Chemistry - Describing Chemical Rxns

​Describing Chemical Reactions

A chemical reaction is the process by which one or more substances are changed into one or more different (new) substances.

• it contains reactants (to the left of the arrow) and products (to the right​ of the arrow)

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​Chem Rxns

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According to the law of conservation of mass, the total mass of reactants must be equal to the total mass of products for any given chemical reaction.

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​Some text here about the topic of discussion.

​Conservation

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An open system has no covering, so this means that gas can escape.

• making it seem that mass is decreasing.

A closed system has no way for gas to escape, so mass remains the same​

​Open vs. Closed Systems

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​Law of Conservation of Mass:

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​The total mass of substances does not change during a chemical reaction.

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Chemical reactions are described by chemical equations.

• A chemical equation represents, with symbols and formulas, the identities and relative molecular or molar amounts of the reactants and products in a chemical reactions.

  • Example: the following chemical equation shows that the reactant ammonium dichromate yields the products nitrogen, chromium(III) oxide, and water.

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​Describing Rxns

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The following changes usually indicate a chemical reaction has occurred.

• ​Evolution of Heat and Light – a change in matter that releases energy as both heat and light (it is strong evidence that a reaction has taken place)

​Indications of Chemical Rxns

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• ​​Some reactions only release heat or light.

• The release of heat or light by itself is not always a sign of a reaction because some physical processes also release either heat or light.

The following changes usually indicate a chemical reaction has occurred.

• Production of Gas – the evolution of gas bubbles when two substances are mixed is often evidence of a reaction.

  • Examples: bubbling, fizzing, foaming​

​Indications of Chemical Rxns

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The following changes usually indicate a chemical reaction has occurred.

• Formation of a Precipitate – if a solid appears after two solutions are mixed, a reaction has likely occurred.

​Indications of Chemical Rxns

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• ​A solid that is produced as a result of a reaction in solution and that separates from the solution is known as a precipitate.

The following changes usually indicate a chemical reaction has occurred.

• Color Change – a change in color is often an indication of a reaction.

​Indications of Chemical Rxns

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must be a different color from both starting materials