به انگلیسی

Electrode potentials predict the winner

Science · Chemistry · ages 17-18

Name ______________________ Date ____________

Which set lists the standard conditions for electrode potentials?
- 273 K, 100 kPa, 1.00 mol/dm3
- 298 K, 200 kPa, 2.00 mol/dm3
- 298 K, 100 kPa, 1.00 mol/dm3
What is the standard hydrogen electrode there for?
- It sets 0.00 V for comparison
- It forces every cell to use hydrogen
- It keeps every reaction at 298 K
Under standard conditions, a negative Ecell means the proposed reaction is not thermodynamically feasible as written.
Circle one: True False
In the zinc and copper cell, which metal dissolves and why?
- Copper, since copper ions gain electrons at the positive end
- Zinc, since it gives up electrons at the negative end
- Neither, since zinc and copper both plate out as solid metal
Zinc is -0.76 V and copper is +0.34 V. Work out the standard cell potential in volts.
Answer: ______________
Why are the conditions fixed when measuring standard potentials?
- So the voltmeter always reads in whole numbers of volts
- So values compare fairly with each other
- So every reaction in the table becomes feasible
In a spontaneous standard cell, reduction happens in the half-cell with the more positive reduction potential.
Circle one: True False
A proposed reaction has a standard Ecell of +0.50 V, and the beaker is held at standard conditions, but nothing happens. What is the best explanation?
- The positive sign proves the tabulated electrode data are false
- A large activation barrier slows a feasible reaction
- The reaction must be running backward toward the reactants

Answer key

For grown-ups. Fold this page away before handing over the rest.

298 K, 100 kPa, 1.00 mol/dm3 · The standard conditions are 298 K, 100 kPa for gases and 1.00 mol/dm3 for ions.
It sets 0.00 V for comparison · A single electrode potential cannot be measured on its own, so the standard hydrogen electrode is assigned 0.00 V as a reference.
True · A negative standard cell potential means the reverse direction is thermodynamically favoured under standard conditions.
Zinc, since it gives up electrons at the negative end · Zinc has the lower reduction potential. It is oxidised at the negative electrode, so zinc atoms enter solution as Zn2+ ions.
1.1 · Ecell = E(cathode) - E(anode) = +0.34 - (-0.76) = +1.10 V.
So values compare fairly with each other · Changing concentration, gas pressure or temperature can change a potential. Fixed conditions make the tabulated values comparable.
True · Reduction happens in the half-cell with the more positive reduction potential, at the cathode. In the zinc copper cell, copper ions are reduced and zinc is oxidised.
A large activation barrier slows a feasible reaction · A positive Ecell predicts a feasible direction, not a fast rate. A large activation barrier can keep the mixture unchanged.