Chapter 4 Thermochemistry

Chapter 4

Thermochemistry

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A. ENERGY CHANGES IN CHEMICAL REACTIONS

Thermochemistry:

The study of changes in heat energy which take place during chemical

reactions

STEP TO CONSTRUCT ENERGY LEVEL DIAGRAMS

Step 1 Identify whether the reaction is exothermic or endothermic

Step 2 Draw and label the energy axis

Step 3 Draw the energy level for reactants and products

Step 4 Draw an arrow from reactants level to the products level

Step 5 Write the reactants and products based on the balanced chemical equations

Step 6 Label ∆H as positive or negative

Exothermic reaction Endothermic reaction

Definition A chemical reaction that gives out heat to the surroundings

A chemical reaction that absorbs heat from the surroundings

What happen?

During exothermic reaction, temperature of the surrounding increases.

This is because heat given out from the reaction is transferred to the surroundings.

During endothermic reaction, temperature of the surrounding decreases.

This is because the reactants absorb heat energy from the surroundings.

Heat of reaction, ∆H The change in the amount of heat in a chemical reaction.

∆H negative: heat is given out ∆H positive: heat is absorbed

Energy level diagram

The energy of the products is lower than the

total energy of the reactants

The energy of the products is higher than the

energy of the reactants

Reactants

Products

∆H = negative

Energy Energy

Reactants

Products

∆H = positive

Energy change during formation and breaking of bonds

Usually a chemical reaction involves bond breaking and bond formation.

Bond breaking : always requires energy

Bond formation : always releases energy

Type of Reaction Energy Change Sign of ∆H

Exothermic Energy absorb for bond breaking is less than energy released from bond formation

∆H negative

Endothermic Energy absorb for bond breaking is more than energy released from bond formation

∆H positive

Application of exothermic and endothermic reaction

Cold packs

Contain chemicals that react to absorb heat from surroundings.

Help to reduce high temperature

Help to reduced swelling

Hot packs

Contain chemicals that react to release heat.

Help to warm up something

Help to lessen the pain of aching muscles

Heat of Reaction

The change in the amount of heat in a chemical reaction.

Symbol: ∆H

Different types of reactions

Types of Reaction Heat of Reaction

Precipitation Heat of Precipitation

Displacement Heat of Displacement

Neutralization Heat of Neutralization

Combustion Heat of Combustion

Heat of reaction Definition

Heat of Precipitation The heat change when one mole of a precipitate is formed from their ions in aqueous solution

Heat of Displacement

The heat change when one mole of a metal is displaced from its salt solution by a more electropositive metal

Heat of Neutralization The heat change when one mole of water is formed from reaction between an acid and an alkali

Heat of Combustion The heat change when one mole of a substance is completely burnt in oxygen under standard conditions

The Ways to Calculate Heat of Reaction Step 1: Calculate how many moles of reactant

Number of moles = 𝑀𝑉

1000 use this formula only for:

Heat of precipitation Heat of neutralization

Heat of displacement

Number of moles = 𝑀𝑎𝑠𝑠

𝑅𝑀𝑀 𝑎𝑙𝑐𝑜 ℎ𝑜𝑙 use this formula for:

Heat of combustion only

Step 2: Write the ionic equation for the reaction

Step 3: Calculate the heat given out/heat release

Q = mcθ m → mass

c → 4.2 J g-1 0C-1 θ → change of temperature

Step 4: Calculate the heat of reaction ∆H = -ve (exothermic reaction)

∆H = +ve (endothermic reaction)

i) mass is refer to total volume of two

solution of reactants for (heat of

precipitation & heat of neutralization)

ii) mass is refer to volume of one solutions

for heat of displacement

iii) mass is refer to volume of water that use

for heat of combustion

∆H = 𝑚𝑐𝜃

𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒

From Step 3

From Step 1

B. HEAT OF PRECIPITATION

In an experiment, 50 cm3 of 1 mol dm-3 hydrochloric acid, HCl is added to 50 cm3 of 1 mol dm-3 silver nitrate, AgNO3 solution. The reacting mixture is stirred and the highest temperature obtained is recorded. The results of the experiment are shown below. Initial temperature of hydrochloric acid, HCl = 28.0°C Initial temperature of silver nitrate, AgNO3 solution = 29.0°C Highest temperature of the mixture = 35.5°C Calculate the heat of precipitation of silver chloride, AgCl. [Specific heat capacity of solution: 4.2 J g-1 °C-1. Density of solution: 1 g cm-3] 1. Calculate the number of moles of precipitate formed

2. Write the ionic equation

3. Calculate the heat given out

4. Calculate the heat of precipitation

C. HEAT OF DISPLACEMENT

In an experiment, 1 g of zinc powder is added to 50 cm3 of 0.2 mol dm-3 copper(II) sulphate, CuSO4 solution. The solution is stirred continuously and the highest temperature reached is recorded. The results are as follows: Initial temperature of copper(II) sulphate, CuSO4 solution = 28.0°C Highest temperature of the mixture = 35.5°C What is the heat of displacement of copper in the reaction? Draw the energy level diagram for the reaction. [Specific heat capacity of solution: 4.2 J g-1 °C-1. Density of solution: 1 g cm-3. Relative atomic mass: Zn=65] 1. Calculate the number of moles of copper formed



4. Calculate the heat of displacement

5. Energy level diagram

D. HEAT OF NEUTRALIZATION

In an experiment to determine the heat of neutralization, 100 cm3 of 2.0 mol dm-3 hydrochloric acid, HCl at 29.5 °C is added to 100 cm3 of 2.0 mol dm-3 sodium hydroxide, NaOH solution which is also at 29.5 °C in a plastic cup. The mixture is then stirred and the highest temperature reached is 41.5 °C. Calculate the heat neutralization. Draw the energy level diagram for the reaction. [Specific heat capacity of solution: 4.2 J g-1 °C-1. Density of solution: 1 g cm-3] 1. Calculate the number of moles



4. Calculate the heat of neutralization

5. Energy level diagram

E. HEAT OF COMBUSTION

An experiment is carried out to determine the heat of combustion of methanol. CH3OH. The results of the experiment are as follows: Volume of water used = 100 cm3 Initial temperature of water = 29.0°C Highest temperature of water reached = 51.0°C Mass of spirit lamp and methanol, CH3OH before combustion = 156.55 g Mass of spirit lamp and methanol, CH3OH after combustion = 156.05 g Based on the results, calculate the heat of combustion for methanol, CH3OH and hence construct the energy level diagram for the combustion of methanol, CH3OH. [Specific heat capacity of solution: 4.2 J g-1 °C-1. Density of water: 1 g cm-3. Relative atomic mass: H=1; C=12, O=16] 1. Calculate the number of moles


3. Calculate the heat of combustion

Energy level diagram

Fuel Value FUEL VALUE:

The amount of heat energy given out when one gram of the fuel is completely burnt in excess of oxygen

Unit for fuel value: kJ g-1

The existence of various energy sources

Source Energy Technology used to harness it Advantage Disadvantage

SUN Solar energy Solar cell Clean

Renewable

Inexhaustible

Intermittent

Unreliable

Storage problem

FOSSIL FUELS (petroleum, natural gas)

Heat energy Generator or car engines Convenient

High fuel value

Non-renewable

Depleting

Cause environmental pollution

WATER Hydroelectric energy Hydroelectric power station

Clean

Renewable

Convenient

Economical

Limited in its location

High cost of construction

Destruction of environment

BIOMASS (plant)

Biomass energy Generator or car engines Renewable Small scale

Large scale of land needed

RADIOACTIVE SUBSTANCES (uranium, plutonium)

Nuclear energy Nuclear fission and nuclear fusion

Enormous supply

Non-renewable

Nuclear accident

Produces radioactive waste

Chapter 4 Thermochemistry

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Transcript of Chapter 4 Thermochemistry