Chemical Reactions

Chemical Reactions Processes in which one or

more substances are converted into new substances with different physical and chemical properties Ex: rusted metal, burned toast

Chemical Reactions

Parts of a chemical reaction:

Reactants – substances before reaction occurs

Products – substances produced by the reaction

Chemical Reactions

Reason why reactions occur:

Increase in stability for products

Types of Reactions Combination - two

substances combine to form new substance

A + B AB Ca + Cl2 CaCl2

Types of Reactions Combustion –

substance reacts with oxygen

A + O2 AO CH4 + O2 CO2 + H2O

Types of Reactions Decomposition –

substance breaks into two or more substances

AB A + B MnSO4 MnO + SO3

Types of Reactions Single-replacement –

uncombined element “trades places” with an element in a compound

A +BX AX + B Cd + 2 HCl CdCl2 + H2

Types of Reactions Double-replacement –

two binary compounds “trade partners”

AX +BY AY + BX CaCO3 + 2 HCl

CaCl2 + H2CO3

Chemical Equations Sentences representing a

chemical reaction 2 types

Word equations - gives names of reactants & products

Formula equations – shows chemical formulas and symbols

Chemical EquationsFormula equations…

Symbols used:+ reacts/combines

with, and yields/produces precipitate formed gas emitted

Chemical EquationsFormula equations…

Symbols used:(s) solid state(l) liquid state(g) gaseous state(aq) aqueous state

Chemical EquationsDiatomic molecule reminder…HydrogenH2

NitrogenN2

OxygenO2

FluorineF2

ChlorineCl2

Bromine Br2

IodineI2

Chemical EquationsUnbalanced Examples: Calcium reacts with oxygen

to produce calcium oxide

Carbon reacts with oxygen to produce carbon dioxide

Chemical EquationsUnbalanced Examples: Solid sodium reacts with

chlorine gas to produce solid sodium chloride

Nitrogen gas reacts with hydrogen gas to produce ammonia gas

Chemical EquationsUnbalanced Examples: Sodium hydroxide reacts

with hydrogen chloride to produce water and sodium chloride

Chemical EquationsHow to balance: Write formula equation with correct formulas & symbols Count number of atoms on each side of arrow (make

table if needed) Mass must be conserved (number of atoms on each side

of arrow are equal), use coefficients to adjust Work with one atom or ion at a time Save H & O for last (they tend to be in more than one

place & can be difficult) Recount to ensure that number of atoms are equal on

each side (coefficient ratio needs to be lowest possible numbers)

Chemical Equations

Balancing examples: NaOH + HCl H2O +

NaCl

Ca + O2 CaO

Chemical Equations

Balancing examples: N2 + 3 H2 2 NH3

CH4 + 2 O2 CO2 + 2 H2O

Chemical Equations

Balancing examples: FeCl3 + Na2CO3 NaCl +

Fe2(CO3)3

C2H6 + O2 CO2 + H2O

Chemical Reactions

Reaction Rates Reactions happen if

particles have enough energy.

Chemical ReactionsReaction Rates Factors

Temperature Concentration Surface area/Particle Size Catalysts – substances that

speed up reaction without being used

Chemical Reactions

Reactions are reversible

When rate of forward and reverse reaction are equal, system is at equilibrium.

Chemical ReactionsReactions are reversible Equilibrium is described by

Le Chatelier’s principle If stress is applied to a

system, the system changes to relieve the stress

Chemical Reactions

Reactions are reversible Factors affecting

equilibrium: Concentration Temperature Pressure

Chemical ReactionsReactions are reversible Equilibrium is described by a

constant Keq, which is a ratio of

products to reactants. Keq > 1, products favored Keq < 1, reactants favored

Chemical Equations

Can include heat Endothermic – heat, from

surroundings, enters the system, has positive heat of reaction (H) value, reactants have lower potential energy than products

Chemical Equations

Can include heatEndothermic reaction

Chemical EquationsCan include heatEndothermic reaction: C(s) +2 S(s) + 89.3 kJ CS2(l)

C(s) +2 S(s) CS2(l)

H = 89.3 kJ

Chemical EquationsCan include heat Endothermic – heat, from surroundings, enters the system

Exothermic – heat exits the system and goes to surroundings, has negative heat of reaction (H) value , reactants have higher potential energy than products

Chemical Equations

Can include heatExothermic reaction

Chemical EquationsCan include heatExothermic reaction: CaO(s) + H2O(l)

Ca(OH)2(s) + 65.2 kJ

CaO(s) + H2O(l) Ca(OH)2(s)

H = -65.2 kJ

Chemical Equations

Can include heat For systems with a constant

pressure, heat is also called enthalpy (so H is also change in enthalpy)

Chemical Equations

Can include heat Enthalpy diagrams – visual

representation of a thermochemical equation

Chemical Equations Enthalpy

diagrams – Endothermic reaction

Chemical Equations Enthalpy

diagrams – Exothermic reaction

Chemical Reactions

Generally involve heat Hess’s Law – summary of all

intermediate steps in a reaction, includes enthalpy changes for each step

Chemical Reactions

Generally involve heat Hess’s Law –

Can be used to determine heat of process that cannot be directly measured

Chemical Reactions

Generally involve heat Hess’s Law –

Example: conversion of diamond to graphite is too slow to measure, but combustion can be measured

Chemical Reactions

Generally involve heat Hess’s Law –

Example:C(s, diamond) + O2(g) CO2(g)

H = - 395.4 kJ

Chemical Reactions

Generally involve heat Hess’s Law –

Example:C(s, graphite) + O2(g) CO2(g)

H = - 393.5 kJ

Chemical ReactionsGenerally involve heat Hess’s Law –

Example: difference between the two H values will give the H for converting diamond to graphite

Chemical ReactionsHess’s Law Diagram –

-1.9 kJ

Chemical Reactions Hess’s Law (another look) –

Determine the enthalpy change for the reaction of nitrogen dioxide becoming dinitrogen tetroxide.

2 NO2(g) → N2O4(g)

N2(g) + 2 O2(g) → 2 NO2(g) H = 67.7 kJ

N2(g) + 2 O2(g) → N2O4(g) H = 9.7 kJ

H = -58.0 kJ

Chemical Reactions

Will a reaction occur? A reaction that releases

heat is likely to occur

Chemical ReactionsWill a reaction occur? A reaction that releases heat is

likely to occur Since endothermic reactions

occur, there must be something besides enthalpy to determine if reaction happens

Chemical Reactions

Will a reaction occur? Reactions are dependent on

a combination of enthalpy and entropy

Chemical Reactions

Will a reaction occur? Entropy (S) is the measure of

disorder in a system Law of Disorder - processes

move towards maximum disorder (or chaos)

Chemical Reactions

Will a reaction occur? Entropy can be determined

by comparing entropy of reactants to entropy of products

Chemical Reactions

Will a reaction occur? Spontaneous reactions

Reactions that occur naturally and favor products (may be slow or fast reactions), energy will be released

Chemical Reactions

Will a reaction occur? Spontaneous reaction

examples: Heat released & entropy increased Increase in entropy is more than amount

of heat absorbed Amount of heat released is more than

decrease in entropy

Chemical Reactions

Will a reaction occur? Nonspontaneous reactions

Reactions that do not favor product formation, so they are not likely to occur (but they can happen)

Chemical Reactions

Will a reaction occur? Nonspontaneous reaction

examples: Heat absorbed & entropy decreased Increase in entropy is less than amount of

heat absorbed Amount of heat released is less than

decrease in entropy

Chemical Reactions

Will a reaction occur? Spontaneous reactions have

energy available to do work called free energy.

Chemical ReactionsWill a reaction occur? Spontaneous reactions have

energy available to do work called free energy.

The maximum free energy is called Gibbs free-energy change (G)

Chemical Reactions Gibbs free-energy change

(G) is calculated as:G = H – TST is in Kelvins

Chemical Reactions More Gibbs free-energy

change (G)… Spontaneous reactions G

is negative Nonspontaneous reactions

G is positive