Chapter 10 Chemical Reactions. 10.1 Reactions and Equations Chemical Reaction- process by which the...

Chapter 10

Chemical Reactions

10.1 Reactions and EquationsChemical Reaction- process by which the atoms of one or more substances are rearranged to form different substances

Chemical Reaction

Na + Cl2 = NaCl (Salt)

a new substance is formed!!

Evidence of a Chemical Reaction

1. Temperature change

•Exothermic - Release of energy in the form of heat or light

•Endothermic – absorb heat (feels cold)

2. A Color change

3. Odor

4. Gas bubbles

5. Formation of a solid (precipitate)

Chemical Equations

Reactant- starting substances in a reaction

Product- substances formed during the reaction

Na + Cl2 NaCl

Reactants Products

An arrow reads as yields and separates reactants from products and indicates direction of reaction --points to right

Reactants YIELD ProductsReactants Products(left side) (right side)

When there are two or more reactants or products a plus sign separates each reactant or product.

reactant 1 + reactant 2 --- product 1 + product 2

and yields and

(s) Solid (l) Liquid(g) Gas (aq) Aqueous – dissolved in water

Word Equations

Uses words to indicate the reactants and products of chemical reactions.

Ex: Iron + chlorine → iron (III)

chlorideRead: “iron and chlorine react to

produce iron (III) chloride”

Skeleton Equations

Uses chemical formulas to identify reactants and products

Ex:Fe (s) + Cl2 (g) → FeCl3 (s)

Usually they are not balanced!!

Chemical Equations

Chemical Equation- uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction.

C. Balancing a Chemical Equation

The equation must show that the number of atoms of each reactant and each product are equal on BOTH sides of the arrow.

To balance an equation you must find the correct coefficients for the chemical formulas in the skeleton equation.

2Fe (s) + 3Cl2 (g) → 2FeCl3 (s)

coefficients

Coefficient- number written in front of a reactant or product

1. Is a whole number (not a fraction)

2. Tells smallest number of particles of the substance involved in the reaction

Steps for Balancing Equations

1.Write the skeleton equation for the reaction.

A suggestion:Draw boxes around all the chemical formulas. This is the step that people frequently don't do. If it helps YOU… Ignore them. You're drawing those boxes so that you'll be sure not to mess around with the formulas to balance the equation. Here's what the equation looks like:

2. Count the atoms of the elements in the reactants (before).

3. Count the atoms of the elements in the products (after).

Make an element inventory!!!!!

4. Write/Change the coefficients in front of each of the boxes until the inventory for each element is the same both before and after the reaction. Now, what happens when we put a number in front of a formula? Basically, anything in that box is multiplied by that number.

5 H2 5 x 2 = 10 Hydrogen

Keep changing the coefficients until the number of atoms of each element is equal on both sides of the equation.

H2

We can see that on the left side of the inventory, there is one atom of sodium and on the right there are two. The solution: Stick a "2" in front of the sodium hydroxide on the left side of the equation so that the numbers of sodium atoms are the same on both sides of the equation.

Then update your inventory!! But the others still don't match up. What to do?

Using your amazing powers of mathematics (and hopefully not needing to use a calculator), you can see that two multiplied by the number two becomes four. That's what you need to do. How? Put a "2" in front of the water on the right side of the equation to make the hydrogens balance out. Now that this is done, you should make a new inventory that looks something like this:

Since both sides of the inventory match, the equation is now balanced!

5. NEVER change a subscript!! You can’t change any subscripts because that would change the chemical composition of a molecule, which would change the entire reaction and create something different altogether.

6.Write the coefficients in front of the substances in the equation in their lowest possible ratio.4H2O + 4Fe 2Fe2O3 + 4H2

All the coefficients are divisible by 2Should be: 2H2O + 2Fe Fe2O3 +2H2

7. Check your work!

10.2 Classifying Chemical Reactions

video- 5 chem rx animations.mp4

Synthesis Reaction- a chemical reaction in which two or more substances react to produce a single product

Ex: 2Na (s) + Cl2 (g) → 2NaCl (s)

Two or more reactants become one product

Combustion Reaction-Oxygen combines with a

substance and releases energy in the form of heat and light

Ex: 2H2 (g) + O2 (g) → 2H2O

The products are always Carbon dioxide CO2 and Water

H2O

Decomposition Reaction- a single compound breaks down into two or more elements or new compounds

Ex: 2NaN3 (s) → 2Na (s) + 3N2 (g)

Mercury(II) oxide Mercury + Oxygen

Single-replacement Reaction- a reaction in which the atoms of one element replace the atoms of another element in a compound

Ex: Cu (s) + 2AgNO3 (aq) → 2Ag (s) + Cu(NO3)2 (aq)

element + cmpd element + cmpd

2 Al + 3 CuCl2 2 AlCl3 + 3 Cu

Double-replacement Reaction- positive and negative ions of two compounds switch places.

Ex: KCN (aq) + HBr (aq) → KBr (aq) + HCN (g) cmpd cmpd cmpd cmpd

Precipitate – a solid produced during a chemical reaction in a solution

All Double-replacement reactions produce:

•Precipitate

•Gas

•Water

10.3 Reactions in Aqueous Solutions

When a substance dissolves in water a solution forms.Solution – homogeneous mixture (looks the same throughout)

Solute – substances dissolved in water.Solvent – the most plentiful substance

in the solution, usually water.Aqueous solution – a solution in which

the solvent is water.