Chromium, Cr, and its compounds are widely used in many chemical reactions.

1. (a)Cr exists as four stable isotopes.
2. (a)(i)The most common isotope of Cr is chromium-52. Determine the number of protons, neutrons and electrons in an atom of chromium-52. number of protons .......... neutrons .......... electrons ..........[1]
3. (a)(ii)Describe how an atom of chromium-54 differs from an atom of chromium-52. Refer to numbers of particles in your answer.[1]
4. (a)(iii)The relative isotopic masses of the isotopes of Cr can be determined using mass spectrometry. State what other information is needed to calculate the relative atomic mass, Ar, of Cr.[1]
5. (a)(iv)Atoms of ⁵²Cr, ⁵³Cr and ⁵⁴Cr make up more than 95% of naturally occurring chromium atoms. The Ar of naturally occurring Cr is 51.996. Suggest what these statements imply about the relative isotopic mass of the fourth stable isotope of chromium.[1]
6. (b)The shorthand electronic configuration of chromium is [Ar] 3d⁵4s¹.
7. (b)(i)Complete the full electronic configuration of chromium. .......... 3d⁵4s¹[1]
8. (b)(ii)Deduce the total number of unpaired electrons in an atom of chromium.[1]
9. (c)Acidified dichromate(VI) ions will convert methanal, CH₂O, to carbon dioxide. The movement of electrons to or from relevant species is shown in the following half-equations. half-equation 1 CH₂O + H₂O → CO₂ + 4H⁺ + 4e⁻ half-equation 2 Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
10. (c)(i)Identify the species that is reduced in half-equation 2. Explain your answer.[1]
11. (c)(ii)The oxidation state of the carbon atom in methanal is 0. Calculate the oxidation state of carbon in carbon dioxide.[1]
12. (c)(iii)Construct the ionic equation for the reaction of dichromate(VI) ions with methanal in acidic conditions.[2]
13. (c)(iv)Methanal is a liquid at −30°C but carbon dioxide is a gas at this temperature. Explain why.[2]
14. (d)In acidic conditions, a dynamic equilibrium is established between CrO₄²⁻(aq) and Cr₂O₇²⁻(aq). 2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l) ΔH > 0
15. (d)(i)State what is meant by dynamic equilibrium.[1]
16. (d)(ii)Identify the condition necessary to establish dynamic equilibrium.[1]
17. (d)(iii)CrO₄²⁻(aq) ions are yellow and Cr₂O₇²⁻(aq) ions are orange. State what is observed when the following changes are made to an equilibrium mixture of acidified CrO₄²⁻(aq) and Cr₂O₇²⁻(aq) ions. Explain your answers. • The equilibrium mixture is warmed gently. observation .......... explanation .......... • Dilute HCl(aq) is added to the equilibrium mixture. observation .......... explanation ..........[4]
18. (e)Chromium(IV) fluoride, CrF₄, is a covalent molecule that shows similar chemical properties to SiCl₄. Suggest the type of reaction that occurs when CrF₄ is placed in water. Construct a relevant equation for this reaction. type of reaction .......... equation ..........[2]