Chemical Reactions

Section 1 Chemical Changes

A. Describing chemical reactions- change of one or more substances into new substances

1. Reactants are substances that combine or change

2. New substances that are produced are called products.

B. Conservation of Mass- a law which states that, in a chemical reaction, matter is not created or destroyed; it stays the same

1. Antoine Lavoisier- experimented with mercury(II) oxide and heat

2. Found mass of products (liquid mercury and oxygen gas) equaled mass of reactants

C. Writing equations- a chemical equation uses chemical formulas and symbols to describe a chemical reaction and the product(s) it produces

1. Chemical formula expresses the relationship between elements in the compounds and molecules they make up

2. Coefficients- numbers which represent the number of units of each substance in a reaction

3. Knowing coefficients of chemical reactions allows chemists to use the correct amounts of reactants to predict the amounts of products

4. Subscripts- numbers which represent the number of atoms in a molecule of a particular element.

5. Symbols used to show state of reactants: (s)solid, (aq) aqueous, (g)gas, (clear) liquid.

When a chemical has been dissolved in water, this is denoted by writing (aq) after the chemical name.

D. Metals react with atmosphere in different ways.

Section 2 Chemical Equations

A. Checking for balance- law of conservation of mass requirement

1. A balanced chemical reaction- both sides of equation have the same number of atoms of each element

2. Choosing coefficients- becomes easier with practice; trial and error at first

B. Writing balanced chemical reactions- a four step process

1. Describe the reaction in words.

2. Write the equation using formulas and symbols.

3. Check for balance.

4. Add coefficients where needed for balance.

Section 3 Classifying Chemical Reactions

A. Synthesis reaction- two or more substances form a new substance; A + B ---> C

+ ->

B. One substance breaks down into two or more substances in a decomposition reactionAB ---> A + B

The starting compound is ammonium dichromate. When heated, it begins to decompose into nitrogen gas, water vapor and powdered chromium (III) oxide. It looks like a volcano with ash being spread all over the place.

http://www.youtube.com/watch?v=jowikn6tsyY

->

+

C. Single-displacement reaction- one element replaces another one in a compound:A + BC ---> AC + B or D + BC ---> BD + C

A + BC --> AC + B

+ ->

+

D. A double-displacement reaction results if a precipitate, water, or a gas forms when two ionic compounds in solution are combined; AB + CD --> AD + CB

AB + CD --> AD + CB

++ -

>

Combustion- O2 as a reactant and CO2 and H2O as the product

MgCO3 + 2HCl --> MgCl2 + H2O + CO2

Section 4 Chemical Reactions and Energy

A. Chemical reactions involve energy exchange.

Reaction of sulfuric acid and sugar.

1. Breaking chemical bonds requires energy.

Dr. Pyenta melted some potassium chlorate in a test tube over a Bunsen burner in a hood. Then he dropped in a sugar cube. The result was a considerable jet of flame that burned for 30 seconds or more.

2. Forming chemical bonds releases energy.

B. More energy out

1. Exergonic reactions- energy required to break bonds is less than the energy released from new bonds; energy given off is usually light

2. Exothermic reactions- energy given off in the form of heat

C. More energy in

1. Energonic reactions- more energy is required to break bonds than to form new ones; need energy for the reaction to occur

2. If energy needed is heat; the reaction is endothermic.

vinegar & baking soda

3. A catalyst speeds up a chemical reaction without itself being permanently changed.

Enzymes speed up chemical reactions

4. An inhibitor prevents or slows a chemical reaction or interferes with a catalyst's action