chemistry exam

Question 1

Distinguish between an element and a compound:

Answer 1

Element: cannot be broken down chemically into simpler substances / composed only of atoms with the same atomic number / substance listed on the periodic table. Compound: composed of two or more elements chemically combined.

Question 2

Distinguish between an atom and a molecule:

Answer 2

Atom: smallest particle of an element. Molecule: smallest particle of an element or of a compound that can exist independently / two or more atoms joined chemically.

Question 3

Distinguish between ionic and covalent bonding:

Answer 3

Ionic: involves transfer of electrons / involves attraction between oppositely charged ions / arises where difference in electronegativity values (between the bonding atoms) > 1.7. Covalent: involves sharing of electrons / involves mutual attraction for shared electrons / arises where difference in electronegativity values (between the bonding atoms) ≤ 1.7.

Question 4

Define an atomic orbital:

Answer 4

Region or space (around the nucleus of an atom) where the probability of finding an electron is high / where an electron is likely to be found.

Question 5

Write the s, p electron configuration for a lithium atom:

Answer 5

1s² 2s¹ / [He] 2s¹.

Question 6

Write the s, p electron configuration for a chlorine atom:

Answer 6

1s² 2s² 2p⁶ 3s² 3p⁵ / [Ne] 3s² 3p⁵.

Question 7

Describe how you would carry out a flame test on a sample of LiCl:

Answer 7

(Allow ‘inoculating loop’ or ‘spatula’ for ‘platinum wire’; clear labelled diagram acceptable for some or all points).

Question 8

What colour does LiCl give to a flame?

Answer 8

Red / crimson.

Question 9

Define the electronegativity of an element:

Answer 9

Measure of attraction / force or power of attraction / relative attraction (an atom in a molecule has) for a shared pair of electrons / for electrons in a covalent bond.

Question 10

Plot the electronegativity values versus atomic numbers for elements Li to F and F to I:

Answer 10

(Y-axis labelled electronegativity and X-axis labelled atomic number; points plotted appropriately).

Question 11

Explain the decrease in electronegativity values shown down Group 17:

Answer 11

Additional shell added / increasing atomic radius / more screening or shielding (of nucleus).

Question 12

Use electronegativity values to predict the type of bonding in the compound NF3:

Answer 12

“3.98 – 3.04 / 0.94 → polar / polar covalent.”Question

Question 13

Explain the underlined term:

Answer 13

A solution of known concentration.

Question 14

Name the piece of apparatus shown in Figure 13:

Answer 14

Volumetric flask.

Question 15

Describe how the dilution could have been carried out:

Answer 15

(25 cm³) pipette used (to add vinegar to volumetric flask) / pipette (first) rinsed with deionised or distilled water and (then) vinegar or solution to be measured out / pipette filler used to fill (pipette) to mark (with vinegar) / (volumetric) flask rinsed with deionised or distilled water / deionised or distilled water added (to flask) until near mark / pipette or dropper used to add last few drops of water / until bottom of meniscus is on mark / (flask) stoppered and inverted several times.

Question 16

Describe the correct procedure for rinsing a burette for use in this titration:

Answer 16

Rinse with deionised or distilled water / rinse with acid or (diluted) vinegar / rinse with solution it is to deliver or hold.

Question 17

Describe the correct procedure for filling the burette with the diluted vinegar:

Answer 17

Use funnel / fill above (zero) mark / open tap to fill part below tap / (remove funnel and) adjust (approximately) to zero / use dropper to bring to mark / bottom of meniscus on (zero) mark / no bubbles / clamped vertically.

Question 18

Name a suitable indicator for use in this titration:

Answer 18

Phenolphthalein.

Question 19

What colour change would be observed at the end point of the titration?

Answer 19

Pink to colourless.

Question 20

Explain why it is advisable to use no more than 1 to 2 drops of indicator in a titration:

Answer 20

Indicators are weak acids or weak bases / would interfere with or affect result or accuracy or pH of reaction mixture.

Question 21

Calculate the molarity of the potassium hydroxide solution that contained 3.36 g of KOH per litre:

Answer 21

Mr = 56; 3.36 / 56 = 0.06 (moles per litre) / 0.06 (M).

Question 22

Calculate the concentration of CH3COOH in the diluted vinegar in moles per litre:

Answer 22

Using n1 = (V1 × M1) / (V2 × M2) / 18.65 × M1 = 25.0 × 0.06 / (M2) = 0.080 (M).

Question 23

Calculate the concentration of CH3COOH in the original vinegar in moles per litre:

Answer 23

0.080 × 10 = 0.80 (M).

Question 24

Calculate the concentration of CH3COOH in the original vinegar in grams per litre:

Answer 24

“Mr = 60; 60 × 0.80 = 48 (g per litre).”Question

Question 25

How many sulfur (S) atoms were removed from the stibnite to form 12.0 moles of SO2?

Answer 25

12 × 6 × 10^23 = 7.2 × 10^24 (atoms sulfur).

Question 26

What mass of antimony was extracted from its ore in these reactions?

Answer 26

8 moles (antimony); Ar = 122; 8 × 122 = 976 g (Sb).

Question 27

How many litres of CO, measured at s.t.p., were used up in the second reaction?

Answer 27

12 moles (carbon monoxide); 12 × 22.4 = 268.8 (litres CO).

Question 28

What is the valency of antimony in Sb2O3?

Answer 28

3

Question 29

Draw a (dot and cross) diagram to show the arrangement of the valence electrons in a molecule of hydrogen sulfide (H2S).

Answer 29

2 lone pairs or 4 non-bonding electrons in S valence shell; 2 bond pairs.

Question 30

State the shape of a H2S molecule.

Answer 30

Bent / angular.

Question 31

Does a molecule of gaseous H2S have an overall dipole moment?

Answer 31

Yes.

Question 32

Identify X in the compound XH2 where X does not have an overall dipole moment and is in the second period of the periodic table.

Answer 32

Beryllium / Be.

Question 33

What is the shape of a molecule of XH2?

Answer 33

Linear.

Question 34

What is the bond angle in a molecule of XH2?

Answer 34

180°.

Question 35

State Hess’s law.

Answer 35

The heat change for a reaction is independent of the path followed; it depends only on initial and final states.

Question 36

Use Hess’s law to calculate the heat change for the reaction: 2B (s) + 3H2 (g) → B2H6 (g).

Answer 36

ΔH = 36.4 kJ.

Question 37

What is the heat of combustion of boron (B)?

Answer 37

–636.75 (kJ mol–1).

Question 38

What is the heat of formation of water?

Answer 38

–285.8 (kJ mol–1).

Question 39

According to Brønsted-Lowry theory, define an acid.

Answer 39

Proton donor or H+ donor.

Question 40

According to Brønsted-Lowry theory, define a base.

Answer 40

Proton acceptor or H+ acceptor.

Question 41

Distinguish between a strong acid and a weak acid.

Answer 41

A strong acid has a greater tendency to donate protons; a weak acid has a smaller tendency to donate protons.

Question 42

Define pOH.

Answer 42

pOH = − log[OH–].

Question 43

What is the relationship between pH and pOH?

Answer 43

pH + pOH = 14.

Question 44

Calculate the concentration of the HCl solution in moles per litre if the pH is 2.0.

Answer 44

[HCl] = 0.01 (M).