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Chemical Reactions and Equations
Shaun Williams, PhD
Do any of the pictures below show a chemical reaction? How can you tell?
\[ \text{Aluminum + iron(III) oxide} \rightarrow \text{aluminum oxide + iron} \] \[ \chem{Al(s) + Fe_2O_3(s) \rightarrow Al_2O_3(s) + Fe(s)} \]
# of atoms (reactants) | # of atoms (products) |
---|---|
1 Al | 2 Al |
2 Fe | 1 Fe |
3 O | 3 O |
Therefore, we need to balance the equation with coefficients: \[ \chem{2Al(s) + Fe_2O_3(s) \rightarrow Al_2O_3(s) + 2Fe(s)} \]
\[ \text{Methane + oxygen} \rightarrow \text{carbon dioxide + water} \] \[ \chem{CH_4(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)} \]
Currently, the number of atoms of each element is shown below. These numbers were obtained by multiplying the subscript to the right of the element's symbol by the stoichiometric coefficient.
# of atoms (reactants) | # of atoms (products) |
---|---|
1 C | 1 C |
4 H | 2 H |
2 O | 3 O |
\[ \chem{CH_4(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)} \]
First, we look at the carbon atoms. Since the number of carbon atoms on the reactant side is already equal to the number of carbon atoms on the product side, we don't need to add coefficients.
Next, we look at the hydrogen atoms. Currently, there are four hydrogen atoms on the reactant side and 2 hydrogen atoms on the product side. Thus, we need to add a coefficient of 2 in front of water to make the hydrogen atoms equal.
\[ \chem{CH_4(g) + O_2(g) \rightarrow CO_2(g) + 2H_2O(g)} \]
\[ \chem{CH_4(g) + O_2(g) \rightarrow CO_2(g) + 2H_2O(g)} \]
Finally, we look at the oxygen atoms. Currently, there are 2 oxygen atoms on the reactant side and 4 oxygen atoms (combined from carbon dioxide and water) on the product side. Thus, we add a coefficient of 2 in front of the oxygen gas.
\[ \chem{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)} \]
Now the reaction is balanced!
Class | Reactants | Products | Example |
---|---|---|---|
Decomposition | 1 compound | 2 elements (or smaller compounds) | \( \chem{CD \rightarrow C+D} \) |
Combination | 2 elements or compounds | 1 compounds | \( \chem{A + B \rightarrow AB} \) |
Single-Replacement | 1 element + 1 compound | 1 elements + 1 compound | \( \chem{A + CD \rightarrow C+AD} \) |
Double-Replacement | 2 compounds | 2 compounds | \( \chem{CD + EF \rightarrow CF+ED} \) |
Oxides and halides of the metals Au, Pt, and Hg decompose to the elements. \( \chem{2HgO(s) \rightarrow 2Hg(l) + O_2(g)} \) |
Peroxides decompose to oxides and oxygen gas. \( \chem{2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)} \) |
Metal carbonates, except those of group 1A metals, decompose to metal oxides and carbon dioxide gas. \( \chem{NiCO_3(s) \rightarrow NiO(s) + CO_2(g)} \) |
Oxoacids decompose in a similar way to form nonmetal oxides and water. \( \chem{H_2CO_3(aq) \rightarrow H_2O(l) + CO_2(g)} \) |
Ammonium compounds lose ammonia gas. \( \chem{\left( NH_4 \right)_2SO_4(s) \rightarrow NH_3(g) + H_2SO_4(l)} \) |
Ions | Rule |
---|---|
\( \chem{Na^+ ,\, K^+ ,\, NH_4^+} \) (and other alkali metal ions) | Most compounds of alkali metal and ammonium ions are soluble. |
\( \chem{NO_3^- ,\, C_2H_3O_2^-} \) | All nitrates and acetates are soluble. |
\( \chem{SO_4^{2-}} \) | Most sulfates are soluble. Exceptions are \( \chem{BaSO_4 ,\, SrSO_4 ,\, PbSO_4 ,\, CaSO_4 ,\, Hg_2SO_4 ,\, Ag_2SO_4}\). |
\( \chem{Cl^- ,\, Br^- ,\, I^-} \) | Most chlorides, bromides, and iodides are soluble. Exceptions are \( \chem{AgX ,\, Hg_2X_2 ,\, PbX_2 ,\, HgI_2} \). (\( \chem{X=Cl ,\, Br ,\, I}\)) |
\( \chem{Ag^+} \) | Silver compounds, except \( \chem{AgNO_3 \, and \, AgClO_4} \) are insoluble. \(\chem{AgC_2H_3O_2}\) is slightly soluble. |
\( \chem{O^{2-} ,\, OH^-} \) | Oxides and hydroxides are insoluble. Exceptions are alkali metal hydroxides, \(\chem{Ba(OH)_2 ,\, Sr(OH)_2 ,\, Ca(OH)_2}\) (somewhat soluble) |
\( \chem{S^{2-}} \) | Sulfides are insoluble. Exceptions are compounds of \(\chem{Na^+ ,\, K^+ ,\, NH_4^+}\) and the alkaline earth metal ions. |
\( \chem{CrO_4^{2-}} \) | Most chromates are insoluble. Exceptions are compounds are \(\chem{Na^+ ,\, K^+ ,\, NH_4^+ ,\, Mg^{2+} ,\, Ca^{2+} ,\, Al^{3+} ,\, Ni^{2+}}\). |
\( \chem{CO_3^{2-} ,\, PO_4^{3-} ,\, SO_3^{2-} ,\, SiO_3^{2-}} \) | Most carbonates, phosphates, sulfites, and silicates are insoluble. Exceptions are compounds of \(\chem{Na^+ ,\, K^+ ,\, NH_4^+}\). |
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