Equation and mechanism trainer
Equations
Equation 1 of 7 · 0 correct in this set
Complete combustion of propane, C3H8, in oxygen.
Mechanisms
Hydroxide ions attack bromoethane. Conditions: NaOH or KOH, aqueous, warm (under reflux).
Tap where each electron pair starts (a lone pair or a bond) and then where it goes (an atom, or the space where a new bond forms). Check when you have drawn every arrow for this stage.
2 arrows to draw
Where your marks went
Questions people ask
Do I lose marks for an equation written in a different multiple?
Usually not. AQA mark schemes normally accept any correct multiple of a balanced equation, including fractions, unless the question asks for a particular amount, such as an enthalpy change per mole.
Why do I lose marks on curly arrows?
Each arrow is usually a separate marking point. The common losses are an arrow that starts from an atom instead of a lone pair or a bond, an arrow from a positive charge, and an arrow that ends in the wrong place, such as on the wrong carbon or not at the atom that gains the electrons.
Does the direction of the arrow matter?
Yes. The arrow shows a pair of electrons moving, so it must start where the electrons are (a lone pair, a negative charge or a bond) and point to where they go.
How we work this out
What it does
Write the equation and see at once which element or charge does not balance. Then draw the curly arrows for the AQA mechanisms, one arrow at a time.
Equations, ionic equations and half-equations from the AQA 7405 course, and the curly-arrow mechanisms AQA names, drawn on structures.
Method
- Your equation is read however you write it: any multiple or fractions, with or without state symbols, charges as Fe3+, Fe^3+ or Fe³⁺, and arrows as ->, → or =.
- If it has the right substances but does not balance, the trainer names the first element or the charge that differs between the two sides.
- If it balances but has the wrong substances, it says so without giving the answer away.
- Mechanisms are marked arrow by arrow: where each electron pair starts and where it goes. An arrow from a lone pair and one from the negative charge on the same atom count as the same arrow, as AQA allows.
Limits
- State symbols are not checked here. Add them when a question asks for them.
- The trainer checks one correct answer per question. Where AQA would accept another valid equation, it is listed with the answer.
A tick means the equation or arrow is chemically right as written. It cannot mark written explanations or conditions. Use the practice questions for those.