chemistry exam Flashcards
Distinguish between an element and a compound:
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.
Distinguish between an atom and a molecule:
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.
Distinguish between ionic and covalent bonding:
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.
Define an atomic orbital:
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.
Write the s, p electron configuration for a lithium atom:
1s² 2s¹ / [He] 2s¹.
Write the s, p electron configuration for a chlorine atom:
1s² 2s² 2p⁶ 3s² 3p⁵ / [Ne] 3s² 3p⁵.
Describe how you would carry out a flame test on a sample of LiCl:
(Allow ‘inoculating loop’ or ‘spatula’ for ‘platinum wire’; clear labelled diagram acceptable for some or all points).
What colour does LiCl give to a flame?
Red / crimson.
Define the electronegativity of an element:
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.
Plot the electronegativity values versus atomic numbers for elements Li to F and F to I:
(Y-axis labelled electronegativity and X-axis labelled atomic number; points plotted appropriately).
Explain the decrease in electronegativity values shown down Group 17:
Additional shell added / increasing atomic radius / more screening or shielding (of nucleus).
Use electronegativity values to predict the type of bonding in the compound NF3:
“3.98 – 3.04 / 0.94 → polar / polar covalent.”Question
Explain the underlined term:
A solution of known concentration.
Name the piece of apparatus shown in Figure 13:
Volumetric flask.
Describe how the dilution could have been carried out:
(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.
Describe the correct procedure for rinsing a burette for use in this titration:
Rinse with deionised or distilled water / rinse with acid or (diluted) vinegar / rinse with solution it is to deliver or hold.
Describe the correct procedure for filling the burette with the diluted vinegar:
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.
Name a suitable indicator for use in this titration:
Phenolphthalein.
What colour change would be observed at the end point of the titration?
Pink to colourless.
Explain why it is advisable to use no more than 1 to 2 drops of indicator in a titration:
Indicators are weak acids or weak bases / would interfere with or affect result or accuracy or pH of reaction mixture.
Calculate the molarity of the potassium hydroxide solution that contained 3.36 g of KOH per litre:
Mr = 56; 3.36 / 56 = 0.06 (moles per litre) / 0.06 (M).
Calculate the concentration of CH3COOH in the diluted vinegar in moles per litre:
Using n1 = (V1 × M1) / (V2 × M2) / 18.65 × M1 = 25.0 × 0.06 / (M2) = 0.080 (M).
Calculate the concentration of CH3COOH in the original vinegar in moles per litre:
0.080 × 10 = 0.80 (M).
Calculate the concentration of CH3COOH in the original vinegar in grams per litre:
“Mr = 60; 60 × 0.80 = 48 (g per litre).”Question
How many sulfur (S) atoms were removed from the stibnite to form 12.0 moles of SO2?
12 × 6 × 10^23 = 7.2 × 10^24 (atoms sulfur).
What mass of antimony was extracted from its ore in these reactions?
8 moles (antimony); Ar = 122; 8 × 122 = 976 g (Sb).
How many litres of CO, measured at s.t.p., were used up in the second reaction?
12 moles (carbon monoxide); 12 × 22.4 = 268.8 (litres CO).
What is the valency of antimony in Sb2O3?
3
Draw a (dot and cross) diagram to show the arrangement of the valence electrons in a molecule of hydrogen sulfide (H2S).
2 lone pairs or 4 non-bonding electrons in S valence shell; 2 bond pairs.
State the shape of a H2S molecule.
Bent / angular.
Does a molecule of gaseous H2S have an overall dipole moment?
Yes.
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.
Beryllium / Be.
What is the shape of a molecule of XH2?
Linear.
What is the bond angle in a molecule of XH2?
180°.
State Hess’s law.
The heat change for a reaction is independent of the path followed; it depends only on initial and final states.
Use Hess’s law to calculate the heat change for the reaction: 2B (s) + 3H2 (g) → B2H6 (g).
ΔH = 36.4 kJ.
What is the heat of combustion of boron (B)?
–636.75 (kJ mol–1).
What is the heat of formation of water?
–285.8 (kJ mol–1).
According to Brønsted-Lowry theory, define an acid.
Proton donor or H+ donor.
According to Brønsted-Lowry theory, define a base.
Proton acceptor or H+ acceptor.
Distinguish between a strong acid and a weak acid.
A strong acid has a greater tendency to donate protons; a weak acid has a smaller tendency to donate protons.
Define pOH.
pOH = − log[OH–].
What is the relationship between pH and pOH?
pH + pOH = 14.
Calculate the concentration of the HCl solution in moles per litre if the pH is 2.0.
[HCl] = 0.01 (M).
