Topic 3

* Understandings 1. Temperature and absolute temperature 2. Internal energy 3. Specific heat capacity 4. Phase change 5. Specific latent heat

* Applications and Skills 1. Three states of matter 2. Solids fixed shape & volume made up of particles vibrating wrt each other 3. Liquids no fixed shape, fixed volume made up of particles vibrating and moving in straight lines before colliding with other particles 4. Gases(vapours) no fixed volume or shape particles moving in straight lines before colliding –ideal gases 5. Thermal energy misnamed heat refers to energy transferred from a higher temperature object by conduction, convection, and thermal radiation

* Equations * T (K) = θ (°C) * Specific Heat Capacity Q = mcΔT * Specific Latent Heat Q = mL

* Introduction * Heat originally thought of as a substance(caloric) which flowed between hot and cold objects. * Some thought that a substance (frigoric) flowed from cold to hot objects. * Joule’s paddle wheel experiment showed that mechanical work done on a substance could increase its temperature. * Word calorie remains in our vocabulary

* Intuitively know that relative temperature of two objects determines the direction of energy flow. * Energy passes from hotter object to colder one until they reach the sale temperature i.e. thermal equilibrium. The energy is heat * Temperature is a scalar measured in °C (Celsius)or K (kelvin) with a thermometer

* Types of thermometers: * Liquid-in-glass * Gas expansion * Change in resistance of a wire * Change in emf at junction of two different metals * A thermometric property is one which varies with temperature e.g. expansion of a liquid along glass capilliary tube * Liquid-in glass thermometers not really accurate as the capillary may not be uniform, cross sectional area may vary with temperature, can’t really ensure that liquid temperature is identical with object measured, glass is an insulator so time needed to conduct energy through to the liquid

* Liquid-in-glass thermometers based on sale between two fixed points-ice point and steam point. For Celsius thermometers, these are 0°C and 100°C respectively. * Assumption is that liquid’s length varies linearly with temperature between these points. May not be the case. * Digital thermometers and temperature sensors use thermistors electronic devices whose electrical resistance drops with temperature. * Called “ntc” or negative temperature coefficient of resistance thermistors. * Thermistors respond quickly to temperature changes and are more durable than liquid-in-glass thermometers

* Absolute temperature is defined as zero kelvin at absolute zero where all matter has minimum kinetic energy and K (at specific pressure) the triple point of water where water can exist as liquid water, ice, and water vapour

* Source google images

* T (K) = θ (°C) * Temperature differences in the absolute temperature scale correspond to those in the Celsius scale since a difference of 1°C corresponds to 1 K * Examples: a) Convert -26°C to K * b) Convert 100 K to °C

* Substances are comprised of particles in constant random motion. As energy is transferred to the substance particle separation could increase and they could move faster. When particles move farther apart or closed together, their potential energy increases. When they move faster, their kinetic energy increases. The internal energy of a substance is the total of the potential energy and the random kinetic energy of all the particles in the substance. In a solid, these two are approximately equal but for gases, almost all is kinetic since intermolecular forces are so small.

* Different objects receiving the same amount of energy are not likely to have the same temperature change since they have different heat capacities. The specific heat capacity ( c ) of a substance is the energy transferred to 1 kg of the substance causing its temperature to increase by 1 K * c = Q/(mΔT) where Q is energy in Joules, m is mass in kg and ΔT is temp change in K * The units for c are J kg -1 K -1

* If a block of ice was taken from a freezer and placed on a table top, the ice would warm up, melt, and the melted water would warm up. When the ice reaches 0° C, it melts and its temperature remains constant until it has all melted. Then the water warms up. While the ice is melting, the energy added becomes potential energy associated with the forces between neighboring liquid water molecules. The energy required for the phase change is called latent heat

* Specific latent heat of fusion (melting) is the energy required to change the phase of 1 kg of substance from a solid to a liquid without a temperature change. Similarly, * The specific latent heat of vaporization (boiling) is the energy required to change the phase of 1 kg of liquid into a gas without a temperature change. * L = Q/m where Q is energy in J, m is mass in kg. The units for L are J kg -1

* From Chemistry 11 Mr Standring wikispace * See text p. 98

* As energy is transferred to solid, the random kinetic energy of the molecules increases and the intermolecular potential energy increases as the molecules move farther apart. When groups of molecules move far enough away from the influence of their neighbours, the intermolecular bonds are broken and additional energy goes into increasing the potential energy of the molecules. When the groups are sufficiently freed, the solid has melted and additional energy goes into increasing the mean speed and therefore the temperature.

* The average potential energy does not change as the molecules are breaking away and joining together at the same rate. As the liquid reaches boiling point molecules start moving away from each other within their groups. Individual molecules break away and potential energy increases. Here again, the temperature remains constant as the energy supplied goes into the phase change- vapourization. Once the liquid has been completely vapourized, the temperature begins to rise again.

* Worked example * A 10. W heater is activated for 200. s and 4.00 kg of a solid is melted. Determine the specific latent heat of fusion for the solid. * Q = Pt since P = E/t = 10. x 200. = 2000 J * L = Q/m = 2000/4.00 = 500 J kg -1