Observation of burning match or splint – What caused the change in the burning match or splint? NaCl(aq) + HCl(aq) → NaCl(aq) + H2O(l) → NaCl(aq) + H2O(l) → NaCl(aq) + O2(g) → Type of reaction: Combustion Hydrogen Peroxide Reactants Appearance of Reactants Evidence of a Chemical Reaction H2O2(aq) H2O(l) + O2(g) Type of chemical reaction: Decomposition Questions and Problems Q1: What evidence of a chemical reaction might you see in the following cases? Refer to Table 10.1. – Bubbles forming when dropping an Alka-Seltzer tablet into a glass of water – Fading or disappearance of a stain when using bleach – Flame and smoke when burning a match
The Correct Answer and Explanation is:
Here are the correctly filled-in answers for the worksheet, followed by a detailed explanation of the chemical principles involved.
Completed Worksheet Answers
3. Observation of burning match or splint: The match/splint ignites, producing a flame (light and heat), smoke, and turns into black ash/char.
What caused the change in the burning match or splint? A chemical reaction called combustion.
4. 1 Na₂CO₃(s) + 2 HCl(aq) → 1 CO₂(g) + 1 H₂O(l) + 2 NaCl(aq)
5. Type of reaction: Double displacement (or acid-base) reaction.
F. Hydrogen Peroxide
| Reactants | 1. Appearance of Reactants | 2. Evidence of a Chemical Reaction |
| H₂O₂(aq) | Clear, colorless liquid | Formation of gas bubbles (fizzing) |
3. 2 H₂O₂(aq)
→KIKI2 H₂O(l) + 1 O₂(g)
4. Type of chemical reaction: Decomposition reaction.
Questions and Problems
Q1 What evidence of a chemical reaction might you see in the following cases?
a. dropping an Alka-Seltzer tablet into a glass of water: Formation of a gas (bubbles or fizzing).
b. bleaching a stain: A change in color (the stain fades or disappears).
c. burning a match: Emission of light and heat (flame), production of gas (smoke), and a change in color/substance (wood turns to ash).
Explanation
This worksheet explores key concepts in chemistry, including balancing chemical equations, identifying different types of chemical reactions, and recognizing the evidence that a chemical change has occurred.
Balancing Chemical Equations
A fundamental principle in chemistry is the Law of Conservation of Mass, which states that matter cannot be created or destroyed. In a chemical reaction, this means the number of atoms of each element must be the same in the reactants (starting materials) and the products (substances formed). We ensure this by balancing the equation with coefficients—numbers placed in front of chemical formulas.
- In the reaction between sodium carbonate and hydrochloric acid (Na₂CO₃ + HCl), the balanced equation is 1 Na₂CO₃ + 2 HCl → 1 CO₂ + 1 H₂O + 2 NaCl. This ensures there are 2 sodium, 1 carbon, 3 oxygen, 2 hydrogen, and 2 chlorine atoms on both sides of the arrow.
- For the decomposition of hydrogen peroxide (H₂O₂), the balanced equation is 2 H₂O₂ → 2 H₂O + 1 O₂. This gives 4 hydrogen and 4 oxygen atoms on both sides.
Types of Chemical Reactions
Chemical reactions are classified based on how atoms are rearranged.
- Combustion: This is a rapid reaction between a substance and an oxidant, usually oxygen, to produce heat and light. Burning a match is a combustion reaction where the wood and chemicals on the match head react with oxygen.
- Double Displacement: In this reaction, the positive and negative ions of two ionic compounds switch places. The reaction Na₂CO₃ + 2HCl → 2NaCl + H₂CO₃ is a double displacement reaction. Note that the product carbonic acid (H₂CO₃) is unstable and immediately decomposes into water (H₂O) and carbon dioxide (CO₂) gas. Because it involves an acid (HCl) and a base (Na₂CO₃), it is also classified as an acid-base reaction.
- Decomposition: This occurs when a single compound breaks down into two or more simpler substances. The breakdown of hydrogen peroxide into water and oxygen (2H₂O₂ → 2H₂O + O₂) is a classic example of a decomposition reaction.
Evidence of a Chemical Reaction
A chemical reaction involves the formation of new substances with different properties. We can observe this change through several signs:
- Formation of a Gas: As seen with Alka-Seltzer in water, the production of bubbles or fizzing indicates a new gaseous substance is being formed.
- Change in Color: Bleach removes a stain by chemically altering the colored molecules, causing them to become colorless. This visible change in color is evidence of a reaction.
- Emission of Energy: The release of energy as light and heat, such as the flame from a burning match, is a clear sign of an exothermic chemical reaction.
- Formation of a Precipitate: The formation of an insoluble solid when two liquids are mixed.
- Change in Odor: The production of a substance with a distinct smell.
