Module 07: Energy in Reactions

a branch of chemistry concerned with the quantities of heat released or absorbed during chemical reactions

• Temperature • Thermal Energy • Heat

Measure average kinetic energy in particle direct relationship kinetic energy and temperature

Measure total kinetic energy in sample • difference temperature: • Thermal: represents kinetic energy of a sample together • Temperature: average kinetic energy of individual particles

Transfer of thermal energy from one substance to another due to temperature differences

From matter at high temperature to low temperature • depends temperature differences (not in total thermal energy of substances) • continue until both have the same temperature

SJ units: joule (J) • unit kilojoules (kJ) → calories

total heat content of a system equal to the internal energy of the system plus the product of pressure and volume • denoted “q”

ENDOTHERMIC: reactions absorb energy from surroundings → increase net energy (+q) • absorb kinetic energy > convert potential energy • amount energy absorbed: dependent amount of reactants and difference of potential energy EXOTHERMIC: • reactions releases energy to surroundings → decrease net energy (-q)* • stored potential energy released into kinetic energy

Equal heat gained or lost between system and surroundings • gain: +q • lost: -q

• tracks potential energy • y-axis: measure potential energy & x-axis: progress of reaction

• products less PE than reacts • starts higher and ends lower Law of Conservation of Energy: obeyed extra energy not end up is released into the surroundings

• products more potential energy than reactants

the minimum amount of energy colliding particles must have for a chemical reaction to occur Diagrams: representated positive slope that follows reactatns

the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius • intensive property • impacted by mass (how much thermal energy can be absorbed) • J/g °C

C = q/m x △T • C: specific heat • q: heat generated by temperature change • m: mass of substance • △T: change in temperature

q = m x C x △T • q: the heat gained (positive) or lost (negative) by the system, in joules • m: the mass of the sample, in grams • C: the specific heat capacity of the sample, in joules per gram per degree Celsius Jg °C • △T: change in temperature (°C)

• Deposition/Condensation/Freezing: Exothermic (−△H or −q) • Sublimation/Vaporization/Melting: Endothermic (+△H or +q)

• comparing the sum of the enthalpies of reactants to sun of products • Heat of Reaction

△H = Hproducts − Hreactants The heat of reaction (overall change in enthalpy) is found by taking the sum of enthalpies of the products and subtracting the sum of enthalpies of the reactants. • Endothermic: • +△H • energy required break bonds is more than amount of energy released • Exothermic: • -△H • energy required break less amount energy released in forming bonds in product

The enthalpy change value of this reaction tells us that for every mole of methane (CH4) burned, 890.0 kilojoules are given off because the enthalpy change is negative. This is an exothermic reaction. Use the enthalpy change as a conversion factor between moles of methane and kilojoules of energy. 38.5 g CH4÷1 × 1 mol CH4 ÷ 16.05 g CH4 × −890.0 kJ ÷ 1 mol CH4 = −2,135 kJ This shows that as 38.5 grams of methane are burned, 2,140 kilojoules of energy are released.

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