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

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 .

q = mc∆t This can be summarized with following equation: Change in temperature Quantity of heat Specific heat capacity Mass of the substance 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

∆H = q = mc∆t We can extend this to say the following: 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: Read pg. 298 – 305 2. Page 300 #1 – 5 3. Page 302 # 8 – 13