C7- Energy changes
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Conservation of energy meaning
Energy is conserved in reactions, neither created nor destroyed
Exothermic reaction including energy and bonds?
A reaction that releases energy to the surroundings. Releases more energy by forming bonds than is needed to break bonds.
Endothermic reaction including energy and bonds?
A reaction that absorbs energy from the surroundings. Requires more energy to break bonds than it releases by forming bonds.
How to investigate temperature changes
Measure substances into a polystyrene cup, place lid with thermometer through it, record initial temp and calculate temp change
2 uses for exothermic reactions
Hand warmers and self heating cans
1 use for endothermic reactions
Instant cold packs
What are the typical reactants in self-heating cans
Calcium oxide+water
What are the typical reactants in instant cold packs
Ammonium nitrate dissolved in water
What does a reaction profile show?
Relative energies of the reactants and products
Exothermic reaction profile Products level? Energy transferred to..?
Products at a lower level than reactants Energy transferred to surroundings
Endothermic reaction profile Products level? Energy transferred to..?
Products at a higher level than reactants Energy absorbed from surroundings
Activation energy
Minimum amount of energy needed for particles to successfully collide
Endothermic form/break bonds?
Breaks bonds, requires energy
Exothermic form/break bonds?
Forms bonds, releases energy
Bond energy Definition? Unit? Key fact?
Energy required to break the covalent bond between two atoms kJ/mol Always greater than 0
How to calculate the overall energy change from bond energies?
Calculate energy absorbed by reactants. Multiply moles by the total bond energies in each reactant. Calculate energy released by products. Multiply moles by the total bond energies for each product. Subtract product energies from reactants.
What is a spectator ion?
An ion that doesn’t change in a reaction
Higher voltage when testing reactivity means a…
Greater difference in reactivity between the two metals
How to investigate the voltage produced by different metals with copper
-Pour 25cm3 and 1mol/dm3 of electrolyte solution in a beaker -Place one electrode of copper and one of silver -Connect to a voltmeter and record reading -Repeat with different metals
Hydrogen fuel Word equation? Advantages? Disadvantages?
-Hydrogen + oxygen—-> water -No pollutants produced, can be used for dif uses -Highly flammable, hard to store
Why do alkaline batteries stop working? Why can’t they be recharged?
As the alkali is eventually used up The reaction isn’t reversible