HEAT AND ENERGYChemistry: Endothermic and Exothermic Reactions

Standards:7b. Students know chemical processes can either

release (exothermic) or absorb (endothermic) thermal energy.

7c. Students know energy is released when a material condenses or freezes (exothermic) and is

absorbed when a material evaporates or melts (endothermic).

What is Energy?

Energy

Kinetic energy

(EK)

Potential energy

(EP)

Energy due to motion

Energy due to postition (stored energy)

---> The ability to do work

Total Energy = Kinetic Energy + Potential Energy

E = EK + EP

Kinetic energy & potential energy are interchangeable

Ball thrown upwards slows &

loses kinetic energy but gains potential energy

The reverse happens as it falls back to the ground

Law of Conservation of Energy:

the total energy of the universe is constant and can neither be created nor destroyed; it can

only be transformed.

Systems & Surroundings

the world is divided into a system and its surroundingsA system is the part of the world we want to study (e.g. a

reaction mixture in a flask)The surroundings consist of everything else outside the

system

SYSTEM

CLOSED

OPEN ISOLATED

OPEN SYSTEM: can exchange both matter and energy with the surroundings (e.g. open reaction

flask, rocket engine)

CLOSED SYSTEM: can exchange only energy with the surroundings (matter remains

fixed) e.g. a sealed reaction flask

ISOLATED SYSTEM: can exchange neither matter nor

energy with its surroundings (e.g. a thermos flask)

What is heat?

HEAT is the energy that transfers from one object to another when the two things are at different temperatures and in some kind of contacte.g. kettle heats on a stove cup of tea cools down (loses energy as heat)

Heat is energy in transit

UNITS OF ENERGY

S.I. unit of energy is the joules (J)Heat and work ( energy in transit) also measured in

joules

1 kJ (kilojoule) = 103 J

Calorie (cal): 1 cal is the energy needed to raise the temperature of 1g of water by 1oC

1 cal = 4.184 J

ENTHALPY (H)

Heat content of a substance

H = Hproducts - Hreactants

H difference of heat content of products and reactants

First Law of Thermodynamics: *the internal energy of an isolated system is

constant*energy is always conserved

Signs (+/-) will tell you if energy is entering or leaving a system

+ indicates energy entering a system- indicates energy leaving a system

EXOTHERMIC & ENDOTHERMIC REACTIONS

Exothermic process: a change (e.g. a chemical reaction) that releases heat.

Burning fossil fuels is an exothermic

reaction

Endothermic process: a change (a chemical reaction) that requires (or absorbs) heat.

Photosynthesis is an endothermic reaction

(requires energy input from sun)

Forming Na+ and Cl- ions from NaCl is

an endothermic

process

Measuring Heat

reaction

reaction

Exothermic reaction, heat given off & temperature of

water rises

Endothermic reaction, heat taken in &

temperature of water drops

All chemical reactions either release or absorb

heat

Exothermic reactions: Reactants products + heat as energy

Endothermic reactions: Reactants + heat as energy products

e.g. burning fossil fuels

e.g. boiling water

Exothermic energy level diagram

Exothermic energy level diagram

∆H (‘delta H’) is the symbol for the ‘change in energy’.

In an exothermic reaction the products have less energy than the reactants.

∆H is negative for an exothermic reaction.

Endothermic energy level diagram

Endothermic energy level diagram

In an endothermic reaction the products have more energy than the reactants.

∆H is positive for an endothermic reaction.

Concept Check

Look at the chemical reaction below.

6 CO2 + 6 H2O + energy --> C6H12O6 + 6 O2

Which statement BEST describes the chemical reaction?

A. The reaction absorbs energy

B. The reaction releases energy

C. The reaction neither releases nor absorbs energy

D. The reactants have energy but the products are energy-free

Concept Check

Look at the chemical reaction below.

6 CO2 + 6 H2O + energy --> C6H12O6 + 6 O2

Which statement BEST describes the chemical reaction?

A. The reaction absorbs energy

B. The reaction releases energy

C. The reaction neither releases nor absorbs energy

D. The reactants have energy but the products are energy-free

EvaporationEnergy has to be supplied to a liquid to enable it to

overcome forces that hold molecules together• endothermic process (positive)

MeltingEnergy is supplied to a solid to enable it to vibrate more vigorously until molecules can move past each other and

flow as a liquid• endothermic process ( positive)

FreezingLiquid releases energy and allows molecules to settle

into a lower energy state and form a solid• exothermic process ( negative)

(we remove heat from water when making ice in freezer)

Bond Strengths

• bond breaking requires energy • bond making releases energy

Lattice (ex. NaCl)

energy is required to break up solids

Concept Check

Which term BEST describes the energy process involved in forming a chemical bond?

A. Endothermic

B. Exothermic

C. Kinetic

D. Potential

Concept Check

Which term BEST describes the energy process involved in forming a chemical bond?

A. Endothermic

B. Exothermic

C. Kinetic

D. Potential

Concept Check

Which term BEST describes the energy process involved in breaking a chemical bond?

A. Endothermic

B. Exothermic

C. Kinetic

D. Potential

Concept Check

Which term BEST describes the energy process involved in breaking a chemical bond?

A. Endothermic

B. Exothermic

C. Kinetic

D. Potential

Concept Check

Why must heat be absorbed in order for water to boil?

A. To make more molecules

B. To form the bonds for the gas

C. To break the bonds of the liquid

D. To reduce the motion of molecules

Concept Check

Why must heat be absorbed in order for water to boil?

A. To make more molecules

B. To form the bonds for the gas

C. To break the bonds of the liquid

D. To reduce the motion of molecules

• WHY DO THINGS

DISSOLVE?

Substances dissolve because energy and matter tend to disperse (spread out in

disorder)

• If it dissolves and solution heats up : exothermic• If it dissolves and solution cools down:

endothermic

Second Law of Thermodynamics: *the disorder (or entropy) of a system tends

to increase*any physical or chemical change must result in an increase in the entropy of the universe

ENTROPY (S)

• Entropy is a measure of disorder• Low entropy (S) = low disorder

• High entropy (S) = greater disorder

Total entropy change

entropy change of system

entropy change of

surroundings+=

Dissolving

disorder of solution

disorder of surroundings

• must be an overall increase in disorder for dissolving to occur

1. If we freeze water, disorder of the water molecules decreases , entropy decreases

2. If we boil water, disorder of the water molecules increases , entropy increases (vapor

is a highly disordered state)