Conservation Of Mass Including The Quantitative Interpretation
Visão Geral
Set a strip of magnesium alight on a balance pan and something disquieting happens: the reading climbs. Burn a candle on the same balance and the reading falls. Two reactions, two opposite verdicts, and a beginner would be forgiven for concluding that matter can be conjured up and thrown away at will. It cannot. Both readings are honest, both are misleading, and the single idea that resolves them is the most dependable accounting rule in the whole of chemistry.
This lesson turns that rule into a working tool. You will learn to write a reaction as words, then as symbols, then as a balanced equation carrying state symbols, and you will see why an equation that balances is really a statement about atoms being rearranged rather than created. Then comes the payoff. A balanced equation is not only a picture of a reaction: it is a recipe with quantities attached, and you will use it to predict the mass of a product before the reaction is run. You will finish by looking at why the chemist at the bench almost never collects quite as much as the arithmetic promised, and at how the two Combined Science chemistry papers turn each of those skills into marks.
Objetivos
- Chemical reactions can be represented by word equations or by symbol equations. Students should be able to write word and balanced symbol equations for reactions in the specification.
- Information about the states of reactants and products can be included in chemical equations. Students should be able to use the state symbols (g), (l), (s) and (aq) in equations where appropriate.
- No atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
- The masses of reactants and products can be calculated from balanced symbol equations. Students should be able to calculate the mass of a reactant or product from information about the masses of the other reactants and products in the reaction and the balanced symbol equation.
- Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because: the reaction may not go to completion because it is reversible; some of the product may be lost when it is separated from the reaction mixture; some of the reactants may react in ways different from the expected reaction.
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- Which state symbol is used for a substance that is dissolved in water? A. (s) B. (l) C. (g) D. (aq) Answer: D
- Which of these is the correctly balanced equation for the reaction between sodium and chlorine? A. Na + Cl2 -> NaCl B. 2Na + Cl2 -> 2NaCl C. Na + 2Cl -> NaCl2 D. 2Na + 2Cl2 -> 2NaCl Answer: B
- 50 g of calcium carbonate is heated until it has completely decomposed into calcium oxide and carbon dioxide. 28 g of calcium oxide is formed. What mass of carbon dioxide is given off? A. 11 g B. 22 g C. 28 g D. 78 g Answer: B
- A coil of magnesium is burned in an open crucible. The mass of the solid in the crucible increases. What is the reason? A. Atoms of magnesium are created during the reaction. B. Oxygen atoms from the air join the magnesium and stay in the crucible. C. The magnesium oxide formed is a denser substance than magnesium. D. Heating any solid always increases its mass. Answer: B
- A chemist obtains a smaller mass of product than the mass calculated from the balanced equation. Which statement is a valid explanation? A. Some of the atoms were destroyed during the reaction. B. Some of the product was lost when it was separated from the reaction mixture. C. The law of conservation of mass does not apply to reactions that make a solid. D. The relative formula masses change during a reaction. Answer: B
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