C5- Energy changes
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• what happens to energy in chemical reactions
• Energy is conserved • the amount of the energy in the universe at the start of the reaction is equal to the amount at the start of the reaction
What must happen for a reaction to transfer energy to surroundings
• the product molecules mss have less energy than the reactants
What does the conservation of energy mean
• the amount of energy in the universe never changes
What is an exothermic reaction
• one that transfers energy to the surroundings usually by heating - temperature around increases
What are examples of exothermic reactions and how do we use them everyday
• Exothermic reactions include combustion, oxidation and neutralisation • Everyday uses include self heating cans and hand warmers
What is an endothermic reaction
• A reaction that takes energy from surroundings - temperature of surroundings will decrease
What are the examples and everyday examples of exothermic reactions
• Thermal decomposition and reaction of citric acid and sodium hydrogen carbonate • sport injury pack are based on exothermic reactions
How cam energy transfer be measured- practical
• take the temperature of the reagents - mix them in a polystyrene cup and measure to temperature at the end • Loss of energy to surroundings • can also measure the effect that variables have on energy transfer e.g concentration
• What are reaction profiles
• Diagrams that show the relative energies of reactants and products in a reaction and how energy changes over course of a reaction
What is activation energy
• Chemical reactions can only occur when reacting particles collide with each other with sufficient energy. the minimum energy that particles must have to react is called activation energy
What does a reaction profile look like in an exothermic reaction
• Products at l;over energy than reactants - Difference in height represents energy change in reaction per mole
What does a reaction profile look like for an endothermic reaction
• Products are at a higher energy than the reactants - Difference is height represents the overall energy change during the reaction in moles
What does the difference in height represent in reaction profiles
• The overall energy change during the reaction per mole
• How do bonds relate to energy during a reaction
• Energy must be supplied to brea bonds in the reactants - Energy is related when bonds in products are formed
What happens in exothermic reactions regarding bond breaking
What happens in endothermic reactions regarding bind breaking
• The energy needed to break existing bonds is greater than the energy released from forming new bonds
What type of reactions are bond breaking and making
• Bond breaking - endothermic - bond forming- exothermic
What is the overall energy change in a reaction
• The difference between the sum of energy needed to break bonds and energy released when bonds are formed is overall energy change • Difference between energy to break bonds and energy in formation of bonds
Hood you calculate the energy change in a reaction
• Find energy required to break original bonds • Find energy released by forming new bonds • Find overall energy change for reaction using this equation - • Overall energy change = Energy required to break bonds- energy released by forming bonds
What is bond energy
• The amount of energy in a bond and thus the amount needed to break it