Energy changes Flashcards
Why do chemical reactions occur?
So that elements can achieve a more stable energy state by gaining a full outer shell of electrons
Where is energy transferred in exothermic reactions?
In exothermic reactions energy is given out to the surroundings so the temperature of the surroundings increases
Examples of exothermic reactions: (3)
Combustion, oxidation, and neutralisation reactions
Where is energy transferred in endothermic reactions?
In endothermic reactions energy is taken in from the surroundings so the temperature of the surroundings decreases
Examples of endothermic reactions: (3)
Electrolysis, thermal decomposition reactions and the first stages of photosynthesis
What happens to the heat content of exothermic reactions?
heat content of the system falls
What happens to the heat content of endothermic reactions?
heat content of the system increases
How to investigate the temperature change of a reaction?
- Using a measuring cylinder, place 25 cm3 of the NaOH solution into the calorimeter
- Measure and record the temperature of the solution
- Add 5 cm3 of the dilute HCl and stir
- Measure and record the highest temperature reached by the mixture
- Repeat steps 1 – 4 increasing the amount of acid added by 5 cm3 each time
What happens when bonds break ?
Energy is needed to break bonds which is absorbed from the reaction surroundings, so bond breaking is an endothermic process
What happens when bonds form?
For forming bonds as it releases energy back to the surroundings in an exothermic process
In endothermic reactions is the energy change positive or negative and why?
The change in energy is positive since the reactants have more energy than the products as more energy is required to break the bonds than that gained from making the new bonds
In exothermic reactions is the energy change positive or negative and why?
The change in energy is negative since the reactants have less energy than the products as more energy is released when new bonds are formed than energy required to break the bonds in the reactants
How to find energy change?
Energy change = Energy taken in - Energy given out
What is a simple cell design?
Consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by wire, creating a complete circuit
How does a zinc and copper cell work?
- Zinc is the more reactive metal and forms ions more easily, readily releasing electrons
- The electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes
- The electrons then flow around the circuit to the copper electrode which is now the more positive electrode
What causes a voltage to be produced?
The difference in the ability of the electrodes to release electrons causes a voltage to be produced
How to create a higher voltage?
The greater the difference in the metals reactivity then the greater the voltage produced
The electrolyte used also affects the voltage as different ions react with the electrodes in different ways
How do batteries work?
Batteries work by connecting two or more cells in series, which combine to give a larger overall voltage
How to investigate the variables that affect temperature changes in reacting solutions?
liquids
- Place the polystyrene cup inside the glass beaker to make it more stable.
- Measure an appropriate volume of each liquid, eg 25 cm3.
- Place one of the liquids in a polystyrene cup.
- Record the temperature of the solution.
- Add the second solution and record the highest or lowest temperature obtained.
- Change your independent variable and repeat the experiment. Your independent variable could be the concentration of one of the reactants, or the type of acid/alkali being used, or the type of metal/metal carbonate being used.
How to investigate the variables that affect temperature changes in reacting solutions?
solids
- Place the polystyrene cup inside the glass beaker to make it more stable.
- Measure an appropriate volume of the solution, eg 25 cm3.
- Measure an appropriate mass of the solid, or select a suitable sized piece of metal.
- Place the solution in a polystyrene cup.
Record the temperature of the solution. - Add the solid and record the highest or lowest temperature obtained.
- Change your independent variable and repeat the experiment. Your independent variable could be the surface area of the solid, or the type of acid being used, or the type of metal being used.
When do non-rechargeable batteries run out and why?
Over time the electrodes degrade as the reactions that occur there are irreversible
Cells produce a voltage only until one of the reactants is used up and when this occurs the battery dies or goes flat
The products formed cannot be reverted back into reactants as the reaction is irreversible and the battery must be replaced
How do rechargeable batteries work?
In rechargeable batteries the reactions are reversed by connecting the cells to an external electrical supply
This reverses the chemical reactions taking place allowing the cycle to be repeated
Adv and disadv of non-rechargeable cells? (2)
- Cheap
- Disposal of lots of cells which contain toxic chemicals
Adv and disadv of rechargeable cells? (4)
- Can be used many times
- Cheaper than fuel cells
- Takes more time to recharge
- More expensive than non-rechargeable cells
How do hydrogen fuel cells work?
A fuel donates electrons at one electrode and oxygen gains electrons at the other electrode. As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell
Adv and dis of the hydrogen fuel cells work? (6)
- No pollution or disposal issues
- No power is lost in transmission
- Produce more energy than either petrol or diesel
- Materials used are expensive
- High pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle
- Fuel cells are affected by low temperatures, becoming less efficient
What is the reaction for the anode, cathode and the overall reaction? what charge are they?
anode: - negative 2H2 → 4H+ + 4e– cathode: - positive 4H+ + O2 + 4e– → 2H2O overall reaction: 2H2 + O2→ 2H2O
What happens to the hydrogen in a hydrogen and oxygen fuel cell?
Hydrogen enters at the anode where it is oxidised and oxygen enters at the cathode where it is reduced
