CH 1000 MOD 2 REVIEW

CH1000 Fundament als of Chemistry Module 2 – Chapter 8

Chemical Equations

• Chemists use chemical equations to: • Summarize a chemical reaction by displaying the substances reacting and

forming. • Indicate specific amounts of materials consumed or produced during the

reaction. • Reactants: substances consumed during the reaction. • Products: substances formed during the reaction.

• Atom balance must be maintained in all chemical reactions. • All atoms from reactants must appear as part of products.

a A + b B c C + d D

The coefficient 1 is not written in a balanced equation.

Chemical Equations

1. Reactants and products are separated by an arrow. 2. Reactants are on the left side of the arrow, products are on the right. 3. Whole number coefficients are placed in front of substances to

balance the atoms in the equation. 4. The numbers indicate the units of the substance reacted or formed

during the reaction. 5. Information about the reaction (temperature, time) may be placed

above or below the reaction arrow. 6. The physical state is written in brackets after the formula of the

substance. (g) for gas, (l) for liquid, (s) for solid, (aq) for aqueous

a A + b B c C + d D Reactant

s Products

Symbol Summary

Symbol Significance Produces (points towards products)

(s) Solid (written after substance) (l) Liquid (written after substance) (g) Gas (written after substance) (aq) Substance dissolved in an aqueous

solution Heat is added (above or below reaction arrow)

Δ

Law of Conservation of Mass

• The total mass of substances in a chemical reaction must remain constant.

water hydrogen + oxygen 100.0 g 11.2 g 88.8 g

100.0 g total of productsreactants

In any chemical reaction: Mass of reactants = Mass of products

Writing and Balancing Chemical Equations

A balanced chemical equations contain the same number of each kind of atom on both sides of the equation.

1. Write a word equation for the reaction.

2. Write the correct formula for each substance (unbalanced):

3. Balance the equation a) Count the number of each atom on the reactants and

products side and determine what requires balancing.

b) Balance each element sequentially, using whole numbers. It is often best to balance metals first.

mercury(II) oxide mercury + oxygenΔ

HgO Hg + O2 Δ

Hg: 1 O: 1

Hg: 1 O: 2

HgO Hg + O2 Δ

Oxygen atoms need balancing on the reactants side.

2 HgO Hg + O2 Δ

Hg: 2 O: 2

Hg: 1 O: 2

Now Hg atoms need balancing on the products side.

Writing and Balancing Chemical Equations

4. Check after adding coefficients that all atoms still balance. Adjust as needed (a 2 is needed in front of Hg).

5. Do a final check to make sure all atoms now balance on both sides of the equation.

2 HgO 2 Hg + O2 Δ

Hg: 2 O: 2

Hg: 2 O: 2

Note: always use the smallest whole numbers!

4 HgO 4 Hg + 2 O2 Δ

Balanced but incorrect form!

Information in a Chemical Equation

© 2014 John Wiley & Sons, Inc. All rights reserved.

Information from a Chemical Equation

• From the chemical equation below, how many moles of oxygen are needed to burn 2 molecules of propane (C3H8)?

• a) 5 molecules of oxygen • b) 6 molecules of oxygen • c) 10 molecules of oxygen • d) 15 molecules of oxygen

C3H8 + 5 O2 3 CO2 + 4 H2O

For every 1 molecule of propane, 5 molecules of O2 are needed to fully

react. Two molecules of propane would then

require 2 x 5 = 10 molecules of oxygen.

Types of Chemical Equations

1. Combination reactions 2. Decomposition reactions 3. Single displacement reactions 4. Double displacement reactions 5. Oxidation-reduction (redox) reactions

(Chapter 17)

Reactions are classified into subtypes to aide in predicting

the products of chemical reactions.

Reactions are classified into five major categories:

Combination Reactions Two reactants combine to give a single product. A + B AB

Decomposition Reactions

A single reactant breaks down (decomposes) into two or more products

AB A + B

Single Displacement Reactions

One element (A) reacts with a compound (BC) to replace one element in the compound, giving a new element (B) and a different compound (AC).

General Types of Single Displacement Reactions

Double Displacement Reactions

Two compounds exchange partners with one another to yield two new compounds.

AB + CD AD + CB

General Types of Double Displacement Reactions

Double Displacement Reactions

Two compounds exchange partners with one another to yield two new compounds.

AB + CD AD + CB

General Types of Double Displacement Reactions Writing Reaction Equations Practice 1. Write the reaction equation between aqueous

solution of hydroiodic acid and sodium hydroxide.

2. First convert names to chemical formulas and determine the type of reaction.

HI (acid)/NaOH(base)

Neutralization Reaction acid + base salt + water HI (aq) + NaOH (aq) NaI (aq) + H2O (l) Salt formula must charge balance (Na+ and I–)

Heat in Chemical Reactions

Terminology

Energy transfer and changes accompany any chemical reaction

Heat of reaction: quantity of heat actually produced during a chemical reaction. Units: kilojoules (kJ) or kilocalories (kcal)

Exothermic reactions: release heat. H2 (g) + Cl2 (g) 2 HCl (g) + 185 kJ Heat can be treated as a product

Endothermic reactions: absorb heat. N2 (g) + O2 (g) + 181 kJ 2 NO (g) Heat can be treated as a product

C (s) + O2 (g) CO2 (g) + 393 kJ 1 mol of C reacts with 1 mol of O2 to provide 1 mol of CO2 and 393 kJ of heat are released.

Heat in Chemical Reactions Equations Practice

Heat as an Energy Transfer

Vehicle in Nature

Graphical Representations of

Endothermic Reactions

•Products are at a higher potential energy than reactants. •Activation energy: Amount of energy needed to initiate a chemical reaction.

Reaction Coordinate Diagram

Graphical Representations of

Exothermic Reactions

•Products are at a lower potential energy than reactants. •Activation energy: Amount of energy needed to initiate a chemical reaction.

Reaction Coordinate Diagram

Reading Review

How do you know if a reaction is a combustion reaction?

What is an endothermic reaction?

What is an exothermic reaction?

What are the four types of chemical equations?.

How do you know if an equation is balanced?