

What is the name of the salt formed from magnesium and sulfuric acid?

Magnesium sulfate

What is the name of the salt formed from zinc oxide and nitric acid?

Zinc nitrate

What is the name of the salt formed from calcium carbonate and hydrochloric acid?

Calcium chloride

Describe the chemical test for hydrogen

Insert a lit splint;

a squeaky pop is heard if hydrogen is present

Describe the chemical test for carbon dioxide

Bubble through limewater;

it turns cloudy if carbon dioxide is present

When a soluble salt is prepared from an acid and an insoluble reactant, why is excess of the insoluble reactant added?

To ensure all the acid reacts

When a soluble salt is prepared from an acid and an insoluble reactant, how and why is the excess reactant removed?

By filtration; to leave a pure salt solution

What method must be used to prepare a salt from an acid and a soluble reactant? Why?

Titration; because both are soluble so exact volumes are needed to avoid excess

Name the method that could be used to prepare a sample of soluble copper sulfate from insoluble copper oxide and sulfuric acid?

Filtration

What 3 steps are required when producing a pure dry salt from an acid and alkali?

Titration to find exact volumes;

Mix correct volumes;

Evaporate water to form crystals

Describe how to carry out an acid-alkali titration

Add measured acid to flask with indicator;

Fill burette with alkali;

Add alkali to find endpoint;

Repeat carefully;

Record concordant results

Most common chlorides are soluble. What are the two exceptions?

Silver chloride and lead chloride are insoluble

True or false? ‘All nitrates are soluble’

True

Fill in the gap: ‘All common sodium, potassium and ammonium salts are ______’

Soluble

Most common sulfates are soluble. What are the three exceptions?

Lead sulfate, calcium sulfate and barium sulfate are insoluble

Most common carbonates and hydroxides are insoluble. What are the three exceptions?

Sodium, potassium and ammonium carbonates/hydroxides are soluble

What salt is produced when lead reacts with sulfuric acid? Will a precipitate form?

Lead sulfate; Yes, because it is insoluble

How could you prepare a pure, dry sample of an insoluble salt?

Mix solutions; Filter; Wash residue; Dry

What is an electrolyte?

An ionic compound in molten or aqueous state

Why can an electrolyte carry charge?

It has mobile ions that can carry charge

What is electrolysis?

The decomposition of an electrolyte using electrical energy

What is the cathode and anode?

Cathode - negative electrode; Anode - positive electrode

Where do charged ions in the electrolyte move to during electrolysis?

Cations go to cathode; Anions go to anode

What happens at the anode during electrolysis?

Anions lose electrons to form elements

What happens at the cathode during electrolysis?

Cations gain electrons to form elements

Name the processes that occur at each electrode during electrolysis (higher only)

Anode - oxidation; Cathode - reduction

What is formed at each electrode in electrolysis?

Anode - non-metal; Cathode - metal or hydrogen

How can you predict whether a metal or hydrogen will form at the negative electrode?

If metal is more reactive than hydrogen, hydrogen forms; if less reactive, metal forms

What is formed at each electrode during the electrolysis of copper chloride solution?

Anode - chlorine; Cathode - copper

What is formed at each electrode during the electrolysis of sodium sulfate solution?

Anode - oxygen; Cathode - hydrogen

What is formed at each electrode during the electrolysis of molten lead bromide?

Anode - bromine; Cathode - lead

Predict what will be formed at each electrode during the electrolysis of molten zinc chloride

Anode - chlorine; Cathode - zinc

What is formed at each electrode during the electrolysis of sodium chloride solution?

Anode - chlorine; Cathode - hydrogen

What is formed at each electrode during the electrolysis of water acidified with sulfuric acid?

Anode - oxygen; Cathode - hydrogen

What are the half equations for the reactions occuring at the cathode and anode during the electrolysis of copper chloride? (higher only)

Anode: 2Cl- → Cl2 + 2e-; Cathode: Cu2+ + 2e- → Cu

What does oxidation mean in terms of electrons? (higher only)

Loss of electrons

What does reduction mean in terms of electrons? (higher only)

Gain of electrons

Describe how electrolysis of copper sulfate can be used to purify copper

Impure copper anode, pure copper cathode; Cu dissolves from anode and deposits on cathode; impurities fall as sludge