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While reading my chemistry textbook I came across the phrase:

forward reaction is favoured by decrease in temperature in exothermic reactions.

I was tripped off by the term 'forward reaction', can someone explain in layman terms what is meant by this?

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    $\begingroup$ The forward reaction is the reaction as written, towards products, in the direction of the reaction arrow. Naturally, the reverse or backwards reaction is in the opposite direction. $\endgroup$
    – Zhe
    Commented Jan 14, 2019 at 15:51

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This phrase comes from Le Châtelier's Principle. Given a reversible reversible reaction, you may (arbitrarily) choose which chemicals to consider the reactants/products, since the reaction goes from both the reactants to the products and from the products to the reactants. Simply put, the forward reaction starts with what you consider to be the reactants. The reverse reaction starts with the products and ends with the reactants. If you flip the equation these names change.

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  • $\begingroup$ So in all, if the word 'reaction' was used instead of 'forward reaction' it wouldn't have made much a difference? $\endgroup$ Commented Jan 15, 2019 at 3:53
  • $\begingroup$ Not all reactions are reversible. That is key. But essentially, yes. $\endgroup$
    – ringo
    Commented Jan 15, 2019 at 3:54
  • $\begingroup$ So the reaction favoured ahead in case of a reversible reaction is forward reaction? $\endgroup$ Commented Jan 15, 2019 at 3:56
  • $\begingroup$ In the reaction $\ce{A <=> B}$, if B is favored, the forward reaction is favored. $\endgroup$
    – ringo
    Commented Jan 15, 2019 at 4:00
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It's arbitrary and may be a cultural misunderstanding. Most English speaking cultures regard left to right on a page as moving forward and reverse is going from right to left. Thus for a reversible reaction such as producing ammonia from hydrogen and nitrogen: $$\text{(reactants)}\qquad\ce{H2 + N2 <=> 2NH3}\qquad\text{(product)}\tag{reversible reaction}$$

the forward reaction is forming product(s) on the right from reactant(s) on the left $$\text{(reactants)}\qquad\ce{H2 + N2 \color{blue}{->} 2NH3}\qquad\text{(product)}\tag{forward reaction}$$

where as the reverse reaction is forming reactant(s) from product(s) $$\text{(reactants)}\qquad\ce{H2 + N2 \color{red}{<-} 2NH3}\qquad\text{(product)}\tag{reverse reaction}$$


If I were to produce hydrogen and nitrogen from ammonia, the reaction would look more like:

$$\text{(reactant)}\qquad\ce{2NH3 <=> H2 + N2}\qquad\text{(product)}\tag{reversible reaction}$$

the the forward reaction is: $$\text{(reactants)}\qquad\ce{2NH3 \color{blue}{->} H2 + N2 }\qquad\text{(product)}\tag{forward reaction}$$

and the reverse reaction is: $$\text{(reactants)}\qquad\ce{NH3 \color{red}{<-} H2 + N2}\qquad\text{(product)}\tag{reverse reaction}$$

The reactions are the same but because the order of the reversible reaction flipped, which reaction is considered the forward and and reverse has also flipped

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  • $\begingroup$ On my screen, the annotations on the right side of each equation run over into the links in the side bar... $\endgroup$
    – Zhe
    Commented Jan 14, 2019 at 20:45

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