1. Unit 3: Energy Changes & Rates of reaction

2. Basic Definitions
Thermochemistry: the study of energy changes that accompany physical or chemical changes of matter. Thermal Energy: the energy available from a substance as a result of the motion of its molecules; the sum of the kinetic energies of all of the particles of a system (can’t measure this but can measure heat released or absorbed).

3. Basic Definitions
System: any sample under observation (could be the coffee in a coffee cup)
Chemical System: the set of reactants and products that are being studied (usually represented by a chemical equation)
Surroundings: all matter around the system being studied that is capable of absorbing or releasing energy.

4. Types of Systems OPEN SYSTEM: A chemical system in which
energy and matter can move
between the system and its
surroundings (e.gg a reaction
taking place in a beaker or a
test tube).

5. Types of Systems CLOSED SYSTEM:
One in which matter can not move in or out but energy may.
(e.g. once the lid is on and when heated, the steam cannot leave (matter) but the heat is passed through the pot)

6. Types of Systems cont’d
ISOLATED SYSTEM:
A system in which neither mass or energy can move in or out; this would be the ideal way to measure energy changes in a chemical system.
This is difficult to do, so to measure thermal energy changes, we use calorimeters to be isolated systems even though they are really closed systems
A thermos is almost an isolated system but it does lose or gain heat over time….

7. Measuring Energy Changes
Calorimetry is the process through which energy changes in a chemical system are measured.
The fancy calorimeter
The AHS science lab version

8. Measuring energy changes
Hard to measure thermal energy changes but you can measure: changes in T, Volume of or mass contained in the system and pressure of system on surroundings and vice-versa Temperature – the measure of the average kinetic energy of all the particles of a sample of matter. So to indirectly measure thermal energy, we measure heat (Q) – how much is leaving or entering a system

9. Factors affecting energy changes
The factors that affect energy change in a chemical system are mass (m in g), temperature change (∆T in C) and the specific heat capacity of a substance (c).
Specific heat capacity
The amount of energy required to raise the T of 1 gram of a substance by 1oC. (units = J/g/C)

10. Factors affecting energy changes
Heat change in a chemical system (enthalpy) can be calculated using the formula:
Q=mc ∆ T
∆ T – the change in T from the beginning to the end (Tfinal – Tinitial)
The type of substance is taken care of by the c value which is the specific heat

11. Endothermic vs Exothermic
Energy changes are classified into endothermic or exothermic based on how energy flows between the chemical system and its surroundings.
If energy is lost to the surroundings, the change is EXOTHERMIC.
In an exothermic change,
the value of Q will be
negative and the temperature
of the system will decrease.

12. Endothermic vs. exothermic
If energy is absorbed from its surroundings, the change is ENDOTHERMIC.
In an endothermic change, the value of Q will be positive and the temperature of the system will increase.