Combination Reactions

Mahi Tuatahi

Write the ionic formula for these compounds:

1. Sodium chloride
2. Magnesium sulfate
3. Sodium sulfate
4. Magnesium chloride
5. Iron (III) oxide

Whakatika

1. Sodium chloride: \(NaCl\)
2. Magnesium sulfate: \(MgSO_{4}\)
3. Sodium sulfate: \(Na_{2}SO_{4}\)
4. Magnesium chloride: \(MgCl_{2}\)
5. Iron (III) oxide: \(Fe_{2}O_{3}\)

Ngā Whāinga Ako

• Recognise a combination reaction
• Define a combination reaction
• Know some common observations of combination reactions

Combination (Synthesis) Reactions

Chemical reaction where the atoms of one element reacts with the atoms of another element to form a single compound.

\[ \begin{aligned} A + B &\longrightarrow AB \end{aligned} \]

Combination Reactions with Oxygen

• Sometimes called oxidation reactions
• When heat/light are produced it is known as combustion
• Most require heat for the reaction to occur (think activation energy)
• The product is more stable than the reactants
• The flame can have a unique color
  • E.g. Copper makes green flames and sulfur makes blue ones

Tauria Tahi / Example One

When iron rusts (oxidates), the iron atoms combine with oxygen atoms (from the air) to form iron (III) oxide.

\[ \begin{aligned} \text{iron} + \text{oxygen} &\rightarrow \text{iron (III) oxide} \newline 4Fe_{(s)} + 3O_{2(g)} &\rightarrow 2Fe_{2}O_{3(s)} \end{aligned} \]

What colour is the product?

Tauria Rua / Example Two

When magnesium metal reacts (combusts) with oxygen atoms (in the air) to form magnesium oxide. This happens in fireworks!

\[ \begin{aligned} \text{magnesium} + \text{oxygen} &\rightarrow \text{magnesium oxide} \newline 2Mg_{(s)} + O_{2(g)} &\rightarrow 2MgO_{(s)} \end{aligned} \]

What colour is the product?

Combination Reactions and Ionic Compounds

When metal elements combine with non-metal elements:

1. Valence electrons are transferred from the metal to the non-metal.
2. The metal forms a positive ion & the non-metal forms a negative ion.
3. The ions are held together by electrostatic forces of attraction (positive-negative).

Pātai: NCEA 2018

Lead iodide can be formed by two different reactions:

1. Reaction One: Heating lead metal in iodine vapour.
2. Reaction Two: Adding lead nitrate solution to sodium iodide solution.

Elaborate on these reactions. In your answer, you should:

• identify each type of chemical reaction and justify your answer
• refer to electron transfer, where appropriate
• write a balanced symbol equation for each reaction

Whakatika / Answer

identify each type of chemical reaction and justify your answer

Reaction one is a combination (synthesis) reaction because two elements (lead and iodine) are being combined together to form a single compound (lead iodide). Also, heat is being applied and a metal is present.

refer to electron transfer, where appropriate

Each lead metal atom an electron to two different iondine atoms which becomes negatively charged (\(I^{-}\)), leaving the lead ion positively charged (\(Pb^{2+}\)). These oppositely charged ions then are electrostatically attracted together to form an ionic compound.

write a balanced symbol equation for each reaction

\[ \begin{aligned} \text{lead} + \text{iodine} &\rightarrow \text{lead iodide} \newline Pb_{(s)} + 2I_{(g)} &\rightarrow PbI_{2 (s)} \end{aligned} \]

Mahi Tuatahi

1. Quiz on elements/ions/compounds
2. Describe a combination reaction, give a generic formula and explain what observations would indicate to you that one has occurred.

Anions and Cations

• Anion: A negatively charged ion
  • Form when a non-metal atom gains an electron from a metal \(O^{2-}\)
• Cation: A positively charged ion
  • Typically metals form cations e.g \(Mg^{2+}\)