PREVNEXTSUBMITMenuResourcesGlossaryNotesClear and return to MenuSearch ResultsFilterSlide TextNotesSearch in:Check to includeSearch...TermsDefinitiondefinitionresourcessearchpauseplayreplaysubmitnextpreviousvolumewelcome to the lesson: energy changes in chemical processes and nuclear reactions. next to quickly recap what we learned in the last lesson, chemists are very interested in observing whether a given system absorbs or releases heat during a chemical reaction or change of state. a system is the sample or reaction that a scientist is observing, while the surroundings are everything else that exists around the sample. next the heat content of a substance is also referred to as its enthalpy (h). chemists define chemical processes as either endothermic or exothermic depending on its change in enthalpy (δh) next endothermic reactions absorb heat from their surroundings and have an increase in enthalpy; while exothermic reactions release heat to their surroundings and have a decrease in enthalpy. next slide δh in changes of state sort the following changes of state into endothermic or exothermic. think about whether the system absorbs or releases heat to undergo this change of state. hover your mouse over each image for a definition. melting condensation vapourizing freezing endothermic exothermic melting: changing state from solid to liquid condensation: changing state from gas to liquid freezing: changing state from liquid to solid vapourizing: changing state from liquid to gas correct. for the ice (the system) to melt it must absorb heat from its surroundings, therefore this is an endothermic process in which the enthalpy of the system increases. correct. when gas condenses into liquid, heat is released making it an exothermic reaction in which the enthalpy of the system decreases. incorrect. for the ice (the system) to melt it must absorb heat from its surroundings, therefore this is an endothermic process in which the enthalpy of the system increases. correct. for the water to freeze it must release heat into their surroundings, making this an exothermic process in which the enthalpy of the system decreases. incorrect. when gas condenses into liquid, energy released is making it an exothermic reaction in which the enthalpy of the system decreases. incorrect. for the water to freeze it must release heat into their surroundings, making this an exothermic process in which the enthalpy of the system decreases. correct. for the water (the system) to boil and vapourize into gas it must absorb heat from the surroundings, making this an endothermic process in which the enthalpy of the system increases. next slide incorrect. for the water (the system) to boil and vapourize into gas it must absorb heat from the surroundings, making this an endothermic process in which the enthalpy of the system increases. next slide δh in changes of state during a change of state, the molecules of a substance move closer together or further apart, depending on the type of change. for molecules to move closer together, intermolecular bonds form and to move farther apart, intermolecular bonds are broken. breaking intermolecular bonds requires the absorption of heat by the system and an increase in enthalpy. forming intermolecular bonds results in the release of heat by the system and a decrease in enthalpy. next exothermic changes of state: condensation: gas to liquid freezing: liquid to solid heat is released, enthalpy decreases, δh is negative imagine making ice cubes in a freezer. liquid water is placed in the cold freezer where heat is removed from the water, allowing it to freeze enthalpy next slide enthalpy endothermic changes of state: vapourization: liquid to gas melting: solid to liquid heat is added, enthalpy increases, δh is positive imagine boiling water on a stove-top. heat must be added (δh is positive) to make the water boil and vapourize into gas. next chemical reactions endothermic exothermic in a chemical reaction, intramolecular bonds between atoms are broken and/or formed. as we just learned with changes of state, breaking or forming bonds involves the release or absorption of heat. based on what we have learned already and what you know about chemical bonds, predict the following: 1. breaking chemical bonds between atoms is: breaking chemical bonds is endothermic as energy required to break the bonds. δh is positive. a b energy a + b endothermic exothermic next forming chemical bonds between atoms is exothermic, as energy is released when bonds form. δh is negative. a + b energy a b endothermic exothermic next slide exothermic endothermic 2. forming chemical bonds between atoms is: nuclear reactions nuclear reactions are processes that occur in the nucleus of atoms. there are 2 types of nuclear reactions: fission & fusion. nuclear reactions are always exothermic, therefore, their δh is always negative. these reactions release tremendous amounts of kinetic energy, and this property is used by humans to produce electrical energy as well as destructive energy. click on each type of nuclear reaction for more info. nuclear fission nuclear fusion nuclear fission is the process in which a heavy nucleus splits into 2 lighter nuclei, accompanied by the release of energy. nuclear fission is the process of converting potential energy into kinetic energy that is used in nuclear reactors and nuclear bombs. energy nuclear fission nuclear fusion is the process in which 2 small nuclei combine (or fuse) together to form a single, larger nucleus fusion is the process that occurs on the sun which produces the heat we can feel all the way here on earth. energy nuclear fusion next slide select the choices that represent endothermic processes: vapourization condensation nuclear fission chemical bonds are broken nuclear fusion melting water freezing chemical bonds are formed intermolecular bonds are broken intermolecular bonds are formed continue continue try again select the choices in which the enthalpy decreases (δh is negative). exothermic reaction condensation nuclear fusion chemical bonds are broken melting water freezing chemical bonds are formed intermolecular bonds are broken intermolecular bonds are formed endothermic reaction continue continue try again so, to recap the important points from this lesson, we looked at enthalpy changes during three different processes: changes of state, chemical reactions and nuclear reactions. next during melting or vapourization, intermolecular bonds are broken which requires the input of heat. therefore these changes of state are endothermic, their δh is positive. next during freezing or condensation, intermolecular bonds are formed which causes the release of heat. therefore these changes of state are exothermic, their δh is negative. next during chemical reactions chemical bonds are formed and/or broken. when bonds are formed heat is released and the process is exothermic. when chemical bonds are broken heat is absorbed and the process is endothermic. next nuclear reactions are processes that occur in the nucleus of atoms. these reactions are always exothermic and result in the release of tremendous amounts of energy. finish <html><textformat leading='1' tabstops='[48, 96]' leftmargin='0' indent='0'>Classroom-themed layout features a retro blackboard placeholder. Use this fun and informal layout with your favorite handwriting fonts to complete the schoolhouse-inspired design.</textformat></html><html><textformat leading='1' tabstops='[48, 96]' leftmargin='0' indent='0'>Classroom-themed layout features a retro blackboard placeholder. Use this fun and informal layout with your favorite handwriting fonts to complete the schoolhouse-inspired design.</textformat></html>