Chemical Reactions
Section: Chemistry | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is a Chemical Reaction?
- A chemical reaction is a process where atoms are rearranged to form new substances with different properties
- The reactants are converted into products
- Chemical reaction: a process where atoms are rearranged to form new substances with different properties; the reactants are converted into products
Key Components
- Reactants are the starting materials
- Products are the new substances formed
- Chemical bonds are broken in the reactants and new bonds are formed in the products
Word and Symbol Equations
- A word equation describes a reaction using the names of the substances: Reactants → Products
- A symbol equation describes the same reaction using chemical formulae; balancing symbol equations is not required at this level
- Example:
- Word equation: Magnesium + Oxygen → Magnesium oxide
- Symbol equation: 2Mg + O₂ → 2MgO
Counting Atoms vs Counting Elements in a Formula
- Be careful not to confuse the number of atoms in a formula with the number of different elements in a formula
- Example: barium sulfate, BaSO₄, contains 6 atoms in total (1 Ba + 1 S + 4 O), but only 3 distinct elements (barium, sulfur and oxygen)
- Example: sodium sulfate, Na₂SO₄, also contains 3 distinct elements (sodium, sulfur and oxygen), even though it has a different total number of atoms (7)
- By contrast, sodium chloride, NaCl, and barium chloride, BaCl₂, each contain only 2 distinct elements
Preparing a Pure, Dry Salt
- A soluble salt can be prepared by reacting an acid with an excess of an insoluble reactant (a metal oxide or a metal carbonate), then separating and crystallising the salt from the leftover mixture
Safety
- Dilute acids are corrosive, so wear safety glasses/goggles to stop acid splashing into the eyes (gloves can also be worn to protect the hands)
Method: Making Salt Crystals from an Insoluble Reactant and an Acid
- Step 1: add the insoluble reactant (metal oxide or metal carbonate) to the acid in excess, a little at a time, so that all of the acid is used up and some solid is left over
- Step 2: filter the mixture to separate the leftover excess solid (the residue, trapped on the filter paper) from the salt solution (the filtrate, which passes through)
- Step 3: gently heat/evaporate the filtrate to remove some of the water, concentrating the solution until it is close to saturation (the point where crystals just start to form) - do NOT heat to dryness, as this can decompose the salt or produce a powder instead of crystals
- Step 4: remove from heat and leave the solution to cool slowly, allowing crystals to form
- Step 5: filter again to collect the crystals, then pat them dry between sheets of filter paper
The Law of Conservation of Mass
- The law of conservation of mass states that in a chemical reaction, the total mass of the reactants equals the total mass of the products
- No atoms are created or destroyed
- Law of conservation of mass: in a chemical reaction, the total mass of the reactants equals the total mass of the products - no atoms are created or destroyed
Mass is conserved in a sealed reaction vessel: mass of reactants equals mass of products
Why Mass is Conserved
- Atoms are not created or destroyed during a chemical reaction
- Atoms are simply rearranged into new combinations
- The number and type of atoms stays the same before and after the reaction
- Therefore, the total mass must remain constant
Worked Example: Magnesium Burning
Museli burns magnesium in oxygen: Magnesium + Oxygen → Magnesium oxide (2Mg + O₂ → 2MgO).
- Step 1: 2.4 g of magnesium reacts with 1.6 g of oxygen
- Step 2: Mass of magnesium oxide produced = 2.4 g + 1.6 g = 4.0 g
Practical Investigation
- To demonstrate conservation of mass in the lab:
- place reactants in a sealed container on a balance
- record the initial mass
- allow the reaction to occur
- record the final mass
- the mass should be the same (if the container is sealed)
Common Mistakes
- If a gas escapes (open container), the measured mass will appear to decrease, but the total mass including the escaped gas is still conserved
Apparent Mass Changes
- Sometimes it looks like mass has changed, but it hasn't - this is usually due to gases entering or leaving the system
Apparent Mass Increase
Example: burning magnesium in open air.
- The magnesium gains mass because it combines with oxygen from the air
- The oxygen isn't weighed at the start, so it looks like mass increased
- In reality: Mass of Mg + Mass of O₂ (from air) = Mass of MgO
Apparent Mass Decrease
Example: heating copper carbonate.
- Word equation: Copper carbonate → Copper oxide + Carbon dioxide
- Symbol equation: CuCO₃ → CuO + CO₂
- If the CO₂ gas escapes, the measured mass decreases
- In reality: Mass of CuCO₃ = Mass of CuO + Mass of CO₂ (escaped)
| Situation | What Happens | Explanation |
|---|---|---|
| Sealed container | No mass change | All reactants and products are contained and weighed |
| Open container, gas produced | Apparent mass decrease | Gas escapes but mass is still conserved overall |
| Open container, gas absorbed | Apparent mass increase | Gas from air joins the reaction but wasn't weighed initially |
Calculations Using Conservation of Mass
- We can use the law to calculate unknown masses
- The key is to remember that mass of reactants = mass of products
Worked Example: Decomposition Reaction
Muumbe is told that 10 g of calcium carbonate decomposes to form 5.6 g of calcium oxide and carbon dioxide, and wants to find the mass of carbon dioxide produced.
- Step 1: Mass of reactants = Mass of products
- Step 2: 10 g = 5.6 g + mass of CO₂
- Step 3: Mass of CO₂ = 10 g − 5.6 g = 4.4 g
Worked Example: Combination Reaction
Muchindu is told that 11.2 g of iron reacts with 6.4 g of sulfur, and wants to find the mass of iron sulfide produced.
- Step 1: Mass of products = Mass of reactants
- Step 2: Mass of iron sulfide = 11.2 g + 6.4 g = 17.6 g
Energy Changes in Reactions
- All chemical reactions involve energy changes
- Energy is needed to break bonds and energy is released when new bonds form
- Breaking bonds requires energy (endothermic process)
- Making bonds releases energy (exothermic process)
Exothermic Reactions
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