Heat and Energy of Ractions Chapter 10 Energy World of Chemistry Zumdahl Last revision Fall 2009

Energy is... …the ability to cause a change. o Change in position or temperature …conserved according to the first law of thermodynamics Heat is energy transferred between objects because of temperature difference. The unit is the Joule (J). It is the amount of energy needed to raise the temperature of one gram of water 1 degree Celsius

Heat is not the same as Temperature! Temperature is…  Measure of the average kinetic energy of the particles in a substance. Something is hot because it transfers heat to your hand, it is cold because it pulls heat out of your hand. Heat is transferred due to a temperature difference.

Temperature Scales Three different Scales Celsius Fahrenheit Kelvin Water Freezes 0 0 C Water Boils C 32 0 F F 0 0 F ? 273 K 373 K 0 K ?

Cold Cold = lack of heat Cold things have less kinetic energy, but they still have some energy, they don’t stop moving There is no such thing as cold energy

Absolute Zero is the temperature at which all motion of particles stops. Scientists have yet to reach it.

Temperature is constant during Phase Changes The amount of energy required to melt or freeze something is called the heat of fusion The amount of energy required to vaporize or condense something is called the heat of vaporization

Depositing SublimatingFreezing Boiling Condensing Melting 1. Changes in the states of Matter Heat of fusion Heat of vaporization

State of Matter- Matter can be solid, liquid, gas or plasma depending on how much kinetic energy (or motion) they have. Solid: Particles vibrate in place. It has a definite volume and a definite shape. Liquid: Particles roll around each other. It has a definite volume but no definite shape. Gas: Particles move around colliding with one another. It has no definite volume or shape. Plasma: Particles are so hot and excited that the electrons leave the nucleus and matter is ionized or charged

What’s happening at the molecular level during a phase change? What happens when I add heat to a solid? es-of-matter

Energy in Reactions Exothermic Reactions – A reaction in which energy is released. Feels hot. Endothermic Reactions – A reaction which energy must be provided for it to continue. Absorbs energy and feels cold. Dissolution Reactions – When ionic compounds dissolve in water. Will this be endothermic or exothermic? (remember, breaking bonds requires energy) Endothermic

Energy, potential energy, and Enthalpy. Energy (q) Energy (q) is the ability to do work or produce heat. Kinetic energy is the motion of the molecules and can be measured by taking it’s temperature. Potential energy Potential energy is the energy stored in a substance because of its composition (like the types of bonds it formed). Enthalpy (  H) is the heat released or absorbed in a chemical reaction. Activation Energy is the amount of energy needed to get a reaction started.

Potential energy Heat Exothermic has heat as a product (-) Energy of Reactants Activation Energy Energy of Products The products have less energy than the reactants. It has to go somewhere Released to the surroundings

Potential energy Heat Endothermic has heat as a reactant (+) Heat must be present for the reaction to happen Heat + H 2 O 2 → H 2 O + O 2 H2O2H2O2 H 2 O + O 2 Absorbed from the surroundings (+) Energy of Reactants Energy of Products

Heat Capacity and Specific Heat Heat Capacity (C) Heat Capacity (C) is the amount of heat energy a substance can absorb before the substance will increase its temperature by one degree Celsius. Specific heat (Cs) Specific heat (Cs) of a substance is the amount of heat required to raise the temperature of one gram of that substance by one degree Celsius.

Calculating the Energy of a Reaction Any energy lost by the reaction must be gained by the surroundings. Energy must always be conserved. The sign on Q indicates if energy is leaving or entering the system.

Try it yourself: What is the specific heat of a pure metal that has a mass of 2.8g and requires 10.1J of energy to raise the temperature from 21 o C to 36 o C.

Try this (Remember heat is conserved) : -Q lost = Q gained What is the specific heat of the rock below if the rock begins at 100 o C and is placed in the beaker of water at 20.0 o C with a specific heat capacity of 4.18J/g o C. They both end up at 23.5 o C.

Calculate the heat of a reaction The heat of a reaction (enthalpy ∆H) is measured per mole. (sign of enthalpy indicates the direction heat is flowing) You must used the coefficients of the balanced equation to divide the enthalpy of the equation and make it per 1 mole. If a mass amount is given, you must determine the moles and multiply it by the enthalpy.

20 Enthalpy calculations How much heat is released when 4.50 g of oxygen is burned in methane gas. 1. Write a balance equation. CH 4 + 2O 2 CO H 2 O 2. Look up the enthalpy for that equation  H = -890 kJ 3. Now convert your substance to moles and multiply it by the enthalpy. 4.50g x 1 mol/32.0 g = 4. Divide the enthalpy by the coefficient of what you are concerned with mol x-890kJ = -445 kJ  H= -445kJ/2 = kJ

How much heat will be transferred when 5.81g of graphite react with excess hydrogen? 6C(graphite) + 3H 2 → C 6 H 6 ∆H= 49.03kJ 5.81g C 1 mol C 1 12 g C = 0.484mol C / 6 x kJ = 3.96 kJ ∆ H is the amount of heat released per mole of any substance in the reaction

22 Hess’s Law Enthalpy is a state function. It is independent of the path. We can add equations to to come up with the desired final product, and add the  H Two rules: If the reaction is reversed the sign of  H is changed If the reaction is multiplied, so is  H

23 Given calculate  H o for this reaction Example  Hº= -394 kJ  Hº= -286 kJ  Hº= kJ

24 Example Given Calculate  Hº for this reaction  Hº= +77.9kJ  Hº= +495 kJ  Hº= kJ

25 Standard Enthalpies of Formation Need to be able to write the equations. What is the equation for the formation of NO 2 ? ½N 2 (g) + O 2 (g)  NO 2 (g) Have to make one mole to meet the definition. Write the equation for the formation of methanol CH 3 OH.

26 Since we can manipulate the equations We can use heats of formation to figure out the heat of reaction. Lets do it with this equation. C 2 H 5 OH +3O 2 (g)  2CO 2 + 3H 2 O which leads us to this rule.