Ch. 15: Energy and Chemical Change
Objectives Explain what energy is and distinguish between potential and kinetic energy. Relate chemical potential energy to the heat lost or gained in chemical reactions. Calculate the amount of heat absorbed or released by a substance as its temperature changes.
What is Energy? Energy is the ability to do work or produce heat. Energy exists in 2 basic forms: Potential - energy due to the composition or the position of an object Kinetic - energy of motion
Energy The Law of Conservation of Energy states that in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed. In energy conversions, the total amount of energy present remains constant.
Chemical Energy Chemical systems contain both kinetic energy and potential energy. The kinetic energy of a substance is directly related to the constant random motion of its atoms or molecules and is proportional to temperature. Potential energy of a substance depends upon its composition - the #’s & kinds of atoms, how they’re bonded and arranged.
Chemical potential energy The energy stored in a substance because of its composition is called chemical potential energy. Fuels, like octane, store large amounts of energy in the bonds between atoms. When burned, the potential energy of octane is converted to kinetic energy (the pistons in your car move). Octane (principle components of gasoline) The chemical potential energy of octane results from the arrangements of the carbon and hydrogen atoms and the strength of the bond that join them. When the gas burns, some of the stored energy is converted to work in moving the pistons, which ultimately move the wheels and propel the vehicle. However, much of the energy is released as heat. Which is represented by the symbol q.
Heat Represented by the symbol “q”, heat is the energy that flows from warmer to cooler objects. We recognize heat changes through temperature changes. Heat is measured in various ways. Calorie in the metric system. The SI unit of heat and energy is the joule (J)
Units of Heat In the metric system, a calorie (cal) is the amount of heat required to raise the temperature of one gram of pure water by one degree Celsius. Energy released from food is measured in Calories. 1 Calorie = 1000 cal = 1 kcal
In the SI system The SI unit of heat & energy is the joule (J). 1 J = 0.2390 cal 1 cal = 4.184 J 1000 J = 1 kJ 1000 cal = 1 kcal 1 kcal = 4.184 kJ
Converting Units of Energy 4.00 Cal x 1000 cal = 4,000 cal x 4.184 J = 16,700 J 1 Cal 1 cal
Practice Problems A granola bar contains 142 Cal. Convert this to calories. An exothermic reaction releases 86.5 kJ. How many kilocalories are released? If an endothermic process absorbs 256 J, how many kcal are absorbed? 142,000 calories
Specific Heat The specific heat of any substance is the amount of heat required to raise the temperature of 1 g of the substance by 1 0C. Recall that 1 calorie or 4.184 J is required to raise the temperature of one gram of pure water by 1 degree Celsius. The specific heat (C) of water, then, is 1cal or 4.184 J. g 0C g 0C
Specific Heat Because different substances have different compositions, each substance has its own specific heat. The specific heat of a substance is a measure of how efficiently that substance absorbs heat (without a temperature change).
Specific Heat This means, if the temperature of water (l) is to rise 1 0C, each gram must absorb 1 cal. Each gram of gold, on the other hand, needs 0.03 cal to cause a 1 0C increase in temperature. Water has a high specific heat; metals have a low specific heat. Pg. 520 text
Heat The heat absorbed or released by a substance during a temperature change (no phase changes!) depends on the specific heat of the substance the mass of the substance the amount by which the temperature changes
Q = mCT q or Q = heat absorbed or released C = specific heat of the substance m = the mass of the sample in grams T is the change in temperature in degrees Celsius T is the difference between the final temperature and the initial temperature or, Tf - Ti This means that if the temperature INCREASES, T is POSITIVE and Q is positive; if the temperature DECREASES, T is NEGATIVE and Q is negative.
Practice Problems How many joules of heat will 2.25 x 107 g of water absorb if its temperature increases from 10 0C to 32 0C? The temperature of a sample of iron with a mass of 10.0 g changed from 50.4 0C to 25.0 0C with the release of 114 J of heat. What is the specific heat of iron? What mass of granite will absorb 2.6 x 108 J of energy when its temperature rises 22 C0? The specific heat of granite is 0.0803 J/g 0C.