Phasediagramsthermo09

Definitions • Melting Point – the temperature at which a substance changes from solid to a liquid. • Freezing Point – the temperature at which a substance changes from a liquid to a solid. • Boiling Point – the temperature at which a substance changes from liquid to a gas. • Sublimation – direct change from solid to gas

Heating Curve for Water

Sample Question • When water decreases to a temperature of 2˚C, the molecules begin to realign themselves. Which phase change is the water beginning to undergo? • Freezing (liquid to solid)

• When water increases to a temperature of 98˚C, the molecules begin to move further apart from each other and move more rapidly. Which phase change is the water beginning to undergo? • Boiling (liquid to gas)

A Different Type of Phase Diagram Ques tions • At what pressure will the substance sublimate at 3 ˚C? • At what pressure will the substance boil at 12 ˚C?

• Thermochemistry- heat changes that occur during chemical reactions. • Heat of Reaction= energy released or absorbed in a chemical rxn. • Energy – capacity for doing work or supplying heat. • Heat – (Q) is energy that transfers from one object to another because of temperature difference between them.

• Exothermic – a process that releases heat to its surroundings. The energy released comes from breaking chemical bonds. • Reactants are higher in energy than products.

• Endothermic – a process that absorbs heat from the surroundings. • Products are higher in energy than reactants.

Thermochemical Equations • Thermochemical equations – include the amount of heat absorbed in a reaction. • Heat can be treated like any other reactant or product in a balanced equation. • Joules are the metric unit for heat & all other energy. • 1J = 1 Nm or 1 kg.m2/s2 • EX. C + O2  CO2 + 393.5 kJ

Measuring & Expressing Heat Changes

• Enthalpy – ( H) the heat content of a system at constant temperature & pressure. • Enthalpy changes describe the energy absorbed or released during a reaction. • Endothermic – (+) H • Exothermic – (-) H

Specific Heat • Heat Capacity – the amount of heat needed to change an object’s temperature by 1 degree Celsius or Kelvin. • Mass & composition affect heat capacity.

• Specific Heat Capacity or Specific Heat – the amount of energy needed to raise the temperature of 1 g of something by a degree Celsius/Kelvin. • Water has a high specific heat, metals have low sp. heat.

• Be careful of units. (See ex p. 513) • Specific heat is in J/goC or other energy units such as cal/goC

Enthalpy Equation: Q = mCp T m = Mass Cp = specific heat Q = Heat T = Tfinal - Tinitial

Example Problem • How much heat energy is required to raise the temperature of 10 grams of ice from –100C to 00C? • Q = mCp T • Cp of ice= 2.1 J/g0C • m = 10 g • T = 00C – (–100C) • Q=? • Heat = (2.1 J/g0C)(10g)(100C) • Heat = 210 J

Your Turn! • A 200-gram sample of iron, initially at 00C is heated to 370C. Determine the energy attained by the iron sample if the specific heat capacity of iron is 0.11 cal/g0C. • Cp = 0.11 cal/g0C • m = 200 g • T = 370C – 00C • Q= ? • Heat = (0.11 cal/g0C)(200g)(370C) • Heat = 814 cal

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