C7 Flashcards
Exothermic and endothermic reactions in terms of bonds?
Breaking bonds aBsorbs energy, forRming bonds Releases energy
What is Activation energy?
minimum energy required for a reaction to take place
What kind of reactions are neutralisation reactions?
exothermic
RQ. What equipment do you need to investigate temperature changes?
poly(styrene) cup and thermometer
RQ. Method for investigating temperature changes? safety?
improvements?
• Record initial temperatures of reactants
• mix reactants in cup
• record max/minimum temperatures reached
• Safety = goggles
• improvements= lid for cup, stirring to get even distribution of heat
Uses of exothermic reactions? (2)
Chemical hand warmers, self-heating cans
3 main endothermic reactions?
Photosynthesis, hydrogen carbonate + acid (sherbet), dissolving ammonium nitrate in water
Equation for reaction in self-heating cans?
calcium oxide + water > calcium hydroxide
Uses of endothermic reactions?
Chemical cold packs
How do chemical cold packs work?
They contain ammonium nitrate and water, when ammonium nitrate dissolves, it absorbs energy from surroundings
What is Bond energy?
Energy needed to break a bond between 2 atoms
What units are bond energies measured in?
kJ/mol
You can use bond energies to work out…
the energy change for most reactions
When new bonds are formed between atoms….
energy is released
Energy must be supplied to…
break chemical bonds
What is an exothermic reaction?
One that gives out energy to the surroundings
What is an endothermic reaction?
One that takes in energy from the surroundings
In terms of energy, what happens when bonds are broken?
Energy is required (endothermic)
In terms of energy, what happens when bonds are formed?
Energy is released (exothermic)
How do you remember whether energy is released or required when bonds are being rearranged?
Bendy Mexicans:
B(endo)ROKEN
M(exo)ADE
What does it mean if the energy of the products is lower than the reactants?
Energy has been released and the reaction is exothermic
What does it mean if the energy of the products is higher than the reactants?
Energy has been taken in and the reaction is endothermic
How does a catalyst increase the rate of reaction?
By lowering the activation energy
READ ONLY:
How do you work out total energy change?
• total for breaking bonds: no. of bonds x breaking bond energy
• total for making bonds: no. of bonds x making bond energy
• total energy change: breaking bonds - making bonds
What does it mean if you get a positive number when working out energy change?
The reaction is endothermic
What does it mean if you get a negative number when working out energy change?
The reaction is exothermic
In terms of energy change, what do exothermic and endothermic reactions always have?
• endothermic always have positive energy changes
* exothermic always have negative
How can a simple cell be made?
By connecting two different metals in contact with an electrolyte
What do batteries consist of to provide a greater voltage?
Two or more cells connected
What chemicals are cells made from?
• carbon rod with metal cathode
• manganese oxide
• ammonium chloride
• zinc casting anode
What is the electrolyte in a cell made of?
Ammonium chloride
What is the anode made of in a cell?
Zinc casting
What does the voltage produced by a cell depend on?
The type of metals making up the cathode and anode and electrolyte
What happens when two metals are dipped in salt solution and joined by a wire?
The more reactive metal will donate electrons to the less reactive metal to form an electrical cell
Which metal will lose their electrons in a simple electrical cell?
The more reactive metal
What happens when there is a greater difference between reactivity of metals in a cell?
There will be a higher voltage produced
Are alkaline batteries rechargeable?
No
What happens when one of the reactants has been used up in non-rechargeable batteries?
The chemical reactions stop
What happens to the chemical reactions in rechargeable batteries?
They are reversed using an external electrical current
How do hydrogen fuel cells work?
• oxygen is supplied to the positive electrode and diffuses through the graphite to react to form hydroxide ions - accepting electrons from the external circuit
• hydrogen is supplied to the negative electrode and diffuses through the graphite electrode and reacts with hydroxide ions to form water
• overall charge left in fuel cell
What hydrogen fuel cells offer an alternative to?
Fossil fuels and rechargeable cells and batteries
Advantages of hydrogen fuel cells?
• don’t need to be electrically recharged
• no pollutants produced
• can be a range of sizes for different uses
• do not get less efficient as they run
• can be a source of drinking water
Disadvantages of hydrogen fuel cells?
• hydrogen is flammable and difficult to store
• hydrogen is sometimes produced for the cell by non renewable sources
• produce a relatively low voltage so several are needed together
What are fuel cells useful as an energy source for aircraft?
• provide water
• efficient
• no moving parts
• lightweight
Half equation for the hydrogen supplied to a hydrogen fuel cell? (anode)
H2 -> 2H+ + 2e-
Half equation for the oxygen supplied to a hydrogen fuel cell?
02 + 4H2+ 4e- > 2H20
Equation for the two electrode reactions in a hydrogen fuel cell?
2H2 + 02 →> 2H20
