Topic 5 Energy Changes Flashcards
What is an exothermic reaction?
Transfers energy to surroundings so temperature of surroundings increases
What are 3 examples of exothermic reactions?
Combustion
Neutralisation
Many oxidation reactions
What are 2 everyday uses of exothermic reactions?
Hand warmers
Self-heating cans
What is an endothermic reaction?
Takes in energy from surroundings so temperature of surroundings decreases
What are 2 examples of endothermic reactions?
Thermal decomposition
Reaction of citric acid and sodium hydrogencarbonate
What is an everyday use of an endothermic reaction?
Sports injury packs
What do reaction profiles show?
Relative energies of reactants and products, activation energy and overall energy change
What is the activation energy?
Minimum amount of energy that particles must have to react
When can chemical reactions only occur?
Reacting particles collide with each other with sufficient energy
How is an exothermic reaction represented on a reaction profile?
Products at lower energy level than reactants
How are endothermic reactions represented on a reaction profile?
Products at higher energy level than reactants
What does the initial rise in energy on a reaction profile represent?
Activation energy
What happens during a chemical reaction?
Old bonds broken and new bonds formed
What is needed to break bonds in the reactants?
Energy must be supplied
What type of process is bond breaking?
Endothermic
What happens when new bonds in the products are formed?
Energy released
What type of process is bond formation?
Exothermic
In an exothermic reaction, what is the energy released from forming new bonds greater than?
Energy needed to break existing bonds
In an endothermic reaction, what is the energy needed to break bonds greater than?
Energy released from forming new bonds
What can be calculated from bond energies?
Energy needed to break bonds and energy released when bonds are formed
How can you calculate the overall energy change in a reaction?
Sum of energy needed to break bonds in reactants minus sum of energy released when new bonds formed in products
How can a simple cell be made?
Connecting 2 different metals/electrodes in contact with electrolyte
How is a cell set up?
Electrodes connected by wire, voltmeter connected
What 2 things is the voltage produced by a cell dependent on?
Electrolyte
Type of electrode
What does the electrolyte in a cell contain that reacts with the electrodes?
Ions
What causes the pd of a cell?
Different metals react differently with same electrolyte
What is the rule for pd of a cell?
Bigger difference in reactivity of electrodes, bigger voltage of cell
What is a battery?
Two or more cells connected in series, provide greater voltage
Why does a battery have a greater voltage?
Voltages of cells combined
What is an example of non-rechargeable batteries?
Alkaline Batteries
When does electricity stop being produced in non-rechargeable cells/batteries?
Reactants used up
Why can rechargeable cells/batteries be recharged?
Reaction can be reversed by connecting an external electric current
What are 2 types of electrical energy vehicles can use?
Batteries
Hydrogen-oxygen fuel cells
What are 2 disadvantages of using batteries (compared to using fuel cells)?
Rechargeable batteries eventually need replacing
Batteries store less energy than fuel cells- need recharging more often
What are 2 disadvantages of using hydrogen fuel cells (compared to batteries)?
Hydrogen is explosive (when mixed with air), hard to store safely
Hydrogen is gas- takes up more space to store
What is a fuel cell supplied with?
Fuel and oxygen/air
How is a pd produced in a fuel cell?
Fuel oxidised electrochemically within fuel cell
What are hydrogen fuel cells a potential alternative to?
Rechargeable batteries/cells
What is the overall reaction in a hydrogen fuel cell?
Hydrogen oxidised to produce water
