Energy changes in chemical reactions

in this video we'll look at change that accompanies All chemical reactions the change in energy one of the signs that chemical reaction has taken place is that there is some kind of temperature change caused by the reaction when chemical reaction occurs there's always transfer of energy either the reaction produces energy or it absorbs energy from the surroundings this is result of chemical bonds forming and breaking and we'll go into the detail of that later in the course when we look at the energy changes in reactions it helps if we divide the universe into the reaction that we're studying and everything else and we can then look at whether the energy is Flowing from the reaction to the rest of the universe which we can just call the surroundings or the other way around if chemical reaction releases energy then as the reactants are turning into the products energy flows from the Rea re acting chemicals out into the surroundings and this energy is now free to move through the rest of the universe this energy usually takes the form of heat so we detect the energy change by the surroundings getting hotter such reaction is called exothermic heat to the outside if chemical reaction absorbs energy where is it going to get the energy from it has to come from the surroundings that is from somewhere in the rest of the universe energy flows from the surroundings into the reacting chemicals and is incorporated or stored as chemical energy in the molecules of the products of that reaction because energy is being removed from the surroundings to the reaction You observe this as the surroundings cooling down you may also observe it as the reaction only being able to go if you provide continual supply of extra heat from the outside such reaction is called endothermic heat to the inside where the inside is the newly formed molecules there are number of ways in which energy can be released in an exothermic reaction in combustion the most obvious way is that the energy is released in the form of heat but notice that flame gives light too so combustion reactions often release light as well as heat in fact both light and heat are forms of the same kind of energy electromagnetic radiation in luminescent reactions the energy is released at visible light without heat which is why these reactions are some sometimes called cold light glow sticks and the flashing of fireflies and gloworms are examples of luminescent reactions chemical explosives release energy as mechanical energy and sound as well as heat and light touch powder is very unstable compound that explodes when it's touched it gives off heat but the most obvious form of energy released by this reaction is sound energy which is kind of mechanical energy respiration is the chemical reaction that living things use to turn glucose and oxygen into carbon dioxide water and energy that's combustion reaction but in the bodies of organisms it occurs in slow and controlled fashion so there's no flame involved nevertheless it does produce heat which is where the heat comes from that keeps us warm blooded animals warm and fireworks produce all sorts of energy at once heat light and sound endothermic reactions are sometimes not as obvious as exothermic ones because the energy is being absorbed and that's harder to see perhaps the most wellknown kind of endothermic reaction is the kind that makes cold packs cold instant cold packs have two or more chemicals in separate pouches inside larger pouch when the inside pouches are broken and the reactants mix an endothermic reaction occurs and the chemicals absorb heat from the surroundings which could include for example your sprained ankle another example of endothermic reactions is the decomposition of copper carbonate in fact this is the case for lot of carbonate chemicals when green copper carbonate powder is heated it decomposes to give black copper oxide and carbon dioxide gas this reaction only happens if it's heated up and it's that heating energy that gets absorbed as the copper carbonate is transformed into the products you may at this point wonder why the copper carbonate has to be heated and yet the cold pack reaction just happens as soon as the chemicals are mied well that has to do with the activation energy of the reaction and we'll deal with that in more detail later on another vitally important endothermic reaction is photosynthesis where carbon dioxide gas and water are combined to give glucose and oxygen and this is done by plants this is exactly the opposite reaction to respiration so whereas respiration produced energy as the glucose and oxygen were turned into carbon dioxide and water in photosynthesis energy has to be put in to turn the carbon dioxide and water into glucose and oxygen and that energy comes from the Sun so remember EXO means outside so heat goes to the outside so in these reactions the surroundings get hotter and Endo or inside heat goes to the inside the chemicals themselves and that heat doesn't actually go towards making the chemicals hotter instead it's stored as chemical energy in the bonds of the newly formed molecules so energy disappears from the surroundings gets locked up in the chemical bonds and the surroundings get colder if you have trouble remembering which is which perhaps think of biology analogy insects spiders crabs creatures like that they have an exoskeleton this means their hard supportive structures are on the outside so EXO means outside whereas vertebrates including us have skeletons on the inside and the proper name for this is an endoskeleton so Endo means inside okay so here's your task for this video I'm describing reaction here it's sulfuric acid plus zinc giving zinc sulfate and hydrogen gas I'd like you to write and balance the chemical equation for the reaction now zinc's transition metal but I'm telling you you can assume that the zinc ion has two plus charge in fact zinc is almost always 2+ and then I'm asking you think about for this reaction what are the surroundings that is when you do this reaction what or where will you most obviously observe the change in temperature and then the third question is is this reaction exothermic or endothermic and what evidence can you give for this from the description of the reaction