Unit 5: Chemical Reactions and Equations
Reading a chemical equation as a real rearrangement of atoms, balancing it correctly, and sorting reactions into the patterns they actually follow.
10 topics · 33 questions
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5.1 Reading and Writing Chemical Equations
- How it worksHow do the reactants on the left rearrange into the products on the right in a chemical equation?
- Common mistakeWhy doesn't a plus sign between two reactants mean they're simply added together unchanged?
- Going furtherWhat happens to a chemical equation's notation when a reaction needs heat to get started?
Your turn1 of 2
A chemical equation like 2H2 + O2 makes 2H2O uses an arrow pointing from reactants to products, never an equals sign the way a math equation would. Why does an arrow instead of an equals sign separate the two sides of a chemical equation?5.2 Balancing Chemical Equations
- How it worksHow do you adjust the coefficients in front of formulas until every element matches on both sides?
- Worked exampleFind the balanced equation for the combustion of methane, CH4 + O2 makes CO2 + H2O.
- Common mistakeWhy doesn't changing a subscript instead of a coefficient actually balance an equation?
- Common mistakeWhy doesn't writing O instead of O2 in an equation still correctly represent the way oxygen gas actually exists?
- Going furtherWhat happens to the balanced coefficients when you double every amount of reactant you start with?
Your turn1 of 2
An unbalanced chemical equation needs a coefficient of 6 placed in front of a chemical formula that contains 4 atoms of a particular element in each formula unit. Once balanced with that coefficient, how many total atoms of that element does this term of the equation contribute?5.3 Conservation of Mass in a Reaction
- How it worksHow does a balanced equation show that every atom present at the start is still present at the end?
- Worked exampleFind the mass of carbon dioxide produced when 16 grams of methane react completely with 64 grams of oxygen.
- Common mistakeWhy doesn't a burning log losing visible mass violate the law of conservation of mass?
Your turn1 of 2
In a sealed container, 141 grams of one reactant combine completely with 132 grams of a second reactant. According to the law of conservation of mass, what is the total mass of the products formed?5.4 Synthesis (Combination) Reactions
- How it worksHow do two or more simple substances combine into one larger product in a synthesis reaction?
- Worked exampleFind the product formed when solid sodium reacts with chlorine gas in a synthesis reaction.
- Common mistakeWhy isn't every reaction that produces one substance automatically a synthesis reaction?
Your turn1 of 2
Solid sodium metal and chlorine gas react together vigorously the instant they touch, rushing directly into a single compound, sodium chloride, rather than reacting slowly or gradually. Why do two separate elements sometimes rush together into a single compound the moment they touch?5.5 Decomposition Reactions
- How it worksHow does a single compound absorb energy and break apart into two or more simpler products?
- Worked exampleFind the products formed when calcium carbonate decomposes when heated.
- Common mistakeWhy isn't decomposition simply the reverse of every synthesis reaction you could run?
Your turn1 of 2
A bottle of hydrogen peroxide left sitting on a shelf slowly breaks down into water and oxygen gas over time, even without anyone actively doing anything to it. Why does hydrogen peroxide slowly fall apart into water and oxygen just sitting on a shelf?5.6 Single-Replacement Reactions
- How it worksHow does a more reactive element kick a less reactive element out of a compound in a single-replacement reaction?
- Worked exampleFind the products formed when solid zinc is added to a solution of copper(II) sulfate.
- Common mistakeWhy doesn't every metal added to a solution automatically replace the metal that's already in it?
- Going furtherWhat happens when you try a single-replacement reaction with a metal that's less reactive than the one in solution?
Your turn1 of 2
Dropping a strip of zinc metal into a blue copper sulfate solution slowly fades the blue color to clear while a reddish copper coating builds up on the zinc strip. Why does a piece of zinc dropped into copper sulfate solution slowly turn the solution clear?5.7 Double-Replacement Reactions and Precipitates
- How it worksHow do the positive and negative ions in two dissolved compounds swap partners in a double-replacement reaction?
- Worked exampleFind the precipitate formed when a solution of silver nitrate is mixed with a solution of sodium chloride.
- Common mistakeWhy doesn't every double-replacement reaction produce a visible precipitate?
Your turn1 of 2
Mixing two clear, colorless solutions together, one containing dissolved silver nitrate and the other dissolved sodium chloride, instantly produces a cloudy white solid suspended in the liquid. Why does mixing two clear solutions sometimes instantly cloud up into a solid?5.8 Combustion Reactions
- How it worksHow does a hydrocarbon combine with oxygen to release energy as carbon dioxide and water form?
- Worked exampleFind the balanced equation for the combustion of propane, C3H8 + O2 makes CO2 + H2O.
- Common mistakeWhy isn't every reaction that produces heat and light automatically a combustion reaction?
Your turn1 of 2
A lit candle placed under a sealed glass jar burns for a short while and then goes out on its own, well before the wax has been used up. Why does burning a candle always need oxygen to keep the flame alive?5.9 Predicting the Products of a Reaction
- How it worksHow does recognizing a reaction's type tell you what pattern its products will follow?
- Worked exampleFind the products formed when magnesium metal reacts with hydrochloric acid.
- Common mistakeWhy doesn't guessing a reaction's products at random ever replace knowing its reaction type?
Your turn1 of 2
A chemist can write out the expected products of a reaction between two named substances before ever actually mixing them together in a lab. Why can chemists predict what a reaction will make before they ever run it?5.10 The Activity Series and Predicting Reactivity
- How it worksHow does the activity series rank metals by how easily they give up electrons?
- Worked exampleFind whether a single-replacement reaction occurs when copper metal is placed into a solution of magnesium chloride.
- Common mistakeWhy doesn't a metal's activity ranking depend on how shiny or dense the metal looks?
Your turn1 of 2
Dropping a small piece of potassium metal into water triggers an immediate, vigorous, sometimes flaming reaction, while a piece of gold left sitting in water shows no reaction at all, even after a very long time. Why does potassium react violently with water while gold barely reacts with anything?