Grade 10

Chemical Reactions

What really happens when substances react, how to write it down, how much product to expect and where the energy goes.

6 lessons

  1. 1How can you tell a chemical change from a physical one?Ask whether a new substance has appeared. In a physical change the substance stays what it was and only its shape or state altered, and you can usually reverse it. In a chemical change different substances form with different properties, and getting back needs another reaction rather than just cooling or drying.30m
  2. 2Why may you change a coefficient in an equation but not a number inside a formula?Because the coefficient says how many units take part, while the number inside the formula says what the substance is. Changing a coefficient means taking more of the same substance; changing a number inside a formula means swapping it for a different substance. You balance by adding units, not by inventing new compounds.35m
  3. 3What is a mole, and why do chemists count atoms by weighing them?A mole is a pack of a fixed number of particles — about 6.02 times ten to the twenty-third. A single atom is far too small to weigh and there are far too many to count in any real sample, but the mass of a whole pack sits comfortably on an ordinary balance. The molar mass is read straight off the periodic table, so weighing becomes counting.40m
  4. 4How do you predict how many grams of product a given mass of reactant will give?In three steps. Convert the reactant's mass to moles, use the coefficient ratio in the balanced equation to cross to moles of product, then convert back to grams. The equation is a recipe written in moles, and the mole is what lets you enter and leave it with a balance.40m
  5. 5Why does one flask warm up while another goes cold?Because breaking bonds costs energy and forming bonds releases it, and the difference between the two totals decides. If more was released than spent, the surplus leaves as heat and the reaction is exothermic. If breaking cost more, the shortfall is taken from the surroundings and the flask cools — an endothermic reaction.35m
  6. 6What actually happens when an acid meets a base?The hydrogen ion the acid released and the hydroxide ion the base released join to make water. What is left in solution is a salt — the base's positive ion with the acid's negative ion. That is why neutralisation always gives a salt and water, and why the pH moves towards seven.40m