topic 5- energy changes

what is an exothermic reaction?

A reaction that transfers energy from the reaction molecules to the surroundings

What happens to the temperature of the surroundings around an exothermic reaction?

Temperature of the surroundings increases

an example of types of exothermic reactions?

• combustion (burning fuels)

• neutralisation reactions

• many oxidation reactions

examples of exothermics every day uses?

• hand warmers

• self heading cans

what would an energy profile diagram look like for an exothermic reaction?

why did the products have less energy than the reactants in an exothermic reaction?

Because energy has been transferred from the reaction to the surroundings

what are endothermic reactions?

endothermic reactions are ones that take in energy from the surroundings

what are some types of endothermic reactions?

• reaction between citric acid and sodium hydrogen carbonate

• thermal decomposition

What happens to the temperature of the surroundings around an endothermic reaction?

The temperature of surroundings decrease

what would the energy profile of an endothermic reaction look like?

examples of uses of endothermic reactions?

• sports injury packs No

what is the definition of activation energy?

The minimum amount of energy that particles must have to react

what happens to the bonds in an endothermic reaction?

bonds break

why do the bonds break in an endothermic reaction?

energy must be supplied to break existing bonds

what happened to the bonds in an exothermic reaction?

bonds form

why are bonds formed in an exothermic reaction?

Because energy is released

How would you carry a practical to determine temperature change?

• Use a measuring cylinder to measure 30 cm³ of dilute hydrochloric acid

• Transfer the acid into a polystyrene cup

• stand the Polystyrene cup inside a beaker (stops the cup from falling over)

• Use a thermometer to measure the temperature of the acid

• use a measuring cylinder to measure 5 cm³ of sodium hydroxide solution

• transfer this into the polystyrene cup

• fit plastic lid onto the cup and place the thermometer inside

• use a thermometer to gently stir the solution

• because the reaction is exothermic (neutralisation reaction), it will release energy so the temperature of the solution will increase

• Record the temperature reached

• Now rinse out and dry the polystyrene cup

• Repeat the whole experiment with using 10 cm³ of sodium hydroxide solution

• do this multiple times with increasing the solution by 5 cm³ each time

• once you have done this, repeat the whole experiment again, so you have two sets of results

• you can then find out on me for each volume of solution

• you can then plot a graph of your results

what would the graph of your results look like?

why after a certain point does the maximum temperature reached start to decrease?

Because as you add a greater volume of solution so the energy release is now spread into a greater volume.

why is a polystyrene cup used?

It’s a good thermal insulator so it reduces heat loss

why do you put a lid on the top?

Reduces heat loss to the air

what is an electrochemical cell?

A basic system made up of two different electrodes in contact with an electrolyte

why are the electrodes usually metals?

Because they must be able to conduct electricity

what is the electrolyte?

A liquid that can conduct electricity (contains ions which react with the electrodes)

What happens when we take two different metals and placed them into an electrolyte?

We can produce electricity

what does an electric cell look like?

why can a cell only produce electricity for a certain amount of time?

Eventually, the chemicals in the cell run out and the reaction stops

what must the metals be to produce electricity?

Must use metals with different reactivities

what happened when the electrodes get connected by a wire?

The charge is able to flow and electricity is produced

what is the size of the potential difference depend on?

The difference in reactivity between the two metal

what is a battery?

Contains two or more cells connected in series to produce a greater voltage

Image of a battery?

why do batteries run out?

Because the reactants in the batteries run out so no more electricity is produced (irreversible)

examples of batteries that can run out

alkali batteries

why can rechargeable batteries be recharged?

Because the chemical reactions can be reversed

what do fuel cells generate?

electricity

what happens in a fuel cells?

we react a fuel with either pure oxygen or air, inside the fuel cell a chemical reaction takes place which produces an electrical current

what is the only waste product from a fuel cell?

water

diagram of a fuel cell?

what is the advantage of fuel cells in terms of their life span compared to rechargeable batteries?

• hydrogen fuel cells will produce electricity for as long as you provide hydrogen

• Rechargeable batteries run out and need to be recharged.

what is the advantage of fuel cells producing water as a waste product?

• Hydrogen fuel cells can be a source of drinkable water eg on space-craft.

disadvantages of fuel cells using the gas hydrogen?

• Hydrogen fuel cells run on hydrogen which is an explosive gas and is very difficult to store safely.

what is the difference in potential difference produced between fuel cells and rechargeable batteries?

• Hydrogen fuel cells produce a relatively low potential difference or voltage so several are needed together.

• • Rechargeable batteries can produce a greater potential difference than a hydrogen fuel cell.