PREVNEXTSUBMITMenuResourcesGlossaryNotesClear and return to MenuSearch ResultsFilterSlide TextNotesSearch in:Check to includeSearch...TermsDefinitiondefinitionresourcessearchpauseplayreplaysubmitnextpreviousvolumewelcome to this introductory lesson on thermochemistry. we are going to begin this unit by introducing some of the important terms and concepts that we will be using in the next few lessons next the focus of this unit is on energy changes and how they affect chemical reactions. we will be looking at different types of reactions and how they affect and are affected by their surroundings. next by the end of this lesson, you should be familiar with terms and concepts including: system, surroundings, enthalpy, exothermic and endothermic. next energy all substances have a certain amount of energy associated with them. either in the form of kinetic energy, which is the energy of motion, or potential energy, which is energy that is stored within an object. next tnt next slide picture an unlit stick of dynamite, it is storing a tremendous amount of potential energy inside of it. when the stick is lit, this potential energy is converted into kinetic energy in the form of an explosion. heat that is given off from the explosion is a type of kinetic energy that is present in all substances. click on the dynamite below to ignite it and convert the potential energy into kinetic energy. tnt law of conservation of energy in chemical processes, which include chemical reactions and changes of state, energy is released into the surroundings or absorbed from the surroundings. energy can never be created or destroyed, only transformed from one form into another. this is known as the law of conservation of energy: energy cannot be created or destroyed. the total amount of energy in an isolated system remains constant. next after explosion 4350 joules of kinetic energy 160 joules of potential energy = 4,510 joules of total energy the total amount of energy remains the same before and after the explosion, only the type of energy has changed, from potential to kinetic. next slide before explosion: 10 joules of kinetic energy 4500 joules of potential energy = 4,510 joules of total energy tnt next system & surroundings in this unit we will look at energy changes and transfers during chemical processes to examine these changes, we need to define which objects are absorbing and which are releasing energy, so scientists divide chemical processes into the system and surroundings. next the system is the sample you are examining. for example, if you are examining water boiling, click the button to see the system system the surroundings are everything else around the system. click to see the surroundings system surroundings surroundings surroundings surroundings surroundings surroundings surroundings surroundings when investigating water boiling, as we see to the right, the water itself is the system (green box) and everything else around it is the surroundings. next slide system & surroundings: chemical reactions if we are examining a chemical reaction taking place in water, the chemical reactants and products are the system, and the water is the surroundings. click to see the of the system and surroundings of a chemical reaction in water. diagram system surroundings products ↔ reactants water next slide system and surroundings 1. a chemist is examining the evaporation of a liquid in a flask, identify the system by clicking anywhere within the system. 2. the erlenmeyer flask would be considered part of the system: 3. the scientists' hands would be considered part of the surroundings. true false true false next slide ok, so to recap what we just learned, all substances have a certain amount of energy inside of them. chemists are very interested in observing the changes of energy in a system 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 slide heat = enthalpy heat is a form of kinetic energy related to the molecules that make up a substance when heat is added to a system, its molecules move faster and its temperature increases, and when heat is removed from a system, its molecules move more slowly and its temperature decreases the heat content of a substance is called its enthalpy, which is represented by the symbol: h chemical reactions and changes of state involve the absorption or release of heat by the system, which causes a change in enthalpy. a change in enthalpy is represented by the symbol: δh (pronounced as "delta h") next h h h h h h h h next slide if you leave a beaker of hot water (the system) on a desk top, it will begin to cool. in other words, the water loses heat to the surroundings. this loss of heat from the system to the surroundings is called enthalpy change δh next endothermic & exothermic reactions exothermic describes a chemical process where heat is released from the system to the surroundings (exo- means out of). for example, an explosion is an exothermic reaction. next exothermic endothermic next slide endothermic describes a chemical process where heat is absorbed by the system from the surroundings (endo- means inside). for example melting ice is an endothermic process. next a good memory tool to remember the difference between endo- and exo-thermic is that exo- sounds like "exit" so heat exits the system; while endo- sounds like "indoor" or "into" so heat comes into the system. when distinguishing between exothermic and endothermic, we are always considering whether the system is absorbing (endothermic) or releasing (exothermic) heat look at the images below, which do you think represents an exothermic process and which do you think represents and exothermic process? click on each to find out the answer. a good memory tool to remember the difference between endo- and exo-thermic is that exo- sounds like "exit" so heat exits the system; while endo- sounds like "indoor" or "into" so heat comes into the system. system surroundings system surroundings endothermic reactions & enthalpy in an endothermic reaction, the system gains heat from its surroundings. therefore its enthalpy changes. we can represent the change in enthalpy for a reaction using an energy diagram. the diagram below represents the change in enthalpy (δh) of a chemical reaction(the system). looking at the graph below, do you think the products of an endothermic reaction will be higher or lower than the reactants? click on the graph to find out. enthalpy (h) reactants products δh enthalpy (h) reactants in an endothermic reaction, the enthalpy of the system increases and the products of the reaction will have a higher enthalpy value than the reactants. in an endothermic reaction, the δh is positive. next slide exothermic reactions & enthalpy in an exothermic reaction, the system loses heat (enthalpy) to the surroundings. therefore its enthalpy changes. the diagram below represents the enthalpy of a chemical reaction (the system). do you think the products of an exothermic reaction will be higher or lower than the reactants, in other words will the change in enthalpy ? click on the graph to find out. enthalpy (h) reactants products δh enthalpy (h) reactants in an exothermic reaction the enthalpy of the system decreases, in which case, the products will have a lower enthalpy value than the reactants. therefore, in an exothermic reaction, the δh is negative. next slide true false true or false: in an exothermic reaction, energy is destroyed. that's right! the law of conservation of energy states that energy cannot be created or destroyed. the total amound of energy in a reaction always remains the same. continue incorrect. the law of conservation of energy states that energy cannot be created or destroyed. the total amound of energy in a reaction always remains the same. continue products have lower enthalpy than reactants drag the described process into the proper bin. heat is absorbed by the system enthalpy increases heat is released by the system products have a higher enthalpy than the reactants enthalpy decreases endothermic exothermic continue continue try again ok, lets recap the important concepts we just learned. heat is a form of kinetic energy that can be added or removed from a system. when it is added, the temperature of the system increases and when it is removed, the temperature decreases. next enthalpy is a measure of the heat content of a substance. when a substance goes through a chemical reaction or change of state, it absorbs or releases heat, and therefore its enthalpy changes. we represent this change in enthalpy as δh next we define chemical reactions and changes of state as either endothermic or exothermic depending on whether heat is absorbed or released by a system. in other words, it depends on whether the change in enthalpy is positive or negative. next in an endothermic process, a system absorbs heat and its enthalpy increases, the δh is positive. whereas in an exothermic process, a system releases heat to its surroundings and its enthalpy decreases, in other words, the δh is negative. next thanks for joining me for this lesson, if you have any questions, please contact me right away! finish <html><textformat leading='1' tabstops='[48, 96]' leftmargin='0' indent='0'>Classroom-themed layout features a retro blackboard placeholder. 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