4.5 energy changes Flashcards
exothermic reactions (examples)
transfers energy to the surroundings, so the temperature of the surroundings increases
Product molecules must have less energy than
the reactants, by the amount transferred (as energy is transferred to surroundings)
examples of exothermic reactions are: combustion, many oxidation reactions, and neutralisation
every day uses -> self heating cans (for coffee), hand warms
endothermic reactions
takes in energy from the surroundings, so the temperature of the surroundings decreases
examples of endothermic reactions: thermal decomposition (breaking down a substance using heat)
and the reaction of citric acid and sodium hydrogencarbonate
some sports injury packs use endothermic reactions
Required practical 4: investigate the variables that affect temperature changes in reacting solutions
- Measure 25cm³ of hydrochloric acid using measuring cylinder, and pour into a polystyrene cup
- Put the polystyrene cup inside a beaker, stopping the cup from falling over and spilling acid/alkali
- Measure and record temperature of the hydrochloric acid.
- Measure 5cm³ of sodium hydroxide and add to polystyrene cup. Quickly close the cup with the lid and stir gently with the thermoter through the hole (so heat energy is distributed evenly throughout the solution)
- Record the maximum change in temperature; when it stops going up anymore
- Repeat the whole experiment several times by adding 5cm³ extra of sodium hydroxide each time until 40cm³
- Calculate a mean value for the maximum temperature reached for each sodium hydroxide volume
RP4: graph
As we increase the volume of sodium hydroxide solution, the maximum temperature reached increases -> adding more particles of sodium hydroxide reacts w/ hydrochloric acid
exothermic reaction -> more energy is released -> maximum temperature reached increases
However at a certain volume of sodium hydroxide, the maximum temperature starts to decrease (so much sodium hydroxide, not enough hydrochloric acid and some of the sodium hydroxide is unable to react-> energy released by reaction has reached a maximum)
When we add large volumes of sodium hydroxide, maximum temperature decreases
RP4
Why use a polystyrene cup with a lid
safety precautions (wear goggles)
Variables
measuring the temperature, so want to reduce any heat losses
polystyrene is a good thermal insulator -> reduces heat loss through sides and bottom
lid reduces heat loss to the air
independent variable - volume of sodium hydroxide solution
dependent variable - maximum temperature reached
control variable - volume of hydrochloric acid and type of concentration
reaction profiles
Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
activation energy - the minimum amount of energy needed by the particles to react
reaction profiles can be used to show the relative energies of reactants and products, the activation energy, and overall energy change of a reaction
energy change of reactions
during a chemical reaction, energy must be supplied to break bonds in the reactants, and energy is released when bonds in the products are formed
The energy needed to break bonds and energy released when bonds are formed can both be calculated from bond energies
Sum of energy taken in to break bonds – sum of energy released to form bonds = overall energy change
Bond energy calculations
energy must be supplied to break bonds -> endothermic (positive)
energy is released when new bonds are formed -> exothermic (negative)
- exothermic -> energy released when forming bonds is greater than energy used to break them
(overall energy has been released) - endothermic -> energy used to break bonds is greater than the energy released when forming them
(overall energy has been taken in)
Cells and Batteries
Cells contain chemicals which react to produce electricity
Eventually, these chemicals run out and reaction stops, meaning cells can only produce electricity for a certain period of time
cells only produce electricity if we use metals with different reactivities
The greater the difference between the reactivity of the metals, the greater potential difference produced by the cell
-> the electrolyte also affects the potential difference
A simple cell can be made by connecting two different metals in contact with an electrolyte
Batteries are two or more cells connected together in series to provide greater voltage
The voltage produced by a cell depends on a number of factors including; electrode and electrolyte
non-rechargeable & rechargeable cells and batteries
NR - Chemical reactions stop when one of the reactants has been used up (no more electricity is produced).
no way to reverse these reactions (irreversible) -> non-rechargeable
Alkaline batteries are non-rechargeable
R - can be recharged, can reverse chemical reactions when an external electrical current is supplied
Fuel cells
- very efficient way of producing electrical energy
Fuel cells can provide voltage continuously, as long as they’re supplied by an external source of fuel (usually hydrogen) and pure oxygen or air
In a hydrogen fuel cell:
Hydrogen and oxygen are supplied to the fuel cell
Fuel is oxidised electrochemically in the fuel cell to produce a potential difference
The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water
Hydrogen fuel cells are a potential alternative to rechargeable cells and batteries
electrolyte
a solution that can conduct electricity eg a solution of an ionic compound
evaluate the use of hydrogen fuel cells in comparison with rechargeable cells and batteries (advantages)
hydrogen fuel cells only use the fuels hydrogen and oxygen to produce electricity, and the only waste product is water (non-polluting)
comparison of hydrogen fuel cells with rechargeable batteries:
- HFC will produce electricity for as long as your provide hydrogen, whereas rechargeable batteries run out and need to be recharged
- HFC don’t get less efficient the longer the run, whereas RB can store less electricity the more charging cycles they go through; eventually need to be replaced
- HFC can be a source of drinkable water, such as on space-craft
disadvantages of HFC
- run on hydrogen, explosive gas and hard to store safely, whereas there are no dangerous fuels needed in rechargeable batteries (some types of RB can catch fire if not manufactured properly)
- HFC produce relatively low amount of potential difference, so several are needed together
however, rechargeable batteries can produce a greater potential difference than a HFC
