Chemical Reactions

Chapter 10Reactions and Equations

Evidence of Chemical Reactions

• How many indicators of a chemical reaction can you list?

1. 2. 3. 4. 5.

ANSWERS

1. Temperature change2. Color change3. Texture change4. Precipitate formation5. Bubbling, fizzing6. Gas released

Representing Chemical Reactions

• Chemical equations show REACTANTS on the left and PRODUCTS on the right side of an arrow.

Reactant 1 + Reactant 2 → Product 1 + Product 2

• A skeletal equation uses chemical formulas rather than words to identify the reactants and the products. For example, the skeleton equation for the reaction between iron and chlorine is represented below.

• Iron(s) + chlorine(g) → iron (III) chloride (s)

• Fe(s) + Cl2 (g) → FeCl3(s)

One iron atom One iron atom Two chlorine atoms Three chlorine atoms

Practice Problems

• Write skeleton equations for each word equation on page 279 of text.

Answers

• H2 (g) + Br2 (g) → 2HBr (g)

• 2CO(g) + O2(g) → 2CO2 (g)

• 2KClO3 (s) → 2KCl(s) + 3O2(g)

Balancing Equations

Fe(s) + Cl2 (g) → FeCl3(s)

One iron atom One iron atom Two chlorine atoms Three chlorine atoms

To accurately represent a chemical reaction by an equation, the equation must show how the law of conservation of mass is obeyed. The equation must show that the number of atoms of each reactant and each product is equal on both sides of the arrow.

•

•

+ →

+ → →

•

2 iron atoms 6 chlorine atoms 2 iron atoms, 6 chlorine atoms

Fe Cl

Cl

Fe

Cl

ClCl

Fe

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Fe

Cl Cl

Steps for balancing equations1. Write the skeleton equation for the reaction.

2. Count the atoms of the elements in the reactants.

3. Count the atoms of the elements in the products.

4. Change the coefficients to make the number of atoms of each element equal on both sides of the equation.

NEVER CHANGE THE SUBSCRIPT in a chemical formula to balance an equation because doing so changes the identity of the substance.

5. Write the coefficients in their lowest possible ratio.

6. CHECK YOUR WORK! Make sure that the chemical formulas are written correctly. Then check that the number of atoms of each element is equal on both sides of the equation.

Practice Problems

• Write chemical equations for each of the word equations on page 282 of text.

4. In water, iron (III) chloride reacts with sodium hydroxide , producing solid iron (III) hydroxide and sodium chloride.

5. Liquid carbon disulfide reacts with oxygen gas, producing carbon dioxide gas and sulfur dioxide gas.

6. Solid zinc and aqueous hydrogen sulfate react to produce hydrogen gas and aqueous zinc sulfate.

ANSWERS

• FeCl3 (aq) + NaOH(aq) → Fe(OH)3 (s) + NaCl(s)

• CS2(l) + O2(g) → CO2(g) + SO2 (g)

• Zn(S) + H2SO4 (aq) → H2 (g) + ZnSO4(aq)

Did you balance?FeCl3 (aq) + NaOH(aq) → 3Fe(OH)3 (s) + 3NaCl(s)

CS2(l) + 3O2(g) → CO2(g) + 2SO2 (g)

Zn(S) + H2SO4 (aq) → H2 (g) + ZnSO4(aq)

(already balanced)

Symbols→ yields, produces, makes, forms, etc.

(s) Solid

(t) Liquid

(aq) aqueous

(g) gas, also may appear as ↑

↔ equilibrium, (reaction can also go in reverse)You may also see this as →

←