Language of rates of reaction

This applies to both A-level and GCSE. It is amazing how many marks are thrown away when writing about the collision theory.

Increasing temperature, concentration and surface area all increase the frequency of collisions. 

It is not enough to say only there are more collisions as there is no reference to time.

More collisions per second is fine though.

A greater chance of collisions is not good enough as probability is not the same as frequency (This is sometimes accepted though.)

 

When talking about increased concentration then say there are more particles per unit volume.  

More particles on its own is not good enough.

 

When talking about temperature the particles have more energy and move faster. They therefore collide more frequently. More particles will also collide with more energy than the activation energy. Leading to more successful collisions.

Be careful with activation energy. Sometimes candidates write particles have more activation energy. This is not correct though.

Activation energy  is the minimum energy needed for the collision to be successful. Particles can’t have more or less activation energy.  Particles can only have more or less energy than the activation energy. ( just two missing words completely changes the meaning)

If the particles have less energy than the activation energy they bounce off each other and do not react. If the particles have more energy than the activation energy then they react.

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Language of bonding

The golden rule in writing about bonding is to be clear about what particles are attracted to each other and what that attraction is.

Do not use the words atoms, ions, and molecules carelessly. A wrongly chosen word can throw away all your marks.

If a question is asking about the high melting point of magnesium oxide, a student will not get much credit for just saying it has strong bonds or strong ionic bonds.

A good answer might be: MgO has a giant lattice structure with strong electrostatic forces of attraction between oppositely charged ions. These require a lot of energy to break.

Similarly if talking about hydrogen bonding in ethanol it is essential to make to make it clear that you know  it exists between  ethanol molecules.

Statements like ethanol has hydrogen bonding, or contains hydrogen bonds are too vague, and potentially hint at a misunderstanding. Some students mistakenly believe the hydrogen bond in ethanol is the actual O-H covalent bond within the molecule. Any confusion with covalent bonds will result in losing all the marks in the question.

A good way of phrasing it is that ethanol forms hydrogen bonds between  molecules.

 

 

Welcome to ChemRevise (or welcome back)

It’s the start of a new academic year so welcome back.

I get lots of questions about whether ChemRevise notes say everything that is needed to do well at A-level. I ought to point out when I originally wrote these notes they were aimed as brief revision notes for my own students. My expectation was that they had done two years of hard  work, answering many questions and fully understood all that we have covered in class. They cannot be a replacement for this hard work.  There is no quick route to success at A-level. These notes can make revision more manageable and they have been used successfully by many thousands of students.

I have more recently made detailed notes in the section called text book that could help deepen your understanding. Some of these chapters have questions.

I hope you have a good year.

Updates to A-level Textbook

I had a bit of a spurt of effort on my e-text book project over Easter and it is now all in the same format. It is very much an ongoing project though. At the moment it as an amalgamation of all the written material I have consolidated from various sources of mine. It is not therefore syllabus specific. Some material may be the same as what is on the revision guides. Some has more extensive explanations or I have tried to look at things from a different angle. Some material is also extension to A-level. Over time I will add to the explanations to turn the revision guide bits into something more detailed. I will also add to the questions that appear in some chapters.