Chemistry b Flashcards
How is ‘a mole’ significant in chemistry ?
When you have that precise number of carbon-12 atoms it will weigh 12g. It works for every element (Ar) or compound (Mr)
What is ‘a mole’ ?
It is the name given to the number:
602,300,000,000,000,000,000,000
What is concentration ?
A measure of how crowded things are
What is concentration measured in ?
Moles per dm^3
(^3 = cubed)
Or Grams per dm^3
What are titrations ?
They allow you to find out exactly how much acid is needed to neutralise a quantity of alkali
What indicators are used in titrations ?
- Phenolphthalein - alkali=pink acid=colourless
* Methyl Orange - alkalis=yellow acid=red
Why is universal indicator not used ?
It changes colour gradually, but we need definite colour change.
Explain the process of a titration experiment
Put some alkali in a flask with indicator. Gradually add the acid a bit at a time using the burette. The indicator will change as it becomes neutralised. Record the amount of acid used using the marks along the burette.
1 dm^3 is equal to what ?
1000cm^3 or a litre
See revision guide for titration calculations
Page 88
How can you measure energy transfer ?
By taking the temperature at the start and at the end of the reaction. The difference gives us the amount of energy transferred
What is biggest problem with energy measurements ?
Some of the energy will be loft to the surroundings
How can you reduce heat lost to the surroundings when measuring heat exchange ?
- Insulate the equipment holding the liquid
* Put a lid on to reduce heat loss through evaporation
What is an exothermic reaction ?
A reaction which GIVES OUT energy to the surroundings, usually in the form of heat and shown by a RISE in temperature
What is an endothermic reaction ?
A reaction which TAKES IN energy from the surroundings, usually in the form of heat and shown by a FALL in temperature
What happens during a chemical reaction ?
Old bonds are broken and new bonds are formed
Is breaking bonds an endothermic reaction ?
Yes. Energy must be supplied to break existing bonds, so energy is TAKEN IN by the surroundings.
If energy is released to form bonds, what type of reaction is it ?
Exothermic
If the reaction is endothermic overall, what does this tell us ?
The energy required to break old bonds is greater than the energy released when new bonds are formed.
What is calorimetry ?
Different fuels have different amounts of energy. To measure the amount of energy released when a fuel is burnt, you can simply use the flame to heat up water. This is known as calorimetry
What equation will tell you the energy transferred by different fuels ?
Energy transferred = mass of water x specific heat capacity of water x temperature change
What is the unit energy change is recorded in ?
Joules
Why is the energy transfer figure usually lower than the real energy transferred ?
Because a lot of energy is wasted heating the container or the air around the container
What does the difference of heights between the reactants and the products represent on an energy level diagram ?
The energy change
