Chapter 4

Thermochemistry

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A. ENERGY CHANGES IN CHEMICAL REACTIONS Thermochemistry: …………………………………………………………………………………………………….

……………………………………………………………………………………………………

STEP TO CONSTRUCT ENERGY LEVEL DIAGRAMS

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Exothermic reaction Endothermic reaction

Definition

What happen?

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

Energy level diagram

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

Heat of Displacement

Heat of Neutralization

Heat of Combustion

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

2. Write the ionic equation

3. Calculate the heat given out

4. Calculate the heat of precipitation

5. Energy level diagram

D. HEAT OF NEUTRALIZATION

E. HEAT OF COMBUSTION

Fuel Value FUEL VALUE: …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… Unit for fuel value: ………………………………..

The existence of various energy sources

Source Energy Technology used to harness it Advantage Disadvantage

SUN

FOSSIL FUELS (petroleum, natural gas)

WATER

BIOMASS (plant)

RADIOACTIVE SUBSTANCES (uranium, plutonium)