For each reaction, show how electrons are gained and lost:
2Na (s) + Br2(l) 🡪 2NaBr (s)
2Ca (s) + O2(g) 🡪 2CaO (s)
Both are done the same way. You must have a working knowledge of the oxidation states of the elements in the reaction. The first one is:
2Na(s) + Br2(l) 🡪 2NaBr(s)
Na is zero on the left and 1+ EACH on the right for total of 2+.
Br is zero on the left and 1- EACH on the right for a total of 2-.
So Na going from zero on the left to 2+ on the right is a lost of 2 electrons.
Br going from zero on the left to 2- on the right is a gain of 2 electrons.
In the given reactions, let's break down the process of how electrons are gained and lost:
Reaction 1: 2Na (s) + Br2 (l) → 2NaBr (s)
In this reaction, sodium (Na) atoms lose one electron each, and bromine (Br) gains one electron each. The step-by-step electron exchange is as follows:
- Sodium (Na) atoms lose one electron each and are oxidized to form Na+ ions:
2Na (s) → 2Na+ (aq) + 2e-
- Bromine (Br) atoms gain one electron each and are reduced to form Br- ions:
Br2 (l) + 2e- → 2Br- (aq)
Overall, the electrons are transferred from the sodium atoms to the bromine atoms, resulting in the formation of sodium bromide (NaBr).
Reaction 2: 2Ca (s) + O2 (g) → 2CaO (s)
In this reaction, calcium (Ca) atoms lose two electrons each, and oxygen (O) gains two electrons each. The step-by-step electron exchange is as follows:
- Calcium (Ca) atoms lose two electrons each and are oxidized to form Ca2+ ions:
2Ca (s) → 2Ca2+ (aq) + 4e-
- Oxygen (O2) molecules gain four electrons in total and are reduced to form O2- ions:
O2 (g) + 4e- → 2O2- (aq)
Overall, the electrons are transferred from the calcium atoms to the oxygen molecules, resulting in the formation of calcium oxide (CaO).
To determine how electrons are gained and lost in a redox (reduction-oxidation) reaction, we need to identify the oxidation and reduction half-reactions.
In the given reactions:
1. 2Na (s) + Br2(l) 🡪 2NaBr (s)
The oxidation half-reaction involves the species losing electrons, while the reduction half-reaction involves the species gaining electrons.
Oxidation half-reaction:
2Na (s) 🡪 2Na+ (aq) + 2e-
In this reaction, the sodium (Na) atoms lose two electrons to form sodium ions (Na+).
Reduction half-reaction:
Br2(l) + 2e- 🡪 2Br- (aq)
In this reaction, the bromine molecule (Br2) gains two electrons to form bromide ions (Br-).
2. 2Ca (s) + O2(g) 🡪 2CaO (s)
Oxidation half-reaction:
2Ca (s) 🡪 2Ca2+ (aq) + 4e-
In this reaction, the calcium (Ca) atoms lose four electrons to form calcium ions (Ca2+).
Reduction half-reaction:
O2(g) + 4e- 🡪 2O2- (aq)
In this reaction, oxygen molecules (O2) gain four electrons to form oxide ions (O2-).
Now, if we look at the overall reaction, we can see that the electrons gained in the reduction half-reaction are equal to the electrons lost in the oxidation half-reaction.
In the first reaction, each sodium atom loses one electron, while each bromine molecule gains two electrons. Therefore, there is a transfer of one electron from sodium to bromine.
In the second reaction, each calcium atom loses two electrons, while each oxygen molecule gains four electrons. Therefore, there is a transfer of two electrons from calcium to oxygen.
This demonstrates how electrons are gained and lost in the given reactions.