Method (for more experimental details see method 1.3a1)

The reaction equation is: Zn(s)  +  Cu²⁺(aq)  ===>  Zn²⁺(aq)  +  Cu(s)

This is a redox displacement reaction i.e. a more reactive metal displacing a less reactive metal.

 

Method

About 3 g of fine zinc powder is added to exactly 25.0 cm³ of copper(II) sulfate solution of 1.0 mol dm^–3 concentration, which can be pipetted in using a suction bulb. As long as its about 3 g, you be sure it is in excess, so the calculation can be based on the moles of copper(II) ion.

The temperature is plotted every 30 seconds, including a few minutes before adding the zinc powder to establish a base line and then continue reading for another 10 minutes.

The reaction is very exothermic and the temperature should rise rapidly and the solution should become colourless.

A graph is then plotted of temperature versus time and the maximum temperature rise deduced by interpolation (see exemplar graph below.

 

Observations and graph

Typical temperature- time graph shape for an exothermic reaction.

If the temperature rise was 52^oC calculate the enthalpy change for the zinc – copper sulfate displacement reaction

The temperature rise is obtained using the extrapolated purple lines to correct as much as possible for heat energy loss as the calorimeter and contents start cooling down as soon as the hydrochloric acid reacts with the sodium hydroxide!

mol Cu²⁺(aq) = 1.0 x 25/1000 = 0.025 mol;  mol Zn = 3/65 = 0.046 mol (showing zinc to be well in excess)

q = m c ΔT  (m in g, c = 4.18 Jg^-1K^-1, ΔT temperature rise in ^oC)

q = 25 x 4.18 x 52 = 5434 J,  5.434 kJ

Scaling up to 1 mole of copper ions: ΔH = 5.434 x 1/0.025 = 217 kJ mol^–1

Since the temperature rose, this displacement reaction is exothermic

ΔH_displacement(Zn + CuSO₄) = –217 kJ mol^–1  (only accurate to 3 sf)

  (only accurate to 3 sf)

Comments

(i) Theoretical calculation from data book information

Zn(s)  +  Cu²⁺(aq)  ===>  Zn²⁺(aq)  +  Cu(s)

ΔH^θ_reaction,298 = ∑ΔH^θ_f,298(products) – ∑ΔH^θ_f,298(reactants)

ΔH^θ_{displacement reaction,298} = ΔH^θ_f,298(Zn²⁺(aq)) – ΔH^θ_f,298(Cu²⁺(aq))

ΔH^θ_{displacement reaction,298} = –152.4 – (+64.4) = –216.8 kJ mol^–1

 

(ii) Enthalpy values for theoretical calculations

Enthalpies of formation of elements in the normal standard stable states at 298K/101 kPa are zero and so can be excluded from the calculation (though you can put them in formally if you want).

The enthalpies of formation of the ions are measured with respect to the formation of the hydrogen ion, H⁺(aq) which is arbitrarily assigned an enthalpy of formation value of zero.

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