C4.2 - identifying the products of chemical reactions Flashcards
How do you detect carbon dioxide gas?
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.
What is the balanced equation for the reaction between calcium hydroxide (limewater) and carbon dioxide ?
Ca(OH)2 (aq) + CO2(g) → CaCO3(s) + H2O (l)
What is limewater .
It is a calcium hydroxide solution
Is blue litmus paper used to test for acids or alkalis ?
Acids
Is red litmus usually used to test for acids or alkalis
Alkalis
How do you detect chlorine gas?
- Use a drop of water to dampen a piece of blue litmus paper
- Hold the paper near to a container that holds the substance
- If chlorine is present the litmus paper turns red then white
Why does damp blue litmus paper turn red and then white when chlorine is present?
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
How do you carefully smell substances in the lab?
- With the container away from your nose breath in enough ear to almost fill your lungs.
- 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. )
Why do you hold the container away from your nose when smelling a substance?
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
How do you detect hydrogen gas?
- place a lighted splint near the mouth of the container of gas
- if hydrogen is present, it should ignite with a squeaky pop
How do you detect oxygen gas?
Place a glowing splint near the mouth of the container of gas
If oxygen is present the splint should relight
Write a word equation for the catalytic decomposition of hydrogen peroxide, H2O2.
Water + oxygen —> hydrogen peroxide
Write a balanced symbol equation for the catalytic decomposition of hydrogen peroxide, H2O2.
2H2O —> O2 + 2H2O
Write a word equation and a balanced symbol equation for the combustion of hydrogen.
Hydrogen + oxygen —> water
2H2 + O2 —> 2H2O
Explain why hydrogen pops with a lighted splint.
Hydrogen is highly flammable so when hydrogen reacts with oxygen in the air it creates a small explosion which results in the pop sound.
Write a word equation for the reaction between zinc and hydrochloric acid.
Zinc + hydrochloric acid —> zinc chloride + hydrogen
Write a balanced symbol equation for the reaction between zinc and hydrochloric acid.
Zn+ 2HCl —> ZnCl2 + H2
Write a word equation and a balanced symbol equation for the reaction between calcium carbonate and hydrochloric acid.
Calcium carbonate + hydrochloric acid —> calcium chloride + water + carbon dioxide
CaCO3 (s) + 2HCl(aq) —> CaCl2 (aq) + H2O (l) + CO2 (g)
What is a flame test?
It is a test that helps to identify metal ions in compounds from the colour that they produce in a flame
What happens when metal ions are heated in terms of energy transfer
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.
What is the flame test colour for lithium?
Red
What is the flame test colour for sodium
Yellow
What is the flame test colour for potassium
Lilac
What is the flame test colour for calcium
Orange red
What is the flame test colour for copper
Green-blue
What are the steps for doing a flame test?
- Dip the clean loop into the test powder or solution
- Use the handle to hold the loop in the edge of a roaring blue flame
- Record the flame colour
Why do you clean the wire loop between each flame test?
It ensures it is not contaminated with pervious substances so you will not confuse flame colours or mix the substances
How do you clean a wire loop?
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
What group are soluble in water
Group 1 metal hydroxides
What is the balanced equation for the reaction between copper (II) sulfate and sodium hydroxide
CuSO4 (aq) + 2NaOh (aq) —> Cu(OH)2 (S) —> Na2SO4 (aq)
What colour hydroxide precipitate is does iron (II) form
Green
What colour hydroxide precipitate does iron (III) form
Orange brown
What colour hydroxide precipitate does copper (II) form
Blue
What colour hydroxide precipitate does calcium form?
White
What colour hydroxide precipitate does zinc form
White
How can you tell if the ion present is calcium is zinc from a precipitate test?
If you add an excess sodium hydroxide solution zinc hydroxide dissolves to form a colourless solution but calcium hydroxide does not dissolve
What happens during a sulfate test?
Barium ions react with sulfate ions to produce white, insoluble barium sulfate
What is the ionic equation for a sulfate test?
Ba2+ (aq) + SO42- (aq) —> BaSO4 (s)
What is the method for a sulfate test?
- Add a few drops of dilute hydrochloric acid
- Then add a few drops of barium chloride solution BaCl2 (aq)
- If sulfate ions are present you will get a white precipitate
How do you detect carbonate ions?
Hydrogen ions from dilute acids react with carbonate ions to produce carbon dioxide and water
What is the ionic equation for a carbonate test?
2H+ (aq) + CO32- (aq) —> CO2 (g) + H2O(l) a
What is the method for a carbonate test
- Add a few drops of dilute hydrochloric acid
- If carbonate ions are present bubbles of gas will be produced. You can confirm this is carbon dioxide by bubbling it through limewater
How do you detect halide ions? (Include method)
Silver fluoride is soluble in water but the other silver halides are insoluble.
- Add a few drops of dilute nitric acid
- Add a few drops of silver nitrate solution (AgNO3)
- Observe colour of precipitate formed
If one of these ions is present you get a precipitate
What is the colour of the silver halide precipitate formed by chloride
White
What is the colour of the silver halide precipitate formed by bromide?
Cream
What is the colour of the silver halide precipitate formed by iodide?
Yellow
What is an instrumental method of analysis?
A technique for analysing a substance that depends upon a machine
What are the advantages of an instrumental method of analysis?
Sensitivity
Accuracy
Speed
Why is sensitivity advantageous?
This means it can analyse small amounts of substances.
This is useful if the substance is expensive or difficult to obtain
What is accuracy advantageous?
the instruments are very accurate.
This means they can be calibrated using internationally accepted standards
Why is speed advantageous
Because the instruments can carry out analyses quickly and they can run all the time
What is a gas chromatogram?
A gas chromatogram is a chart that represents the different substances in a mixture,
How do you interpret gas chromatograms?
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
What is the retention time?
It is the time taken for a substance to travel through the chromatography column
What is a mass spectrometer?
A machine that can measure masses of atoms and molecules
What is a mass spectrometer used for?
To analyse the relative amounts of isotopes of an element and the structure of molecules
How does a mass spectrometer analyse molecules?
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
How do you interpret mass spectra?
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
