Chemisty: Acids and Alkali

What is an acid?

A substance that produces H+ ions in water and has a pH less than 7.

How is the acidity measured?

Using the pH scale.

What is an alkali?

A soluble base that produces OH- ions in water and has a pH above 7.

What pH does a neutral solution have?

7

How can the pH of a solution be found?

By using an indicator.

What is an indicator?

A substance that changes colour depending on the pH.

What is the chemical formula for the common lab acid 'hydrochloric acid'?

HCl

What is the chemical formula for the common lab acid 'sulphuric acid'?

H₂SO₄

What is the chemical formula for the common lab acid 'nitric acid'?

HNO₃

What do all acids contain?

Hydrogen ions (H+)

What do all alkalis contain?

Hydroxide ions (OH-)

What is the chemical formula for the common lab alkali 'sodium hydroxide'?

NaOH

What is the chemical formula for the common lab alkali 'potassium hyrdoxide'?

KOH

What is the chemical formula for the common lab alkali 'calcium hydroxide'?

Ca(OH)₂

Name 4 different types of indicators.

Universal indicator, methyl orange, phenolphthalein, litmus (paper)

What is the colour range for universal indicator?

Red for strong acids, orange for weaker acids, green for neutral, blue for weaker alkalis and purple for strong alkalis.

What colour does litmus paper (blue and red) go in an acid?

Blue litmus paper: turns red
Red litmus paper: stays red

What colour does litmus paper (blue and red) go in a neutral solution?

Blue litmus paper: stays blue
Red litmus paper: stays red

What colour does litmus paper (blue and red) go in an alkali?

Blue litmus paper: stays blue
Red litmus paper: turns blue

What colour does phenolphthalein turn in an acid?

Colourless

What colour does phenolphthalein turn in an alkali?

Pink

What colour does phenolphthalein turn in a neutral solution?

Colourless

What colour does methyl orange turn in an acid?

Red

What colour does methyl orange turn in an alkali?

Yellow

What colour does methyl orange turn in a neutral solution?

Orange

What is an ion?

An atom that has become electrically charged by gaining or losing electrons.

What are polyatomic ions?

Ions that are formed when small groups of atoms, held together by covalent bonds, gain or lose electrons.

Name three examples of polyatomic ions.

OH⁻, NO₃⁻, SO₄²⁻

What do acids produce when they dissolve in water?

An excess of hydrogen ions

How is hydrochloric acid formed?

When hydrogen chloride gas is dissolved in water and splits into H⁺ ions and Cl⁻ ions.

What do alkalis produce when they dissolve in water? Give an example.

An excess of hydroxide ions. For example, sodium hydroxide splits into Na⁺ ions and OH⁻ ions in water.

What is the concentration of a substance?

How strong a solution is based on the amount of a dissolved substance in a given volume of a solvent.

What makes a solution higher in concentration?

The higher the number of hydrogen/hydroxide ions in a solution in a certain volume, the higher the concentration.

What does a higher concentration mean in an acid?

The higher the concentration of hydrogen ions, the more acidic the solution is and the lower the pH is.

What does a higher concentration mean in an alkali?

The higher the concentration of hydroxide ions, the more alkaline the solution is and the higher the pH is.

What concentration do neutral solutions have?

They contain low, equal concentrations of hydrogen and hydroxide ions.

What does dilute mean?

To reduce the concentration of a solute in a solution, usually by mixing in more solvent.

Complete the sentence: A concentrated solution contains...

A lot of dissolved solute per unit volume.

Complete the sentence: A dilute solution contains...

Only a small amount of solute.

What is the equation used to calculate the concentration of a solution?

Concentration\= amount dissolved/volume of solution
Units: grams per decimetre cubed, g dm⁻³
(Note: divide cm³ by 1000 to get dm³)

What happens to the pH if the concentration of hydrogen ions in a substance increases by a factor of 10?

The pH decreases by 1

What happens to the pH if the concentration of hydrogen ions in a substance decreases by a factor of 10?

The pH increases by 1

What happens when a weak acid is dissolved in water?

It only partially disassociates into hydrogen ions.

What happens when a strong acid is dissolved in water?

It completely disassociates into hydrogen ions.

What happens as a solution becomes more acidic?

The concentration of hydrogen ions increases.

What happens as a solution becomes more alkaline?

The concentration of hydrogen ions decreases.

What is the difference between a strong acid and a weak acid?

A strong acid completely disassociates into hydrogen ions when dissolved in water while a weak acid only partially disassociates.

What does the pH of an acid depend on?

The type of acid as well as its concentration.

What do the chemical properties of an acid depend on?

The type of acid as well as its concentration.

What is neutralisation?

A chemical reaction in which an acid and a base react quantitatively with each other.

What is 'indication'?

The visible sign left by a chemical indicator which tells us what a substance is.

What is a base?

A substance that react with acids to neutralise them and form a salt and water only.

What are all metal oxides?

Bases

What is a salt?

An ionic compound produced when the hydrogen ion in an acid is replaced with a metal ion. (Acid+Base)

Where do you get the name of a salt?

It has two parts. The first part comes from the metal, metal oxide or metal carbonate. The second part comes from the acid.

Is a base soluble or insoluble?

Insoluble

Acid + Alkali →

Salt + Water

Metal + Acid →

Salt + Hydrogen

Metal Oxide + Acid →

Salt + Water

Metal Hydroxide +Acid →

Salt + Water

Metal Carbonate + Acid →

Salt + Carbon Dioxide + Water

Explain the process of making hydrated copper(iii) sulfate crystals using dilute sulfuric acid.

1) Heat up the dilute sulfuric acid for a few minutes either one Bunsen burner or using a water bath.
2) Add copper oxide until it's in excess and keep stirring it.
3) Then filter that mixture from step 2.
4) Heat the solution on top of a Bunsen burner under the blue flame from step 3 until a hot, saturated solution forms and half of the solution has evaporated.
5) Then allow the solution to cool and the rest of the water to evaporate so that hydrated copper crystals can form.

When making crystals, how would you know when the copper carbonate is in excess?

When it stops dissolving so you can see a black powder.

What is a conical flask?

A flask that has a shape which allows it to be sealed with a bung for heating purposes, while also allowing space for it to be shaken or stirred without spilling anything.

What is a burette?

A piece of equipment that is used to add small measured volumes of one reactant to the other reacting.

What is a pipette?

A piece of equipment used to accurately measure the volume of a reactant before transferring it to a conical flask.

What is a safety pipette filler?

A piece of equipment used to safely fill pipettes by releasing air, drawing liquid into the pipette and accurately release liquid.

What is the difference between an alkali and a base?

A base is insoluble. An alkali is a soluble base.

Explain the core practical of investigating neutralisation.

1) Using a measuring cylinder, measure out a certain amount of acid and pour it into a beaker
2) Put a piece of universal indicator paper on a white tile and dip the end on a glass rod into a liquid then tap it onto the paper.
3) Wait 30 seconds to fins the pH using the pH chart and rinse the glass rod with water
4) Measure out a certain amount of alkali onto a weighing boat and add it to the beaker and stir.
5) Then estimate and record the pH.
6) Repeat steps 2-5 at least seven times
7) Plot a graph with the acid on the y-axis and alkali on the x-axis to see the amount required to reach pH 7, or neutralisation.

What is the ionic equation for neutralisation?

H⁺ + OH⁻ → H₂O

Using hydrochloric acid and sodium hydroxide as an example, explain what would happen in terms of ions during this reaction.

Na⁺ ions and Cl⁻ ions would remain in the solution after neutralisation. These ions would combine to form the solid sodium chloride after the water evaporates.

Explain the process of titration in detail. (using hydrochloric acid and sodium hydroxide as an example)

First you must wash out the burette with hydrochloric acid to remove any remainders of chemicals from previous usage. Then, using a filter with filter paper, pour a certain amount of acid into the burette while the tap is closed. Record the initial reading. Then using a pipette, measure out an exact measurement of sodium hydroxide and pour it into a conical flask. Then add a few drops of indicator (methyl orange or phenolphthalein) to the flask and place it under the burette on top of a white tile. Then open the tap and slowly add drops of acid to the alkali whilst constantly swirling it. Near the end-point, only add a few drops at a time, and close the tap when it changes colour. Record the final reading. Repeat at least 3 times. Subtract the final reading from the initial reading to find the volume of acid required to neutralise the alkali.

Why is methyl orange or phenolphthalein a good indicator to use for titration?

Because they have obvious colour changes which give you a sharp end-point.

How can you make a pure, dry salt using titration?

Carry out titration, note down the exact volume of acid needed to neutralise the alkali, use the burette to add the correct amount of acid without using the indicator, and evaporate the water from the solution formed.

State the reactivity series from most reactive to least reactive.

Potassium, Sodium, Lithium, Calcium, Magnesium, Aluminium, (Carbon), Zinc, Iron, Tin, Lead, (Hydrogen), Copper, Silver, Gold

What is the chemical test for hydrogen?

The burning splint test. If there is a loud bang when a boiling tube full of gas makes contact with open flame from a burning splint, the gas is hydrogen.

What is the chemical test for carbon dioxide?

The limewater test. If the limewater goes cloudy in the presence of a gas, then the gas is carbon dioxide.

What is the chemical test for oxygen?

The glowing splint test. If the gas is oxygen, it will relight a glowing splint.

How do metals at the top of the reactivity series, such as potassium or sodium, react with acids?

They react explosively with acids.

How do metals in the middle of the reactivity series, such as magnesium or zinc, react with acids?

They react steadily with acids.

How do metals at the bottom of the reactivity series, such as copper or silver, react with acids?

They don't react at all.

Why is effervescence seen in a metal and acid reaction?

Because it happens when hydrogen gas bubbles are produced and hydrogen is a product of metal + acid.

Explain in terms of ions what happens during a metal + acid reaction.

The metal atoms react with the hydrogen ions to form metal ions and hydrogen molecules. The other ions from the acid (e.g. sulfate, chloride) don't change during the reaction, so are known as spectator ions. The hydrogen ions gain electrons to form molecules. The magnesium atoms lose electrons.

What is oxidation?

Loss of electrons

What is reduction?

Gain of electrons

What are spectator ions?

Ions that do not take part in the reaction, therefore you don't include them in an ionic equation for a reaction.

What are the visible signs of an acid reacting with a metal carbonate?

There are bubbles of carbon dioxide and the solid metal carbonate disappears if there is enough acid to react with it.

Explain in terms of ions what happens during an acid + carbonate reaction.

The hydrogen ions from the acid react with the carbonate ions to form water and carbon dioxide molecules.

What are ionic equations? Give an example using magnesium reacting with an acid.

They are balanced equations that only show the ions that changed in that reaction.

Mg + 2H⁺ → Mg²⁺ + H₂

What are ionic half equations? Give two example using hydrogen and magnesium.

They show what happens to the electrons of a substance in a reaction.

2H⁺ + 2e → H₂

Mg → Mg²⁺ + 2e

Which nitrates are soluble?

All nitrates

Which nitrates are insoluble?

None

Which chlorides are soluble?

Most of them

Which chlorides are insoluble?

Silver and lead chlorides

Which sulfates are soluble?

Most of them

Which sulfates are insoluble?

Lead, barium and calcium sulfate (but calcium sulfate is slightly soluble)

Which carbonates are soluble?

Sodium, potassium and ammonium carbonates

Which carbonates are insoluble?

Most of them

What is a precipitation reaction?

When soluble substances in solution react to produce an insoluble precipitate.

What type of reaction is the limewater test for carbon dioxide and why?

A precipitation reaction because insoluble calcium carbonate is formed.