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Chemistry
Reactions & Everyday Chemistry
What happens when substances meet, and why.
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What is the chemical formula for water?
How Atoms Bond and React
This is the machinery underneath the rest of chemistry. Ten questions on bonding, energy and reaction rates, aimed at people who want more than the formulas.
About this topic
Almost everything chemistry describes comes down to what electrons are doing. Hand them over and you get ions that stick together by simple attraction. Share them and you get a molecule with a definite shape. Move them from one substance to another in the middle of a reaction and you have oxidation and reduction, which covers everything from a battery to a rusting gate. The second half of the subject is bookkeeping and speed. Atoms are never created or destroyed in a reaction, only rearranged, which is why equations have to balance. And a reaction that is going to happen eventually can still be made to happen sooner, which is most of what industrial chemistry is about.
Questions & answers
How Atoms Bond and React: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1In an ionic bond, what happens to the electrons involved?
- A They are released into the surroundings
- B One atom transfers them to the other Correct
- C Both atoms share them equally
- D Both atoms share them, but unequally
Answer: B. One atom transfers them to the other
Why: A metal hands electrons to a non-metal, leaving two oppositely charged ions that attract each other. Because the ions are locked in place in a solid, ionic compounds only conduct electricity once they are melted or dissolved.
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2What kind of bond holds the hydrogen atoms to the oxygen atom inside a water molecule?
- A Covalent bonds Correct
- B Ionic bonds
- C Metallic bonds
- D Hydrogen bonds
Answer: A. Covalent bonds
Why: Within the molecule the atoms share electrons, which is a covalent bond. Hydrogen bonds are the weaker attractions between separate water molecules, and they are the reason water stays liquid at temperatures where similar-sized molecules are gases.
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3A reaction that gives out heat to its surroundings is described as what?
- A Endothermic
- B Isothermic
- C Catalytic
- D Exothermic Correct
Answer: D. Exothermic
Why: Forming the new bonds releases more energy than breaking the old ones absorbed, and the surplus leaves as heat. Instant cold packs run the other way, absorbing heat as a salt dissolves.
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4In a balanced chemical equation, what has to be equal on both sides?
- A The number of separate compounds
- B The total number of bonds
- C The number of atoms of each element Correct
- D The number of molecules
Answer: C. The number of atoms of each element
Why: Atoms are only rearranged in a reaction, never created or destroyed, so every element has to be accounted for on both sides. Lavoisier established the point in the 1770s by weighing sealed vessels before and after burning things inside them.
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5An acid reacting with a base produces water and what else?
- A A metal
- B A salt Correct
- C An oxide
- D An alcohol
Answer: B. A salt
Why: The reaction is called neutralisation, and the salt is whatever the leftover ions form. Table salt is only the most familiar example of a very large family.
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6What does a reducing agent do in a reaction?
- A Donates electrons to another substance Correct
- B Accepts electrons from another substance
- C Removes water from the mixture
- D Speeds the reaction up without being used up
Answer: A. Donates electrons to another substance
Why: The reducing agent gives electrons away, which means it gets oxidised itself in the process. The old mnemonic still works: oxidation is loss of electrons, reduction is gain.
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7What shape is a methane molecule?
- A Flat and square
- B Linear
- C A square-based pyramid
- D Tetrahedral Correct
Answer: D. Tetrahedral
Why: The four bonding pairs of electrons repel each other and end up as far apart as possible, at angles of about 109.5 degrees. That same tetrahedral arrangement around carbon is what makes so many organic molecules three-dimensional rather than flat.
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8Avogadro's number gives the number of particles in what?
- A One litre of any gas
- B One kilogram of a substance
- C One mole of a substance Correct
- D One gram of a substance
Answer: C. One mole of a substance
Why: It is roughly 6.022 followed by 23 zeros, and it lets chemists count atoms by weighing them. The mole was long defined so that 12 grams of carbon-12 contained exactly that many atoms.
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9Why does heating a mixture usually make a reaction go faster?
- A The products become more stable
- B Particles collide more often and with more energy Correct
- C The activation energy of the reaction falls
- D The bonds in the reactants are permanently weakened
Answer: B. Particles collide more often and with more energy
Why: Heat makes particles move faster, so collisions happen more frequently and a larger share of them are hard enough to react. As a rough rule of thumb, many reactions roughly double in speed for every 10 degree rise.
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10A reversible reaction that has reached equilibrium is doing what?
- A Running forwards and backwards at the same rate Correct
- B Nothing at all, since both reactions have stopped
- C Running until the reactants are entirely used up
- D Sitting exactly halfway between reactants and products
Answer: A. Running forwards and backwards at the same rate
Why: Both directions carry on, but at matching rates, so the amounts stop changing. Le Chatelier's principle predicts which way the balance shifts if you change the pressure, temperature or concentration.
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