Energy Changes Flashcards
What happens in exothermic reactions
Energy from the reacting chemicals is transferred to the surroundings which often increases in temperature.
What happens in endothermic reactions
Energy from the surroundings is transferred to the reacting chemicals causing the temperatures of the surroundings to decrease
When can chemical reactions only occur
When reacting particles collide with eachother and with sufficient energy
What is activation energy
The minimum amount of energy that particles must have to react
How can you tell from a reaction profile that a reaction is exothermic
In the profile for an exothermic reaction, the overall change is negative. You can tell this because the products have less energy than the reactants, and the arrow showing the overall change in energy points downwards.
How can you tell from a reaction profile that a reaction is endothermic
In the profile for an endothermic reaction, the overall change is positive. You can tell this because the products have more energy than the reactants, and the arrow showing the overall change in energy points upwards.
What type of process is used when breaking bonds
Endothermic
What type of process is used when making bonds
Exothermic
When is a reaction exothermic
if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
When is a reaction endothermic
if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
What are bond energies
the amount of energy needed to break one mole of a particular covalent bond.
How do you calculate the energy change in a reaction
add together the bond energies for all the bonds in the reactants - this is the ‘energy in’
add together the bond energies for all the bonds in the products - this is the ‘energy out’
energy change = energy in - energy out
When an energy change is negative or positive what does it mean
Negative=exothermic
Positive=endothermic
What is the required practical for the temperature change in chemical reactions
1)Measure 25cm3 of hydrochloric acid into a polystyrene cup
2)Place the cup inside the beaker to make it more stable
3)measure and record the temperature of the hydrochloric acid
4)measure 5cm3 of sodium hydroxide and add it into the polystyrene cup
5)quickly put a lid on the cup and gently stir the solution with a thermometer through the hole in the lid
Required practical for the temperature change in chemical reactions
6)When the reading on the thermometer stops changing and becomes fairly constant, record the temperature
7)repeat steps 4 and 5 to add further 5 cm3 of sodium hydroxide to the cup. A minimum total of 40 cm3 needs to be added
8) repeat steps 1-7 to ensure reliability of results
9)calculate the mean maximum temperature reached for each of the sodium hydroxide volumes
