C4.2 - identifying the products of chemical reactions

Question 1

How do you detect carbon dioxide gas?

Answer 1

You test for carbon dioxide by using limewater.

Limewater is a calcium hydroxide solution

It turns cloudy when carbon dioxide is bubbled through it

This change is caused by the calcium hydroxide reacting with the carbon dioxide to form water and a white precipitate of calcium carbonate.

Question 2

What is the balanced equation for the reaction between calcium hydroxide (limewater) and carbon dioxide ?

Answer 2

Ca(OH)2 (aq) + CO2(g) → CaCO3(s) + H2O (l)

Question 3

What is limewater .

Answer 3

It is a calcium hydroxide solution

Question 4

Is blue litmus paper used to test for acids or alkalis ?

Answer 4

Acids

Question 5

Is red litmus usually used to test for acids or alkalis

Answer 5

Alkalis

Question 6

How do you detect chlorine gas?

Answer 6

1. Use a drop of water to dampen a piece of blue litmus paper
2. Hold the paper near to a container that holds the substance
3. If chlorine is present the litmus paper turns red then white

Question 7

Why does damp blue litmus paper turn red and then white when chlorine is present?

Answer 7

Chlorine gas reacts with water to produce an acidic solution which is also an effective bleach.

The acidic solution formed on the damp blue litmus paper turns it red

The bleach then causes the colour to change from red to white

Question 8

How do you carefully smell substances in the lab?

Answer 8

1. With the container away from your nose breath in enough ear to almost fill your lungs.
2. Hold the container a few cm away from your nose and waft the smell towards you. Take a cautious sniff.

(Holding the container away from your nose makes sure you don’t breath in a lot of the substance. )

Question 9

Why do you hold the container away from your nose when smelling a substance?

Answer 9

Holding the container away from your nose makes sure you don’t breath in a lot of the substance.

This also ensures that should any of these substances escape, it will not go up your nose

Question 10

How do you detect hydrogen gas?

Answer 10

• place a lighted splint near the mouth of the container of gas
• if hydrogen is present, it should ignite with a squeaky pop

Question 11

How do you detect oxygen gas?

Answer 11

Place a glowing splint near the mouth of the container of gas

If oxygen is present the splint should relight

Question 12

Write a word equation for the catalytic decomposition of hydrogen peroxide, H2O2.

Answer 12

Water + oxygen —> hydrogen peroxide

Question 13

Write a balanced symbol equation for the catalytic decomposition of hydrogen peroxide, H2O2.

Answer 13

2H2O —> O2 + 2H2O

Question 14

Write a word equation and a balanced symbol equation for the combustion of hydrogen.

Answer 14

Hydrogen + oxygen —> water

2H2 + O2 —> 2H2O

Question 15

Explain why hydrogen pops with a lighted splint.

Answer 15

Hydrogen is highly flammable so when hydrogen reacts with oxygen in the air it creates a small explosion which results in the pop sound.

Question 16

Write a word equation for the reaction between zinc and hydrochloric acid.

Answer 16

Zinc + hydrochloric acid —> zinc chloride + hydrogen

Question 17

Write a balanced symbol equation for the reaction between zinc and hydrochloric acid.

Answer 17

Zn+ 2HCl —> ZnCl2 + H2

Question 18

Write a word equation and a balanced symbol equation for the reaction between calcium carbonate and hydrochloric acid.

Answer 18

Calcium carbonate + hydrochloric acid —> calcium chloride + water + carbon dioxide

CaCO3 (s) + 2HCl(aq) —> CaCl2 (aq) + H2O (l) + CO2 (g)

Question 19

What is a flame test?

Answer 19

It is a test that helps to identify metal ions in compounds from the colour that they produce in a flame

Question 20

What happens when metal ions are heated in terms of energy transfer

Answer 20

When metal ions are heated energy is transferred to their electrons.
This makes their electrons move into higher electron shells.

When they move back to their normal electron shells, energy is transferred to the surroundings as radiation, which you see as light.

Question 21

What is the flame test colour for lithium?

Answer 21

Red

Question 22

What is the flame test colour for sodium

Answer 22

Yellow

Question 23

What is the flame test colour for potassium

Answer 23

Lilac

Question 24

What is the flame test colour for calcium

Answer 24

Orange red

Question 25

What is the flame test colour for copper

Answer 25

Green-blue

Question 26

What are the steps for doing a flame test?

Answer 26

1. Dip the clean loop into the test powder or solution 2. Use the handle to hold the loop in the edge of a roaring blue flame 3. Record the flame colour

Question 27

Why do you clean the wire loop between each flame test?

Answer 27

It ensures it is not contaminated with pervious substances so you will not confuse flame colours or mix the substances

Question 28

How do you clean a wire loop?

Answer 28

By dipping it several times in hydrochloric acid and then rinsing it with distilled water If you hold it in a Bunsen burner flame there will be no change in the colour of the flame

Question 29

What group are soluble in water

Answer 29

Group 1 metal hydroxides

Question 30

What is the balanced equation for the reaction between copper (II) sulfate and sodium hydroxide

Answer 30

CuSO4 (aq) + 2NaOh (aq) —> Cu(OH)2 (S) —> Na2SO4 (aq)

Question 31

What colour hydroxide precipitate is does iron (II) form

Answer 31

Green

Question 32

What colour hydroxide precipitate does iron (III) form

Answer 32

Orange brown

Question 33

What colour hydroxide precipitate does copper (II) form

Answer 33

Blue

Question 34

What colour hydroxide precipitate does calcium form?

Answer 34

White

Question 35

What colour hydroxide precipitate does zinc form

Answer 35

White

Question 36

How can you tell if the ion present is calcium is zinc from a precipitate test?

Answer 36

If you add an excess sodium hydroxide solution zinc hydroxide dissolves to form a colourless solution but calcium hydroxide does not dissolve

Question 37

What happens during a sulfate test?

Answer 37

Barium ions react with sulfate ions to produce white, insoluble barium sulfate

Question 38

What is the ionic equation for a sulfate test?

Answer 38

Ba2+ (aq) + SO42- (aq) —> BaSO4 (s)

Question 39

What is the method for a sulfate test?

Answer 39

1. Add a few drops of dilute hydrochloric acid 2. Then add a few drops of barium chloride solution BaCl2 (aq) 3. If sulfate ions are present you will get a white precipitate

Question 40

How do you detect carbonate ions?

Answer 40

Hydrogen ions from dilute acids react with carbonate ions to produce carbon dioxide and water

Question 41

What is the ionic equation for a carbonate test?

Answer 41

2H+ (aq) + CO32- (aq) —> CO2 (g) + H2O(l) a

Question 42

What is the method for a carbonate test

Answer 42

1. Add a few drops of dilute hydrochloric acid 2. If carbonate ions are present bubbles of gas will be produced. You can confirm this is carbon dioxide by bubbling it through limewater

Question 43

How do you detect halide ions? (Include method)

Answer 43

Silver fluoride is soluble in water but the other silver halides are insoluble. 1. Add a few drops of dilute nitric acid 2. Add a few drops of silver nitrate solution (AgNO3) 3. Observe colour of precipitate formed If one of these ions is present you get a precipitate

Question 44

What is the colour of the silver halide precipitate formed by chloride

Answer 44

White

Question 45

What is the colour of the silver halide precipitate formed by bromide?

Answer 45

Cream

Question 46

What is the colour of the silver halide precipitate formed by iodide?

Answer 46

Yellow

Question 47

What is an instrumental method of analysis?

Answer 47

A technique for analysing a substance that depends upon a machine

Question 48

What are the advantages of an instrumental method of analysis?

Answer 48

Sensitivity Accuracy Speed

Question 49

Why is sensitivity advantageous?

Answer 49

This means it can analyse small amounts of substances. This is useful if the substance is expensive or difficult to obtain

Question 50

What is accuracy advantageous?

Answer 50

the instruments are very accurate. This means they can be calibrated using internationally accepted standards

Question 51

Why is speed advantageous

Answer 51

Because the instruments can carry out analyses quickly and they can run all the time

Question 52

What is a gas chromatogram?

Answer 52

A gas chromatogram is a chart that represents the different substances in a mixture,

Question 53

How do you interpret gas chromatograms?

Answer 53

Each peak represents a substance present in the mixture The areas under each peak show the relative amounts of each substance in the mixture The retention time is different for different substances

Question 54

What is the retention time?

Answer 54

It is the time taken for a substance to travel through the chromatography column

Question 55

What is a mass spectrometer?

Answer 55

A machine that can measure masses of atoms and molecules

Question 56

What is a mass spectrometer used for?

Answer 56

To analyse the relative amounts of isotopes of an element and the structure of molecules

Question 57

How does a mass spectrometer analyse molecules?

Answer 57

The sample molecules are ionised by the machine to form molecular ions. These may break up to form fragments, which the machine can separate and detect

Question 58

How do you interpret mass spectra?

Answer 58

In a mass spectrum: Each peak represents a fragment of the molecule The peak on the far right represents the molecular ion The mass to charge ratio of the molecular ion peak is equal to the Mr of the molecule