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)
