Question

section 1 review
sc.912.p.10.1; sc.912.p.10.7
key ideas

1. list three signs that could indicate that a chemical reaction is taking place.
2. identify whether each of the following is a chemical reaction:

a. melting ice
b. burning a candle
c. rusting iron

1. predict which atoms will be found in the products of the following reactions:

a. mercury(ii) oxide, hgo, is heated and decomposes.
b. limestone, caco₃, reacts with hydrochloric acid, hcl.
c. table sugar, c₁₂h₂₂o₁₁, burns in air to form caramel.

1. list four forms of energy that may be absorbed or released during a chemical reaction.
2. classify each of the following reactions as exothermic or endothermic:

a. paper burning with a bright flame
b. plastics becoming brittle after being left in the sun
c. a firecracker exploding
critical thinking

1. applying concepts calcium oxide, cao, is used in cement mixes. when water is added, energy is released as cao forms calcium hydroxide, ca(oh)₂. what signs indicate a chemical reaction? draw an energy diagram to represent this reaction. which has more chemical energy: the reactants or the products?

Explanation:

Step1: Recall signs of chemical reaction

Change in color, production of gas, formation of precipitate.

Step2: Analyze melting ice

Melting ice is a physical change (change of state), not a chemical reaction.

Step3: Analyze burning candle

Burning candle is a chemical reaction as it involves combustion and new substances are formed.

Step4: Analyze rusting iron

Rusting iron is a chemical reaction as iron reacts with oxygen to form iron - oxide.

Step5: Analyze mercury(II) oxide decomposition

When HgO is heated and decomposes, products will contain mercury (Hg) and oxygen (O) atoms.

Step6: Analyze limestone - hydrochloric acid reaction

CaCO₃ + HCl reaction will have products with calcium (Ca), carbon (C), oxygen (O), hydrogen (H) and chlorine (Cl) atoms.

Step7: Analyze table - sugar burning

When C₁₂H₂₂O₁₁ burns, products will have carbon (C), hydrogen (H) and oxygen (O) atoms.

Step8: Recall forms of energy in chemical reactions

Heat, light, sound, electrical energy.

Step9: Classify exothermic/endothermic reactions

Paper burning is exothermic (releases heat and light), plastics becoming brittle is endothermic (absorbs energy from sun), firecracker exploding is exothermic (releases heat, light, sound).

Step10: Analyze calcium - oxide reaction

Signs of chemical reaction: release of energy (heat), formation of a new substance (Ca(OH)₂). Reactants have more chemical energy as energy is released.

Answer:

1. Change in color, production of gas, formation of precipitate.
2. a. Not a chemical reaction. b. Chemical reaction. c. Chemical reaction.
3. a. Hg and O. b. Ca, C, O, H, Cl. c. C, H, O.
4. Heat, light, sound, electrical energy.
5. a. Exothermic. b. Endothermic. c. Exothermic.
6. Signs: release of energy, formation of new substance. Reactants have more chemical energy.