# Thermochemistry the study of transfers of energy as heat that accompany chemical reactions and physical changes.

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Thermochemistry the study of transfers of energy as heat that accompany chemical reactions and physical changes.

Heat and Temperature Energy absorbed or released as heat in a chemical or physical change is measured in a calorimeter. In some calorimeters, known quantities of reactants are submersed in water and combusted. The energy given off is noted by the temperature change in water.

Heat and Temperature

Heat and Temperature Temperature – a measure of the average kinetic energy of the particles in a sample of matter. The greater the kinetic energy, the greater the temperature. Joules – the SI unit of heat as well as all other forms of energy

Heat and Temperature Heat – the energy transferred between samples of matter because of differences in their temperature Energy transferred as heat always moves spontaneously from matter at a higher temperature to matter at a lower temperature.

Heat Capacity and Specific Heat
Specific heat – is the amount of energy required to raise the temperature of one gram of substance by one Celsius degree or one Kelvin. Joules per gram per Kelvin or joules per gram per Celsius degree Q = (m)(Cp)(ΔT) Cp = specific heat at a given pressure ; q = energy lost or gained; m = mass of sample; ΔT = the change in temperature

Heat Capacity and Specific Heat
EXAMPLE: A 4.0 g sample of glass was heated from 274 K to 314 K, a temperature increase of 40 K, and was found to have absorbed 32J of energy as heat. What is the specific heat of this type of glass?

Heat Capacity and Specific Heat
Given: m = 4.0 g q = 32 J ΔT = 40 K Cp = ??? Q = (m)(Cp)(ΔT) J = (4.0 g)(Cp)(40 K) J = 160 (Cp) J/gK = Cp

Heat Capacity and Specific Heat
How much energy will the same glass sample gain when it is heated from 314 K to 344 K?

Heat Capacity and Specific Heat
(0.20 J/gk)(4.0g)(71K – 41K) (0.8)(30) = 24 J

Now You Try…. Determine the specific heat of a material if a 35 g sample absorbed 48 J as it was heated from 293K to 313K. If 980 kJ of energy are added to 6.2 L of water at 291 K, what will the final temperature of water be?

Heat of Reaction Heat of Reaction
Heat of reaction – the quantity of energy released or absorbed as heat during a chemical reaction.

Heat of Reaction 2 H2 (g) + O2 (g)  2 H2O (g)
In the above equation 2 mol of hydrogen gas is ignited to consume 1 mol of oxygen gas and form 2 mol of water. This is an explosive reaction and kJ are produced. We would re-write this as a thermochemical equation. 2 H2 (g) + O2 (g)  2 H2O (g) kJ

Doubling the reaction would likewise produce double the heat released.
Heat of Reaction Doubling the reaction would likewise produce double the heat released. 4 H2 (g) + 2 O2 (g)  4 H2O (g) kJ Fractional co-efficients are sometimes used in thermochemical equations. H2 (g) + ½ O2 (g)  H2O (g) kJ

Heat of Reaction The physical states of reactants and products must always be included in thermochemical equations because they influence the overall amount of energy exchanged. (s) = Solid (aq) = aqueous (g) = gas (l) = liquid

ΔH = Hproducts - Hreactants
Heat of Reaction The energy absorbed or released as heat during a chemical reaction at constant pressure is represented by ΔH. “H” is the symbol for a quantity called enthalpy. Only changes in enthalpy can be measured Enthalpy change – the amount of energy absorbed or lost by a system as heat during a process at constant pressure. ΔH = Hproducts - Hreactants

2 H2 (g) + O2 (g)  2 H2O (g) ΔH = -483.6 kJ/mol
Heat of Reaction Thermochemical equations are usually written by designating the enthalpy change, rather than the energy as a reactant or product. 2 H2 (g) + O2 (g)  2 H2O (g) ΔH = kJ/mol Note how enthalpy change is a negative number. This means energy is evolved, or given off, during the reaction.

Heat of Reaction 2 H2O (g)  2 H2 (g) + O2 (g) ΔH = + 483.6 kJ/mol
The opposite would look like the following: 2 H2O (g)  2 H2 (g) + O2 (g) ΔH = kJ/mol

Heat of Reaction An exothermic reaction gives off energy, and therefore has a negative enthalpy change.

Heat of Reaction An endothermic reaction receives energy, and therefore has a positive enthalpy change.

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