Chemical Reactions
Senior 1 · Chemistry
Chemical Reactions
Lesson 1 of 5
22 min
Leave an iron bicycle chain outside through a rainy season and it turns orange-brown and flaky. Light a match and the white head flares, smokes and leaves black ash. Tip baking soda into vinegar and the mixture froths. In Unit 3 you called changes like these chemical changes: a new substance forms that was not there before, and the change is not easy to reverse. In this unit you will learn to describe them the way chemists do, as chemical reactions written as equations.
Definition
Chemical reaction: a change in which one or more substances are turned into new substances with different properties. The atoms are rearranged: bonds between atoms are broken and new bonds are made.
Definition
Reactants and products: the reactants are the substances you start with; the products are the new substances formed. Example: when charcoal burns, carbon and oxygen are the reactants and carbon dioxide is the product.
You cannot see atoms rearranging, so you look for evidence. One sign alone is not proof: boiling water gives off bubbles, but that is a change of state, a physical change.
Table 1.1: Signs that a chemical reaction has taken place
| Sign | Example |
|---|---|
| A gas is given off (bubbles or fizzing) | baking soda and vinegar; an antacid tablet dropped in water |
| A colour change | iron rusting; a cut avocado or banana turning brown in the air |
| Energy given out or taken in (the mixture gets hot or cold) | charcoal burning in an imbabura stove |
| Light is given out | a match or magnesium ribbon burning |
| A solid (precipitate) appears in a clear solution | carbon dioxide turning clear limewater milky |
| A new smell | milk souring into ikivuguto; food going bad |
Put 50 cm³ of vinegar in a conical flask and a spatula of baking soda in a small balloon. Stretch the balloon over the neck of the flask without letting the powder fall in, and weigh everything. Then lift the balloon so the powder drops into the vinegar, and weigh again (Figure 1.1 A). Repeat with a second flask left open, with the baking soda tipped straight in (Figure 1.1 B).
Figure 1.1: Weighing a reaction in a closed and an open flask. Typical results from a school laboratory.
Warning
Safety. Vinegar and baking soda are harmless, but wear goggles: the froth can squirt out of the flask. Never do this with a stopper pushed tightly into the flask, because the gas pressure can blow it out.
In the closed flask the carbon dioxide made by the reaction is trapped in the balloon, and the mass stays exactly the same. In the open flask the carbon dioxide escapes into the air, so the flask loses mass; the "lost" 0.88 g is the mass of gas that left. Nothing was destroyed. This result was first shown clearly by the French chemist Antoine Lavoisier in the 1780s, who weighed reactions in sealed glass vessels.
Definition
Law of conservation of mass: in a chemical reaction, matter is neither created nor destroyed. The total mass of the products equals the total mass of the reactants. It holds exactly in a closed system, where no substance can enter or leave.
The law follows from what happens to atoms. In a reaction the atoms are only rearranged into new substances. Every atom present at the start is still there at the end, so the mass cannot change. Figure 1.2 shows this for charcoal burning.
Figure 1.2: When carbon burns, the atoms are rearranged into new molecules, but none is gained or lost.
Worked example
Problem. 2.4 g of magnesium ribbon is burned in a closed container and forms 4.0 g of magnesium oxide. What mass of oxygen reacted?
Solution. By the law of conservation of mass, the mass of the reactants equals the mass of the products:
Answer: 1.6 g of oxygen combined with the magnesium. The product is heavier than the metal, because it contains oxygen taken from the air.
Worked example
Problem. Keza burns 10.0 g of dry wood shavings in an open dish. Only 0.3 g of ash is left. Her friend says: "This proves that mass is destroyed when things burn." Is he right?
Solution. No. The dish was an open system. Wood reacts with oxygen from the air to form carbon dioxide and water vapour, which are gases and escape. If the burning were done in a sealed container and everything were weighed, including the gases, the total mass of products would equal the mass of wood plus the oxygen used.
Important
A chemical reaction rearranges atoms. It never creates or destroys them. This is the reason every chemical equation must be balanced (Lesson 4).
Tip
When a mass seems to change in a reaction, ask: "Did a gas get in or out?" Burning metals gain mass (oxygen gets in); burning wood and fizzing reactions lose mass (gases get out).
TED-Ed: The law of conservation of mass, by Todd Ramsey (5 min). Watch for Lavoisier's sealed-vessel experiments and how the atoms are accounted for.
Practice
Sources and credits
