SCH 4U – Chapter 5.1 Changes in Matter and Energy
Heat and Energy Changes
• Thermal energy is a form of kinetic energy that results from
the motion of molecules.
• This increase in kinetic energy can be experienced as a
physical change such as the melting of a substance.
• When studying energy transfers we make a distinction
between the substances undergoing a change - the chemical
system and the system’s environment called the
surroundings
2 C2H2 + 5 O2 → 4 CO2 + 2H2O + energy
Chemical system
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The surroundings in this reaction include anything that can
absorb thermal energy; the air, water etc.
When the reaction occurs, heat (q) is transferred between
substances.
Heat is energy being transferred between substances.
When heat is transferred between a chemical system and
its surroundings there is a corresponding temperature
change (∆t).
Chemical systems that give energy to their surroundings are
exothermic. They are characterized by an increase in
molecular motion and temperature.
Chemical systems that absorb energy from their
surroundings are endothermic. They are characterized by a
decrease in molecular motion and temperature.
• Chemical systems may be classified as either open or closed.
• Open systems allow both energy and matter to flow into or
out of a system.
• Most explosive reactions can be considered open systems
because it is difficult to contain the matter and energy
produced.
• Isolated systems are ideal systems in which the matter and
energy can’t move out of a chemical system.
• Most calculations of energy changes occur in closed systems
where energy can move in and out of the surroundings but
matter cannot.
• An example of such a system is a calorimeter.
Calorimeter
Open, closed and isolated systems.
Calorimetry
• Is the technological process of measuring energy changes in a
chemical system.
• When a fuel like methane burns heat is transferred into the
surroundings (like water in a beaker).
• If more heat is transferred, the observed temperature rise in
the water is greater.
• Given the same amount of heat, a smaller the mass of water
will undergo a larger temperature increase than a larger
mass.
• Different substances vary in their ability to absorb heat.
• The quantity of heat transferred by a chemical system
depends on: mass (m), temperature change (∆t) and type of
substance .
• This can be summarized with following equation:
q = mc∆t
Quantity of heat
Mass of the substance
Change in temperature
Specific heat capacity
Specific heat capacity - is the amount of energy required to increase
the temperature of 1 gram of a substance by 1 degree celcius. For water
c = 4.18 J / (g•˚C)
Specific heat capacities vary from substance to substance and even for
different states of the same substance. See table 1 page 301
When 600 ml of water in an electric kettle is heated from 20˚C to
85˚C to make a cup of tea how much heat flows into the
water?
Aqueous ethylene glycol is commonly used in car radiators as an
antifreeze and coolant. A 50% ethylene glycol solution has a
specific heat capacity of 3.5 J /(g•˚C). What temperature
change would be observed in a 4 kg solution of ethylene
glycol if it absorbs 250 kJ of heat?
Heat Transfer and Enthalpy Change:
• Chemical systems have many different forms of energy both
kinetic and potential they include:
1) moving electrons within atoms
2) vibration of atoms in chemical bonds
3) rotation and translation of molecules made up of these
atoms.
4) nuclear potential forces of protons and neutrons in the
nucleus.
5) the electronic potential energy of atoms connected by
chemical bonds
• It is not yet possible to measure sum of all of these energies.
• Instead chemists study the enthalpy change or the energy
absorbed or released to the surroundings when a system
changes from reactants to products.
∆H = ±│q surroundings │
• in the reaction:
Zn(S) + 2HCl(aq) → H2(g) + ZnCl2(g)
• Some chemical potential energy is used to increase the kinetic
energy of the products but this is transferred to the
surroundings.
• The enthalpy change equals the heat absorbed by the
surroundings
We can extend this to say the following:
∆H = q = mc∆t
• In order to control variables and allow comparisons energy
changes in chemical systems are measured at standard
conditions of temperature and temperature.
• Enthalpy changes can be either physical, chemical or nuclear
changes (See Table 2 page 304)
Changes in Kinetic and Potential Energy
Homework:
1. Read pg. 298 – 305
2. Page 300 #1 – 5
3. Page 302 # 8 – 13
4. Page 305 # 1 - 6