Chemical Reactions and Physical Changes Flashcards
what’s a chemical reaction…
When elements and/or compounds react to make different elements/compounds.
New substances are always made in a chemical reaction.
what’s a physical change/reaction…
In a physical change no new substances are made -the particles DO NOT change, they are just arranged differently.
Difference between a physical change and a chemical reaction…
A chemical reaction forms new substances but is irreversible whereas a physical change doesn’t form new substances but is arranged differently and is reversible.
list some observations that tell us it is a chemical reaction…
· One or more new substances are made.
· The particles react to form different particles. For example when hydrogen burns, hydrogen molecules react with oxygen molecules to form water molecules.
· Most chemical changes are irreversible.
Examples –
- rusting
- combustion (burning)
- acid reacting with alkali
We can tell that a chemical reaction has happened if there is:
o a colour change
o a temperature change
o a pH change
o effervescence (fizzing and bubbling) because a gas is produced
o a solid produced – the mixture will turn cloudy
list some observations that tell us it is a physical change…
· No new substances are made.
· The particles do not change - they are just arranged differently.
For example ice is made of water molecules, when it melts it is still made of water molecules – the only difference is how the molecules are arranged and how they move.
· There is a change of appearance (it will look different).
· Most physical changes are reversible.
Examples –
- changes of state (melting, evaporating, condensing, freezing, sublimation)
- dissolving
- diffusion
what happens in a chemical reaction in terms of atoms…
A chemical reaction occurs when moving molecules hit each other, breaking their bonds and producing an exchange of atoms that form new products
why is mass conserved (stays the same) in a chemical reaction in terms of atoms…
No atoms are added or taken away from the system, so the mass stays the same.
Even in a chemical reaction when atoms interact and create new products, mass is conserved. This is because the new substances created are composed of atoms that were present in the reactants.
what is the law of conservation…
No atoms are lost or made in a chemical reaction – they are just rearranged. This means that the total mass of the products is exactly the same as the total mass of products. Overall the mass hasn’t change – we say mass has been conserved. This is the law of conservation of mass.
total mass of reactants = total mass of products
e.g.
At the start (reactants) there are two magnesium atoms and two oxygen atoms - so four atoms in total. At the end (products) there are still two magnesium atoms and two oxygen atoms (four atoms in total), they are just arranged differently.
oxidation…
A chemical reaction when an element or compound reacts with oxygen to form an oxide.
e.g. magnesium + oxygen —-> magnesium oxide
sulfur + oxygen —-> sulfur dioxide
Non-metal oxides (e.g. sulfur dioxide) are acidic and metal oxides (e.g. magnesium oxide) are basic
combustion…
The chemical reaction that takes place when something burns.
The elements in the fuel react with oxygen from the air (so it is also an oxidation reaction) and heat energy is given out (it is an exothermic reaction).
Most fuels are hydrocarbons - they contain hydrogen and carbon. The carbon reacts with oxygen to produce carbon dioxide and the hydrogen reacts with oxygen to form water. So the products of combustion are carbon dioxide and water.
e.g.
methane + oxygen –> carbon dioxide + water
CH4 + 2O2 –> CO2 + 2H2O
endothermic…
A reaction that TAKES IN heat energy from the surroundings, so the temperature will decrease.
Examples: thermal decomposition and photosynthesis.
exothermic…
A reaction that GIVES OUT heat energy to the surroundings so the temperature will increase.
Examples: oxidation, combustion, displacement reactions and neutralisation.
what is the difference between a word equation and symbol equation…
Word equations are used to represent chemical equations with the name of the elements while symbol equations are used to represent chemical equations with the symbols of the elements
How do particle diagrams show chemical reactions?
A given chemical reaction can be represented using a particulate diagram, in which the reaction mixture is depicted both before the reaction occurs and after the reaction has proceeded completely as possible
rate of reaction…
The rate of a reaction is the speed – how fast or slow the reaction is.
How to balance an equation…
We balance symbol equations to ensure that there are the same number of atoms of each element on the left-hand side of the equation as there are on the right-hand side. We cannot change the symbols or formulae (so we cannot add, remove or change the small numbers that are already in the equation). We can only write big numbers in front of the formula or symbol to multiply it.
In this image we balance the equations by putting the equal number of atoms on both sides adding numbers in front of elements.
expansion…
When something gets bigger (its volume increases).
This happens when a substance is heated, because the particles have more energy, so move faster and spread out, so the substance takes up more space overall. The individual particles DO NOT expand, they just spread out.
contraction…
When something gets smaller (its volume decreases). It is the opposite of expansion
This happens when a substance is cooled, because the particles have less energy, so move slower and get closer together, so the substance takes up less space overall. The individual particles DO NOT contract, they just get closer together.
gas pressure…
Gases exert pressure on the walls of the container that they are in because when the particles move around they bump into the walls with force.
diffusion…
When particles spread out from a high concentration (one place) to a low concentration (many places).
For example, if you spray air freshener in one corner of the room, eventually the air freshener particles will diffuse (spread out) into the whole room. Diffusion happens in gases and liquids because the particles can move around, but cannot happen in a solid because the particles cannot move. Diffusion is faster in gases than in liquids because the particles are moving faster
