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THERMOCHEMISTRY
ENERGY AND HEAT
➤ Thermochemistry is concerned with the heat changes that occur during chemical reactions
➤ Energy is the capacity for doing work or supplying heat.
➤ ︎ Work is a force times a distance.
➤ ︎ Energy is not visible – it is odorless and tasteless.
➤ Energy can only be detected because of its results.
ENERGY AND HEAT
➤ Energy stored within the chemical structure units of chemical substances is called chemical potential energy.
➤ ︎ Gasoline is a form of stored potential energy, so is sugar.
➤ ︎ Amount of energy stored in a chemical depends on the elements and their arrangements.
ENERGY AND HEAT
➤ Heat, represented by q, is energy that transfers from one object to another due to a temperature difference between them.
➤ ︎ As with energy, heat itself cannot be detected, only the affects of adding or subtracting heat can be detected.
➤ ︎ Adding heat shows a rise in temperature of an object.
➤ ︎ Heat flows from a warmer object to a cooler object.
ENDOTHERMIC AND EXOTHERMIC REACTIONS
➤ Almost all chemical reactions and changes in state involve the release or absorption of heat.
➤ ︎ When looking at the process, we define the system as those objects you are focusing on.
➤ ︎ The surroundings are everything else.
➤ Thermochemistry examines the flow of heat from a system to the surroundings or from the surroundings to the system
THE LAW CONSERVATION OF ENERGY
➤ In any chemical or physical process that energy is neither created nor destroyed.
➤ All energy in a process can be accounted for as work, stored energy or heat.
HEAT AND CHEMICAL REACTIONS
➤ Exothermic Reaction
➤ Reaction releases heat into its surroundings
➤ q is defined as negative.
➤ Endothermic Reaction
➤ Reaction absorbs heat from its surroundings
➤ q is defined as positive
HEAT AND CHEMICAL REACTIONS
➤ A person who ran a race is exothermic
➤ When sitting around a camp fire
➤ The fire is exothermic
➤ The people are endothermic
HEAT CAPACITY AND SPECIFIC HEAT
➤ As food is broken down in your body, heat is generated.
➤ If you ingest 10g of sugar, it is chemically processed into CO2 and H2O and heat is produced.
➤ This is the same amount of heat that would be produced if 10g of sugar were burned in a fire.
HEAT CAPACITY AND SPECIFIC HEAT
➤ calorie - the amount heat needed to raise one gram of pure water by one degree Centigrade & is abbreviated as c.
➤ c refers to energy due to chemical/physical reactions
➤ CO2 & H2O are released in this reaction
➤ Energy in food is represented by the capital letter C
➤ ︎ One Calorie = 1000 calories = 1 kilocalorie
➤ 4.1 Calories = 4.1 kilocalories =4,100 calories in 1 g of sugar
HEAT CAPACITY AND SPECIFIC HEAT
➤ The calorie is also related to the joule – which is the SI unit of heat and energy.
➤ A joule is just less than 1⁄4 of a calorie. ︎ 1J = 0.2390 cal 4.184J = 1cal
HEAT CAPACITY AND SPECIFIC HEAT
➤ The ability of any material to retain heat energy is called that material’s heat capacity.
➤ Specific Heat -
➤ measure of heat capacity,
➤ equal to the quantity of heat needed to raise the temperature of one gram of a substance by one degree Celsius
➤ Specific Heat is represented by the symbols, Cp, & C. The SI units for specific heat are given in J/gC. Specific heat values are also commonly given in cal/gC, where 1 calorie=4.184 Joules.
HEAT CAPACITY AND SPECIFIC HEAT
➤ Compare the heat capacities of concrete and wood. Because the specific heat of wood is twice as great as that of concrete, it takes about twice as much heat to raise the temperature of wood than concrete. This can be verified by comparing the feel of walking on concrete versus walking on wood on a hot, sunny day with bare feet. The concrete feels hotter. The sun gives off energy which is absorbed
➤ Because the wood has a greater specific heat value, it is able to absorb more heat before its temperature rises, and therefore it does not feel as hot as the concrete feels to the bare feet.
HEAT CAPACITY AND SPECIFIC HEAT
➤ General Rule #1 - The greater the specific heat value, the less the temperature will rise when a given heat energy is absorbed.
➤ specific heat also describes the ability of a substance to deliver heat to a cooler object.
➤ General Rule #2 - As the specific heat value decreases, the ability to deliver heat to a cooler object increases.
➤ ex: imagine holding two hot pieces of metal:
➤ X (Cp=2 J/gC)
➤ Y (Cp=3 J/gC).
➤ If a hot piece of metal X was held in one hand and a hot piece of metal Y in the other hand, the hand holding the metal X would get hotter. Because metal X has a specific heat less than metal Y. X transfers heat to a cooler object (your hand) more readily.
➤ Why do different materials possess different specific heat values?
➤ Atoms that have different masses. The mass of each copper atom is larger than the mass of each aluminum atom, for example. Therefore a give mass (such as 58 g of copper) has fewer atoms than the same mass of aluminum.
➤ When heat is added to 58 g of copper, fewer atoms need to be put in motion (remember temperature is related to kinetic energy).
➤ As a result, the specific heat value for copper is lower than the specific heat of aluminum.
HEAT CAPACITY AND SPECIFIC HEAT
➤ General Rule #3 - The larger the metal atom, the lower is specific heat value.
➤ How is the specific heat of a material determined?
➤ Heat transfer or heat flow always occurs in one direction - from a region of higher temperature to a region of lower temperature - until some final equilibrium temperature is reached.
➤ Transfer of energy can be detected by measuring temperature change.
HEAT CAPACITY AND SPECIFIC HEAT
➤ Specific heat can be calculated using the equation:
HEAT CAPACITY AND SPECIFIC HEAT
➤ ∆T, calculated by taking the final temperature minus the initial temperature
➤ ∆T = final temperature - initial temperature = Tf - Ti
➤ The amount of heat (q) delivered by the metal (qmetal) is equal to the mass of the metal (mmetal) multiplied by the specific heat of the metal (Cpmetal) multiplied by the temperature change of the metal (∆Tm e t a l). This relationship is given by Equation 2.
➤ qmetal = (mmetal)(Cpmetal)(∆Tmetal)
➤ The amount of heat absorbed by the water (qwater) is equal to the mass of the water (mwater) multiplied by the specific heat of the water (Cpwater) multiplied by the temperature change of the water (∆Tw a t e r).
➤ qwater = (mwater)(Cpwater)(∆Twater)
HEAT CAPACITY AND SPECIFIC HEAT
➤ The symbol q is a signal showing the direction of heat transfer.
➤ When heat is transferred out of a material, the sign of q is negative.
➤ Conversely, when heat is absorbed by a material, q is positive.
➤ According to the Law of Conservation of Energy, heat delivered by the heated metal (qmetal) is equal to the heat absorbed by the water (qwater) and its surroundings.
➤ qmetal = - qwater ➤ (mmetal)(Cpmetal)(∆Tmetal) = - (m water)(Cp water)(∆T water)
HEAT CAPACITY AND SPECIFIC HEAT
➤ Solve!
➤ If a 58 g sample of metal at 100 C is placed into a calorimeter containing 60 g of water at 18 C, the temperature of the water increases to 22 C.
➤ a. Calculate the amount of heat absorbed by the water in Joules.
➤ b. Determine the identity of the metal by calculating its specific heat.
http://www.etec.bourbon.kyschools.us/wp/escience/wp-content/uploads/2013/10/calorimetry_lab.pdf
