Chemical Equations and Reactions Chapter Eight

Describing Chemical Reactions What is a chemical reaction? -the process by which one or more substances are changed into different substances There are two parts: Reactants -the original substances Products -resulting substances Law of Conservation of Mass -the total mass of the reactants must equal the total mass of the products

How is a chemical reaction expressed? A chemical equation represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction 2Ca + O 2  2CaO reactants yield products

Indications of a Chemical Reaction Evolution of heat and light: presence of both is strong evidence; one by itself is not necessarily a sign as many physical changes also do this

Our Reaction: (evolution of heat and light) Reactants: (known facts) Mg + O 2 (react to yield)  Products: MgO Overall Reaction: Mg + O 2  MgO

Checks and “Balances” for Correctly Written Equations  known facts are represented: all reactants and products identified Mg + O 2  MgO  correct formulas written; diatomics in molecular form  Law of Conservation of Mass satisfied; add coefficients where necessary 2Mg + O 2  2MgO

Indications Production of a gas. The gases produced by factories contribute to air pollution and the formation of acid rain.

Our Reaction (production of a gas) Reactants: Mg + HCl (react to yield)  Products: MgCl 2 + H 2 Overall Reaction: Mg + HCl  MgCl 2 + H 2

Checks and “Balances” for Correctly Written Equations  known facts are represented: all reactants and products identified Mg + HCl  MgCl 2 + H 2  correct formulas written; diatomics in molecular form  Law of Conservation of Mass satisfied; add coefficients where necessary Mg + 2HCl  MgCl 2 + H 2

Indications Formation of a precipitate. A precipitate is considered a solid. It often appears as a cloudy substance in the solution.

Our Reaction (formation of a precipitate) Reactants: CuSO 4 + NaOH react to yield  Products: Cu(OH) 2 + Na 2 SO 4 In this reaction the cations (Cu 2+ and Na + ) simply change places Overall Reaction: CuSO 4 + NaOH  Cu(OH) 2 + Na 2 SO 4

Checks and “Balances” for Correctly Written Equations  known facts are represented: all reactants and products identified CuSO 4 + NaOH  Cu(OH) 2 + Na 2 SO 4  correct formulas written; diatomics in molecular form  Law of Conservation of Mass satisfied; add coefficients where necessary CuSO 4 + 2NaOH  Cu(OH) 2 + Na 2 SO 4

Indications Color change. This often indicates that a new substance has been formed.

Color Change A common example of a chemical reaction indicated by color change is the bright copper of roofs changing to a blue-green color over time. This is actually Cu +1 being changed to Cu +2.

Review Indications of a Chemical Reaction Evolution of heat and light Examples: burning, light sticks, hand warmers Production of a gas Examples: food spoiling, decomposition Formation of a precipitate Examples: rusting, tarnishing, deposits in tea kettle, chunky milk Color change Examples: leaves changing color, bleach on colors

Characteristics of Chemical Equations 1.Must represent known facts. All reactants and products identified. 2.Must contain correct formulas. Diatomics must be in molecular form (H 2, Br 2 …). Sulfur is S 8 and phosphorus is P 4. 3.Law of conservation of mass must be satisfied. To equalize #s of atoms, coefficients are added where necessary. Coefficient: small whole # that appears in front of a formula in a chemical equation.

Look Again! Color change and precipitate (rust) formed. (ppt) When acid rain hits the marble statue it dissolves the marble and gives off carbon dioxide (gas production).

Color change in leaves indicates a chemical change in the production of chlorophyll brought on by a reduction in the amount of daily sunlight. Burning substances of any kind frequently produces gases as indicated in this factory’s pollution.

Heat and light are two of the products in this chemical reaction known as combustion. Precipitate formation is one of the most common indications of a reaction that you will observe in the laboratory. Many of our reactions will be carried out in solution where precipitates at the molecular level can be seen.