Chemical Reactions
Chemical reactions is the topic covering how substances change permanently to form new products, shown using word and symbol equations. It covers conservation of mass, exothermic and endothermic reactions, and required practicals measuring mass changes, temperature changes and rates of reaction, building on knowledge of atoms and compounds.
Method
- Identify signs that a chemical reaction has happened, such as a gas given off, a colour change, a temperature change, or a new solid forming.
- Write and balance word equations and symbol equations for simple reactions.
- Apply conservation of mass to calculate an unknown mass in a reaction.
- Classify reactions as exothermic or endothermic from a temperature change.
- Learn the required practicals: measuring mass changes on heating, temperature changes in reactions, and rates of reaction using a gas syringe.
- Interpret rate of reaction graphs and calculate the mean rate from a graph or table of results.
Worked example
12.0 g of solid copper carbonate is heated strongly until it fully decomposes into copper oxide and carbon dioxide gas, which escapes. The solid remaining (copper oxide) has a mass of 7.7 g. Calculate the mass of carbon dioxide gas released.
- Recall that mass is conserved: mass of reactant = total mass of all products.
- Write down what is known: copper carbonate = 12.0 g; copper oxide (solid remaining) = 7.7 g.
- Set up the calculation: mass of carbon dioxide = mass of copper carbonate - mass of copper oxide.
- Substitute the values: 12.0 - 7.7.
- Calculate: 12.0 - 7.7 = 4.3.
- State the final answer: 4.3 g of carbon dioxide gas was released.
Practice questions
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Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
State two observations that could show a chemical reaction has taken place when a solid is heated.
A student heats a coiled strip of copper in a crucible with a lid, lifting the lid occasionally to let air in. The empty crucible has a mass of 30.00 g. The crucible plus copper has a mass of 30.60 g before heating. After the copper has fully reacted, the crucible plus the black solid formed has a mass of 30.75 g. Calculate the mass of copper used, the mass of copper oxide formed, and the mass of oxygen gained.
A student reacts an excess of magnesium ribbon with 50 cm3 of dilute hydrochloric acid in a conical flask connected to a gas syringe. Results: Time (s): 0, 10, 20, 30, 40, 50; Volume of hydrogen gas (cm3): 0, 18, 30, 38, 42, 42. Identify the independent and dependent variables, calculate the mean rate of reaction between 0 and 30 seconds in cm3/s to 3 significant figures, and explain, in terms of particles, why the rate of reaction decreases as the experiment proceeds and why the reaction eventually stops producing gas.
Free printable worksheet
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