C5 - energy changes Flashcards
Define an exothermic reaction
Energy is given out to the surroundings because more energy was given out by making bonds than was taken in for breaking bonds
Does an exothermic reaction get warmer or colder?
warmer
define an endothermic reaction
Energy is taken in from the surroundings because more energy was taken in for breaking bonds than was given out by breaking bonds
does an endothermic reaction get warmer or colder?
colder
Give two examples of an exothermic reaction
combustion, neutralization reactions
Give two examples of endothermic reactions
the reaction between citric acid and sodium hydrogencarbonate, thermal decomposition (photosynthesis also counts but it isn’t in the book)
give two uses of exothermic reactions
some hand warmers and self-heating cans
give a use of endothermic reactions
some sports injury packs
what are reaction profiles?
diagrams that show the relative energies of the reactants and products in a reaction, and how the energy changes over the course of the reaction
If the reactants are higher than the products in a reaction profile, is the reaction exothermic or endothermic?
exothermic
if the reactants are lower than the products in a reaction profile, is the reaction exothermic or endothermic?
endothermic
what is the name of the initial rise of energy in a reaction profile?
activation energy
what is activation energy?
the minimum amount of energy the reactants need to collide with each other and react. The greater the activation energy, the more energy needed to start the reaction.
what does the overall difference in height of a reaction profile represent?
the overall energy change during the reaction
Does breaking bonds require energy or release energy? Is it exothermic or endothermic?
It requires energy - it is endothermic
does forming bonds require energy or release energy? is it exothermic or endothermic?
It releases energy - it is exothermic
H-H: +436 kJ/mol; Cl-Cl; + 242 kJ/mol; H-Cl: +431kJ/mol
work out the total energy change for the reaction H2 + Cl2 -> 2HCl
-184 kJ/mol
describe an experiment you could do to measure the amount of energy released by a chemical reaction. What is the biggest problem with this reaction? What could this method be used for?
- you can measure the amount of energy released by a chemical reaction (in a solution) by taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction
- the biggest problem with energy measurements is the amount of energy lost to the surroundings
- you can reduce the energy lost by putting the 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
- this method works for neutralisation reactions or reactions between metals and acids, or carbonates and acids
- you can also use this method to investigate what effect different variables have on the amount of energy transferred - e.g. the mass or concentration of the reactants used.
describe an experiment to test the effect of acid concentration on the energy released in a neutralisation reaction between hydrochloric acid (HCl) and sodium hydroxide
- put 25 cm^3 of 0.25 mol/dm^3 of hydrochloric acid and sodium hydroxide in separate beakers
- place the beakers in a water bath set to 25 degrees C until they are both 25 degrees C
- add the HCl followed by the NaOH to a polystyrene cup with a lid
- take the temperature of the mixture every 30 seconds, and record the highest temperature
- repeat steps 1-4 using 0.5 mol/dm^3 and then 1 mol/dm^3 of hydrochloric acid
what is the equation for the overall energy change of the reaction?
overall energy change = energy required to break bonds - energy released by forming bonds
is a negative overall energy change endothermic or exothermic?
exothermic
would an endothermic reaction have a negative or positive overall energy change?
positive
