Unit 10: Energy in Chemical Reactions Cypress Creek High School Chemistry 1L Chapter 10

Energy In this unit, we will study chemical systems & their energy Energy is the ability to do work All of these systems contain kinetic and potential energy Kinetic energy – all particles are in motion Potential energy – stored in the bonds of compounds 2

Chemical Reactions When a substance undergoes a chemical change, it takes part in a chemical reaction. After it reacts, it no longer has the same chemical identity. What are the evidences that a chemical change has occurred?

Evidence for Chemical Changes Color Change Gas produced without heating Precipitate formed New odor develops Large amount of heat or light produced

Collision Theory The following three statements summarize the collision theory. Particles must collide in order to react. The particles must collide with the correct orientation. The particles must collide with enough energy to form an activated complex, which is an intermediate particle made up of the joined reactants. An effective collision is one that results in a reaction; new products are formed. 5

Activated Complex The activated complex is an intermediate particle formed when reactants collide and stick together. Old bonds break while new bonds form. The minimum amount of energy colliding particles must have to form an activated complex is called the activation energy. Particles that collide with less than the activation energy cannot form an activated complex. 6

Exothermic Reactions In exothermic reactions energy is released into the surroundings Potential energy, which is stored in the bonds, converts to kinetic energy when it is released. Exothermic reactions feel warm because thermal energy is entering the surroundings (i.e. your hands) The exothermic reaction releases heat; energy is a product. 7

Endothermic Reactions In endothermic reactions energy is absorbed from the surroundings Kinetic energy converts to potential energy when it is absorbed and is stored in bonds Endothermic reactions feel cold because thermal energy is leaving the surroundings (i.e. your hands) The endothermic reaction absorbs heat; energy is a reactant. 8

ΔHrxn = ΣH(products) - ΣH(reactants) Enthalpy Enthalpy is a measure of heat energy of a system at constant pressure It can’t be measured directly, we measure change in enthalpy. Change in enthalpy is called heat of reaction or ΔHrxn ΔHrxn = ΣH(products) - ΣH(reactants) This information will come from a table “Δ” is the letter delta – it means “change” “Σ” is the letter sigma – it means “sum” “ o ” refers to standard temperature and pressure (STP) Units: kJ/mol

Change in Enthalpy Negative ΔH indicates an energy loss (exothermic) Positive ΔH indicates an energy gain (endothermic)

Chemical Equations Energy Photosynthesis: plants consume carbon dioxide, water, and light energy (from the sun) and convert it into sugar (glucose), oxygen, and water. 6 CO2(g) + 12 H2O(l) + energy → C6H12O6(aq) + 6 O2(g) + 6 H2O(l) Cellular Respiration: animals consume sugar (glucose) and oxygen to produce carbon dioxide, water, and chemical energy (ATP) C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l) + energy These chemical equations are important principles of biology. Which is endothermic and which is exothermic?

Change in Enthalpy All elements in their standard states have an enthalpy of zero Because there is no change involved in their formation Endothermic When N2 and O2 combine to form NO2, the ΔH = +33.2 kJ/mol Exothermic When S and O2 combine to form SO3, the ΔH = -396 kJ/mol.

Check for Understanding! Do elements in their standard states possess zero energy? Why are elements in their standard states assigned enthalpies of zero? What does the +33.2 on the graph tell you? What does the -396 on the graph tell you?