chemistry energy changes topic 5

what happens to energy in a chemical reaction

it's conserved

what is an exothermic reaction

A reaction where energy is transferred to the surroundings, this is usually by heating and is shown by a rise in temperature

what is combustion an example of?

an example of an exothermic reaction. it gives out a lot of energy

what are neutralisation reactions an example of?

an exothermic reaction

what are some examples of everyday uses of exothermic reactions

hand warmers, self heating cans of hot chocolate and coffee

how do some hand warmers work?

uses the exothermic oxidation of iron in air (with a salt solution catalyst) to release energy

what is an endothermic reaction

a reaction that takes in energy from its surroundings. this is shown by a fall in temperature

what kind of reactions are more common?

exothermic reactions

what are some examples of endothermic reactions

- reaction between citric acid and sodium hydrogencarbonate
- thermal decomposition

how are endothermic reactions used everyday?

endothermic reactions are used in some sport injury packs - the chemical reaction allows the pack to become instantly cooler without having to put it in the freezer

what experiment can you use to measure the amount of energy released by a chemical reaction

by taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction

what is the problem with energy measurements

The amount of energy lost to the surroundings

how can you reduce energy loss to the surroundings when taking energy measurements?

by putting the polystyrene cup into a beaker of cotton wool to give more insulation and putting a lid on the cup to reduce energy loss by evaporation

How could you test the effect of acid concentration on the energy released in a neutralisation reaction between hydrochloric acid and sodium hydroxide

1. Put 25cm^3 of 0.25 mol/dm^3 of hydrochloric acid and sodium hydroxide in separate beakers
2. Place the beakers in a water bath set to 25°C until they are both at the same temperature
3. Add the HCl followed by the NaOH to a polystyrene cup with a lid
4. Take the temperature of the mixture every 30 seconds and record the highest temperature
5. Repeat steps 1_4 using 0.5 mol/dm^3

On an energy diagram how can you tell if its an endo or exothermic reaction?

-it's an exothermic reaction when the products are at a lower energy than all the reactants
- it's an endothermic reaction when the products are at higher energy than the reactants

during a chemical reaction what happens to bonds

Old bonds are broken and new bonds are formed

what sort of process is breaking bonds

an endothermic process as energy must be supplied

what process is making bonds

an exothermic process as energy is released when new bonds form

how do you calculate energy change

breaking - making

describe the basic system of an electrochemical cell

- the two electrodes must be able to conduct electricity so are usually metals
- the electrolyte is a liquid that contains ions which react with the electrodes
-the chemical reactions between electrodes and the electrolyte set up a charge difference between the electrodes
- if the electrodes are then connected by a wire the charge is able to flow and electricity is produced. a voltmeter can also be connected to measure the voltage of the cell

what causes different voltages in cells

different metals as they react differently with the same electrolyte. so the type of electrodes affect the voltage of the cell

what makes a bigger voltage in a cell

the bigger the difference of reactivity of the electrodes

how does the electrolyte affect the cell

the electrolyte used jn a cell will also affect the size of the voltage since different ions in solution will react differently with the metal electrodes used

How is a battery formed?

vy connecting two or more cells together in series - the voltages are combined so there is a bigger voltage overall for the battery

what happens over time to normal batteries?

the chemical reactions that happen at the electrodes are irreversible. Over time the reacting particles - the ions in the electrolyte and metal ions on the electrode get used up and turned into the products in the reaction. Once any one of the reactants is used up, the reaction can't happen and so no electricity is produced. the products cant be turned back into the reactants so the cell cant be recharged.

what can happen in rechargeable cells that cant in non-rechargeable batteries?

In a rechargeable cell, the reaction can be reversed by connecting it to an external electric current.

what is a fuel cell?

An electrical cell that is supplied with a fuel and oxygen, and uses energy from the reaction between them to produce electrical energy efficiently.

how is potential difference produced in a fuel cell?

when the fuel enters the cell it becomes oxidised and sets up a potential difference within the cell.

whats the half equation for hydrogen in a hydrogen fuel cell?

H2 -> 2H+ + 2e-

whats the half equation for oxygen in a hydrogen fuel cell?

O2 + 4H+ + 4e- -> H2O

what happens to hydrogen in a hydrogen fuel cell?

hydrogen loses electrons

what happens to oxygen in a hydrogen fuel cell?

oxygen gains electrons

what are the benefits of hydrogen-oxygen fuel cells?

- fuel cells don't produce as many pollutants as other fuels - no greenhouse gases, nitrogen oxides, sulfur dioxides or carbon monoxide. the only by-products are water and heat
-although battery cars dont produce pllutants either their batteries are polluting because they're made from highly toxic metal compounds
-batteries in electric cars are rechargeable but there is a limit to how many times they can be recharged
-batteries are more expensive to make than fuel cells
-batteries store less energy than fuel cells so need to be recharged more often