Equations with ions - Higher
Half equations
A half equationAn equation, involving ions and electrons, that describes the process happening at an electrode. models the change that happens to one substance in a chemical reaction. They are usually used to model the reactions that happen at electrodeA conductor used to establish electrical contact with a circuit. The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode. The electrode attached to the positive terminal of a battery is the positive electrode, or anode. during electrolysis.
Reactions at cathodes
A cathodeThe negative electrode during electrolysis. is a negatively charged electrode. Positively charged ionElectrically charged particle, formed when an atom or molecule gains or loses electrons. gain electronSubatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. at a cathode, forming neutral atomThe smallest part of an element that can exist. or molecules. The ions, chargeProperty of matter that causes a force when near another charge. Charge comes in two forms, positive and negative. For example, a negative charge causes a repulsive force on a neighbouring negative charge. and atoms must balance. For example:
Na+ + e- → Na
Ca2+ + 2e- → Ca
2H+ + 2e- → H2
Question
Balance this half equation: Al3+ + e- → Al
Al3+ + 3e- → Al
Reactions at anodes
An anodeThe positive electrode during electrolysis. is a positively charged electrode. Negatively charged ions lose electrons at an anode, forming neutral molecules. For example:
2Cl- → Cl2 + 2e-
2O2-→ O2 + 4e-
Question
Balance this half equation: Br- → Br2 + e-
2Br- → Br2 + 2e-
Ionic equations
A balanced ionic equationA chemical equation that shows how positively charged ions join with negatively charged ions to make a compound. shows the reacting ionElectrically charged particle, formed when an atom or molecule gains or loses electrons. in a chemical reaction. These equations are often used to model what happens in a precipitation reactionA reaction in which an insoluble solid is formed when certain solutions are mixed.. In these reactions, two solubleAble to dissolve in solvent. For example, sugar is soluble in water because it dissolves to form sugar solution.reactantA substance that reacts together with another substance to form products during a chemical reaction. form an insolubleUnable to dissolve in a particular solvent. For example, sand is insoluble in water. product and a soluble productA substance formed in a chemical reaction..
For example, silver nitrate solution reacts with sodium chloride solution. Insoluble solid silver chloride and sodium nitrate solution form:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
The Na+ ions and NO3- ions remain separate in the sodium nitrate solution and do not form a precipitateA suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction.. Ions that remain separate during a reaction are called spectator ionAn ion that is exactly the same on both sides of an ionic equation.. This means you can ignore them when you write the ionic equation. You only need to model how the solid silver chloride forms:
Ag+(aq) + Cl-(aq) → AgCl(s)
In a balanced ionic equation:
- the number of positive and negative charges is the same
- the numbers of atoms of each elementA substance made of one type of atom only. on the left and right are the same
Question
Explain why this ionic equation is balanced:
Ba2+(aq) + SO42-(aq) → BaSO4(s)
There are the same numbers of atoms of each element on both sides of the equation. The total charge on both sides is also the same (zero).
Question
Balance this ionic equation, which models the formation of a silver carbonate precipitate:
Ag+(aq) + CO32-(aq) → Ag2CO3(s)
2Ag+(aq) + CO32-(aq) → Ag2CO3(s)
Question
Balance this ionic equation, which models the formation of an aluminium hydroxide precipitate:
Al3+(aq) + OH-(aq) → Al(OH)3(s)
Al3+(aq) + 3OH-(aq) → Al(OH)3(s)