Energy, Rusting and Reacting Masses: Quick Revision

A brief note on Energy, Rusting and Reacting Masses. Read it before a test. Try each question at the end before you open its answer.

Energy

• Exothermic: heat given out, the surroundings warm up. Burning, respiration, quicklime with water.
• Endothermic: heat taken in, the surroundings cool. Photosynthesis, heating limestone, a cold pack.
• Heat taken by water: $Q = m c \Delta T$, with $c = 4.18\ \text{J/(g K)}$.

Corrosion and rancidity

• Iron rusts only with both air and water; salt speeds it up. Rust is hydrated iron oxide.
• Prevent it by painting, oiling, galvanising with zinc, or making stainless steel.
• Rancidity is the oxidation of fats and oils. Slow it with antioxidants, airtight packs, cold or nitrogen.

Masses from equations

Grams to moles, the ratio of the coefficients, then moles to grams. With two masses given, find the limiting reactant first.

A worked example

What mass of carbon dioxide forms when $3.6\ \text{g}$ of carbon burns? $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$. Use $\text{C} = 12$ and $\text{CO}_2 = 44\ \text{g/mol}$.

1. Moles of carbon. $\dfrac{3.6\ \text{g}}{12\ \text{g/mol}} = 0.3\ \text{mol}$.
2. Ratio. $1 : 1$, so $0.3\ \text{mol}$ of $\text{CO}_2$.
3. Mass. $0.3\ \text{mol} \times 44\ \text{g/mol} = 13.2\ \text{g}$.

Practice with new numbers

A program to try

The program finds the heat for three reactions from the change in temperature of the water.

Mistakes to avoid

• Using the final temperature in $Q = m c \Delta T$ instead of the change.
• Saying that paint reacts with oxygen. It only covers the iron.
• Using the coefficients as grams. They count moles.
• Forgetting the limiting reactant when two masses are given.

Check yourself

1. Why is rusting called a slow oxidation?

2. $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$. What mass of carbon dioxide comes from $75\ \text{g}$ of calcium carbonate? Use $\text{CaCO}_3 = 100$ and $\text{CO}_2 = 44\ \text{g/mol}$.

3. How much heat warms $120\ \text{g}$ of water by $5\ \text{K}$? Use $c = 4.18\ \text{J/(g K)}$.

4. Why does food last longer in a refrigerator?