Chemistry / Unit 7: Chemical Reactions
7.1 Writing Chemical Equations
Notes
A chemical equation shows reactants (starting substances) turning into products (new substances).
- Format: reactants $\rightarrow$ products. The arrow means "yields."
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous, meaning dissolved in water.
- Law of conservation of mass: atoms are not created or destroyed, so the same atoms appear on both sides.
- The diatomic elements exist as pairs: $\text{H}_2$, $\text{N}_2$, $\text{O}_2$, $\text{F}_2$, $\text{Cl}_2$, $\text{Br}_2$, $\text{I}_2$.
- Example: "Solid sodium reacts with chlorine gas to form solid sodium chloride" is $2\text{Na(s)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{NaCl(s)}$.
Practice
In the equation $\text{Zn(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}$, what does (aq) mean?
Aqueous means dissolved in water.
Which element is written as a diatomic molecule in an equation?
Oxygen gas exists as $\text{O}_2$ molecules.
In a closed container, 10.0 g of reactants react completely. What is the total mass of the products?
Mass is conserved: atoms are only rearranged.
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