Topic 5 Flashcards
What does it mean if the products of a reaction store more energy than the reactants?
Energy has been taken in from the surroundings
What happened to the energy if the products stored less?
Energy was transferred to the surroundings during the reaction
Why does the overall energy not change?
Energy is conserved in reactions, it cannot be created or destroyed only moved around
What happens in an exothermic reaction?
Transfers energy to the surroundings usually by heating, this is shown by the rise in temperature.
Examples of an exothermic reaction
Burning fuels (combustion), neutralisation reactions, oxidation reactions and hand warmers, self heating cans
What happens in an endothermic reaction?
Energy is taken in from surroundings, shown from fall in temperature
(much less common)
example of endothermic reactions
thermal decomposition
used in sports injury packs
Practical
1) measure amount of energy released by a chemical reaction by temperature of the solution at the end of the reaction
2) biggest problem with energy measurements is amount lost during reaction
3) reduce this by putting a polystyrene cup into a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation
4) method works for neutralisation reactions or reactions between metal and acids or carbonates an acids
5) you can also use this method to investigate effect of different variables have on the amount of energy transferred
Actual test
1) 25cm3 of 0.25mol/dm3 of hydrochloric acid and sodium hydroxide in separate beakers
2) place beakers in water bath at 25 degrees C until both same temperature
3) add HCL and NaOH to polystyrene cup with lid
4) take temperature of mixture every 30s and record highest temperature
5) repeat steps 1-4 using 0.5 mol/dm3 and then 1 mol/dm3 of hydrochloric acid
Reaction profiles
show energy changes
