Physical Chemistry (T4)

Question 1

If an acid is reacted with an alkali the end result will always be… And why?

Answer 1

• a neutral salt

- the metal atom from the alkali and part of the acid molecule combine to produce a salt

Question 2

What happens when ammonia (NH3) is exposed to water?

Answer 2

• this produces an alkaline solution, which can then be used to make ammonia salts
• ammonium salts are commonly used in fertilisers

Question 3

What must a salt have to be a carbonate?

Answer 3

• to be a carbonate a salt must have CO3 within it
• normally this is two too many electrons so you may see it written as CO3 -2
• the -2 indicates the negative charge due to the extra electrons

Question 4

What is one of the most commonly used carbonates, how is it acquired and what is it often used for?

Answer 4

• calcium carbonate
• CaCO3
• calcium carbonate is often quarried straight out of the ground within limestone
• it is used to limit acidity in soils or to create many materials used in modern life, such as cement and glass

Question 5

If we mix a metal carbonate with an acid, what is generally produced? Using calcium carbonate and hydrochloric acid as an example, give the word and chemical formulas…

Answer 5

The action often produces carbon dioxide, a salt and water

Calcium carbonate + hydrochloric acid > calcium chlorate + water + carbon dioxide

CaCO3 + 2HCl > CaCl2 + H2O + CO2

Question 6

Calcium carbonate + hydrochloric acid > calcium chlorate + water + carbon dioxide

If we were to replace the calcium with another metal (say, zinc), what would happen? Give the applicable word and chemical equations..

Answer 6

We would get virtually the same reaction:

Zinc carbonate + hydrochloric acid > zinc chlorate + water + carbon dioxide

ZnCO3 (s) + 2HCl (aq) > ZnCl2 (s) + H2O (l) + CO2 (g)

Question 7

The release of carbon dioxide from carbonates can also occur with just the carbonate and added heat. What process is this and how does it work? Use zinc carbonate as an example..

Answer 7

• the release of carbon dioxide also occurs in a process called thermal decomposition

Eg:

Zinc carbonate > zinc oxide + carbon dioxide

ZnCO3 (s) > ZnO (s) + CO2 (g)

Question 8

Metals ….. in the reactivity series have carbonates that need …… of energy to to decompose them. Metals ….. in the reactivity series have carbonates that are ….. decomposed

Answer 8

Metals high up in the reactivity series have carbonates that need a lot of energy to to decompose them. Metals low down in the reactivity series have carbonates that are easily decomposed

Question 9

While some material might not react, we can generally use metals or bases to make salts in water (or aqueous salts)

If we are using metals, some will remain resistant or inert (like gold which is resistant to most acids) or some will not react properly. We could use an insoluble base but this generally involves mixing base to the acid until saturation is reached.

What do we mean by saturation?

Answer 9

• saturation is when the capacity of the acid to react is filled
• the liquid becomes as ‘full’ of the salt as can be and so will stop reacting completely
• when this happens, any remaining solid base is merely filtered off to be used elsewhere

Question 10

In most cases, the simplest way of reacting an acid with a base is…

Answer 10

…a simple use of two aqueous solutions

Question 11

If we want to react a base with an acid but the base is insoluble, what do we do?

Answer 11

• the base is powered and mixed with the acid
• the is to increase something we refer to as the surface area
• this means the acid can get to the surface of as much of the base as possible

Question 12

Why are powdered solids often used in reactions?

Answer 12

• to increase surface area
• if a solid base is powered and added to the acid then the acid can reach as much of the surface of the base as possible
• using blocks would also take longer to react
• for example, think of the difference between a sugar cube and powdered sugar dissolving in water

Question 13

If calcium hydroxide is reacted with hydrochloric acid to produce calcium chloride, the calcium chloride will be in aqueous form.

For application such as dialysis, we need the calcium chloride to be in a crystalline or solid form. How would we go about achieving this?

Answer 13

• crystallisation
• the substance is placed in a shallow tray and gently heated to evaporate off the water and leave pure salt crystals behind

Question 14

In short, what is precipitation?

Answer 14

A process by which soluble metals in water can be removed.

A solid is formed within a liquid through a reaction.

Question 15

How could you use precipitation to purify water?

Answer 15

Ions, or charged atoms are added to bind to materials within a liquid, such as iron (Fe) or other metals that would poison the drinker. These bond with the substances that are not wanted and form into a precipitate.

The metallic ions can then be filtered out or will settle out of the liquid.

Question 16

Using Magnesium and Hydroxide ions, explain how we can use precipitation to purify effluent water that contains particularly unpleasant metallic ions.

Answer 16

Magnesium ions + Hydroxide ions > Magnesium Hydroxide

Mg2+(aq) + 2 OH-(aq) > Mg(OH)2(s)

To be soluble, these metallic atoms need to be ions, so they will always have a + or -.

This reaction adds hydroxide to magnesium to form a solid material which will then settle as solid particles and can be separated from the water.