energy physical chemistry 3ABC Flashcards
go over the different types of reactions like combustion, metals + acids etc, go over all the practicals (47 cards)
what type of reactions are there (2)
exothermic and endothermic
what is an exothermic reaction?
one which gives out energy to the surroundings, usually in the form of heat and is usually shown by a rise in temperature of the surroundings.
(basically the test tube gets warmer - in the reaction chemical energy (stored in the bonds of chemicals) is converted to heat energy, which is released to the surroundings and rthe temp of the mixture and its surroundings go up)
what is an endothermic reaction?
one which takes in (absorbs) energy from the surroundings, usually in the form of heat and is usually shown by a fall in temperature of the surroundings
(if you hold the test tube, it gets cooler beause the products have more energy than the reactants, meaning you need to get the extra energy by using heat from the surroundings. THis is converted into chemical energy and the temperature of the reaction mixture and its surroundings goes down because heat energy has been converted into a different form of energy)
what is enthalpy change?
the overall change in energy in a reaction
symbol for enthalpy change?
ΔH
if the reaction is endothermic the enthalpy change is….
positive because the reaction is taking in energy
if the reaction is exothermic the enthalpy change is….
negative because the reaction is giving out energy.
what is a reaction profile?
it shows energy cchanges
what is the symbol for activation energy
Ea (tiny a)
what is activation energy?
the minimum amount of energy the reactants need to collide with eachother and react
is thermal decomposition exo/endothermic:?
endothermic
name one type of reaction which is definitely exothermic.
any reaction that produces a flame i.e., combustion reactions
name all reactions that are exothermic.
reactions of metals with acids, neutralisation reactions, displacement reactions
what happens to enthalpy change in an exothermic reaction?
the products are more stable than the reactants. As the reaction happens, energy is given out in the form of heat. The energy warms up both the reaction itself and its surroundings. TGhe enthalpy change is negative
what does the term stability?
it is usually used to describe the relative energies of the reactants and the products in a chemical reaction. The more energy a chemical has, the less stable it is.
what are the units for delta H?
it can be written as kJ/mol or kJ mol-1
what happens to enthalpy change in an endothermic reaction?
enthalpy change is positive. this is because the products have more energy than the reactants (remember energy is required and must be put in for the reaction to take place). The products are less stable than the reactants
what is specific heat capacity?
IT IS THE AMOUNT OF HEAT NEEDED TO RAISE THE TEMPERATURE OF 1 GRAM OF A SUBSTANCE BY 1 DEGREES.
it tells us how much energy has to be put in to increase the temperature of something.
what are the units for specific heat capacity?
J/g/ºC
what is the specific heat capacity of water?
4.18 J/g/ºC
When we do calculations this is the number you use for specific heat capacity (you sort off assume the liquid has a similar specific heat capacity to water)
What is specific heat capaqcity directly proportional to?
mass and temp change
what is the equation linking to specific heat capacity?
Q = mc ΔT
heat energy change = mass x specific heat capacity x temp change
what is the technique used to measure the amount of heat absorbed or given out in chemical reactions and physical changes?
calorimetry. it is based ont he idea that is we use the heat form a reaction to heat another substance, like water, then we can use Q=mcΔT to calculate the amount of heat released.
Note here the mass and temp changes are rferring to the substance heated
we can then work out the molar enthalpy change, ΔH, of the reaction in the unit kJ/mol.
when you do calimetry, how do you get the eneergy in kJ?
divide Q, energy given out, by 1000
