IB DP Chemistry Reactivity 1.1 Measuring enthalpy changes HL Paper 2- Exam Style Questions - New Syllabus

IBDP Chemistry HL Paper 2 - All Chapters

Question

Cobalt ions are known to form compounds with characteristic colours.
(a) Determine the electron configuration of the Co²⁺ ion.
(b) When hydrated cobalt ions are combined with concentrated hydrochloric acid, a dynamic equilibrium is established.
[Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l)
                Pink                                                   Blue
(i) Predict the effect on the value of the equilibrium constant, K, and the equilibrium position when solid sodium chloride, NaCl(s), is added to the mixture at constant temperature.
(ii) Heating an equilibrium mixture that is initially pink changes the colour to purplish-blue. Deduce, giving a reason, whether the formation of [CoCl₄]²⁻(aq) is an exothermic or endothermic process.

Most-appropriate topic codes (IB Chemistry 2025):

• Structure 1.3: Electron configurations — part (a)
• Reactivity 2.3: How far? The extent of chemical change — parts (b)(i), (b)(ii)
• Reactivity 1.1: Measuring enthalpy changes — part (b)(ii)
• Reactivity 3.4: Electron-pair sharing reactions — part (b)
• Structure 3.1: The periodic table: Classification of elements — parts (a), (b)
▶️ Answer/Explanation

(a)
The cobalt atom (Co, Z=27) has the electron configuration [Ar] 4s² 3d⁷. The Co²⁺ ion loses two electrons from the 4s orbital first, resulting in the configuration [Ar] 3d⁷.
\(\boxed{[Ar] 3d^7}\) or \(\boxed{1s^2 2s^2 2p^6 3s^2 3p^6 3d^7}\)

(b)(i)
Adding solid NaCl increases the concentration of Cl⁻(aq) ions. According to Le Chatelier’s principle, the equilibrium will shift to the right to reduce this disturbance, favouring the formation of the blue [CoCl₄]²⁻ complex. The equilibrium constant, K, is unaffected by changes in concentration at constant temperature.
\(\boxed{\text{K is unchanged, equilibrium shifts to the right.}}\)

(b)(ii)
Heating the mixture causes a colour change from pink to blue, indicating an increase in the concentration of the product, [CoCl₄]²⁻. According to Le Chatelier’s principle, if increasing temperature favours the forward reaction, the reaction must be endothermic.
\(\boxed{\text{Endothermic, because heating favours the forward reaction.}}\)

More DP Chemistry HL Paper 2 Questions..
Scroll to Top